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LED7707
6-rows 85 mA LEDs driver with boost regulator for LCD panels backlight
Features
Boost section - 4.5 V to 36 V input voltage range - Internal power MOSFET - Internal +5 V LDO for device supply - Up to 36 V output voltage - Constant frequency peak current-mode control - 250 kHz to 1 MHz adjustable switching frequency - External synchronization for multi-device application - Pulse-skip power saving mode at light load - Programmable soft-start - Programmable OVP protection - Stable with ceramic output capacitors - Thermal shutdown Backlight driver section - Six rows with 85 mA maximum current capability (adjustable) - Rows disable option - Less than 10 s minimum dimming time - 2 % current matching between rows - LED failure (open and short-circuit) detection
VFQFPN-24 4x4
Description
The LED7707 consists of a high efficiency monolithic boost converter and six controlled current generators (rows) specifically designed to supply LEDs arrays used in the backlighting of LCD panels. The device can manage an output voltage up to 36 V (i.e. 10 white LEDs per row). The generators can be externally programmed to sink up to 85 mA and can be dimmed via a PWM signal (1 % dimming duty-cycle at 1 kHz can be managed). The device allows to detect and manage the open and shorted LED faults and to let unused rows floating. Basic protections (output over-voltage, internal MOSFET over-current and thermal shutdown) are provided.
Applications

LCD monitors and TV panels PDAs panel backlight GPS panel backlight
Table 1.
Device summary
Order codes LED7707 VFQFPN-24 4x4 (exposed pad) LED7707TR Tape and reel Package Packaging Tube
September 2008
Rev 1
1/48
www.st.com 48
Contents
LED7707
Contents
1 2 Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 2.2 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 3.2 3.3 Maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4 5
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 Boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.1.7 5.1.8 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Over voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Switching frequency selection and synchronization . . . . . . . . . . . . . . . 16 Slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2
Backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2.1 5.2.2 Current generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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LED7707
Contents
5.3
Fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.3.1 5.3.2 5.3.3 5.3.4 FAULT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 MODE pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Open LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Shorted LED fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.1 6.2 6.3 System stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.1.1 Loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.3.1 6.3.2 6.3.3 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Flywheel diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.4
Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7 6.4.8 6.4.9 Switching frequency setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Row current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Inductor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Output capacitor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Input capacitor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Over-voltage protection divider setting . . . . . . . . . . . . . . . . . . . . . . . . . 36 Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Power dissipation estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.5
Layout consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7 8 9
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3/48
Typical application circuit
LED7707
1
Figure 1.
VIN
Typical application circuit
Application circuit
L
Slope Compensation OVP selection
VOUT
MLCC +5V
AVCC VIN LX OVSEL SWF
ROW1 ROW2 ROW3 ROW4 ROW5 ROW6 Switching Frequency selection
LDO5 Internal MOS OCP BILIM Rows current selection RILIM SS COMP SGND
LED7707
FAULT
SLOPE
MODE
SYNC
DIM
EN
PGND
Up to 10 WLEDs per row
Dimming Fault Enable Faults Management Selection Sync Output
AM00579v1
4/48
LED7707
Pin settings
2
2.1
Pin settings
Connections
Figure 2. Pin connection (through top view)
FAULT SYNC
DIM
EN
SS
LX
19 18
24
COMP RILIM BILIM FSW MODE AVCC
1
OVSEL PGND
LED7706
ROW6 ROW5 ROW4
6 7 12
13
ROW3
LDO5
VIN
SLOPE
SGND
ROW1
ROW2
5/48
Pin settings
LED7707
2.2
Pin description
Table 2.
N 1 2 3
Pin functions
Pin COMP RILIM BILIM Function Error amplifier output. A simple RC series between this pin and ground is needed to compensate the loop of the boost regulator. Output generators current limit setting. The output current of the rows can be programmed connecting a resistor to SGND. Boost converter current limit setting. The internal MOSFET current limit can be programmed connecting a resistor to SGND. Switching frequency selection and external sync input. A resistor to SGND is used to set the desired switching frequency. The pin can also be used as external synchronization input. See Section 5.1.5 on page 16 for details. Current generators fault management selector. It allows to detect and manage LEDs failures. See Section 5.3.2 on page 23 for details. + 5 V analog supply. Connect to LDO5 through a simple RC filter. + 5V LDO output and power section supply. Bypass to SGND with a 1 F ceramic capacitor. Input voltage. Connect to the main supply rail. Slope compensation setting. A resistor between the output of the boost converter and this pin is needed to avoid sub-harmonic instability. Refer to Section 6.1 on page 26 for details. Signal ground. Supply return for the analog circuitry and the current generators. Row driver output #1. Row driver output #2. Row driver output #3. Row driver output #4. Row driver output #5. Row driver output #6. Power ground. Source of the internal Power MOSFET. Over-voltage selection. Used to set the desired 0 V threshold by an external divider. See Section 5.1.4 on page 15 for details. Switching node. Drain of the internal Power MOSFET. Dimming input. Used to externally set the brightness by using a PWM signal. Enable input. When low, the device is turned off. If tied high or left open, the device is turned on and a soft-start sequence takes place. Fault signal output. Open drain output. The pin goes low when a fault condition is detected (see Section 5.3.1 on page 23 for details). Synchronization output. Used as external synchronization output. Soft-start. Connect a capacitor to SGND to set the desired soft-start duration.
4
FSW
5 6 7 8 9
MODE AVCC LDO5 VIN SLOPE
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
SGND ROW1 ROW2 ROW3 ROW4 ROW5 ROW6 PGND OVSEL LX DIM EN FAULT SYNC SS
6/48
LED7707
Electrical data
3
3.1
Electrical data
Maximum rating
Table 3.
Symbol VAVCC VLDO5 AVCC to SGND LDO5 to SGND PGND to SGND VIN VLX VIN to PGND LX to SGND LX to PGND RILIM, BILIM, SYNC, OVSEL, SS to SGND EN, DIM, SW, MODE, FAULT to SGND ROWx to PGND/ SGND SLOPE to VIN SLOPE to SGND Internal switch maximum RMS current (flowing through LX node) PTOT Power dissipation @ TA = 25 C Maximum withstanding voltage range test condition: CDF-AEC-Q100-002- "human body model" acceptance criteria: "normal performance"
Absolute maximum ratings (1)
Parameter Value -0.3 to 6 -0.3 to 6 -0.3 to 0.3 -0.3 to 40 -0.3 to 40 -0.3 to 40 -0.3 to VAVCC + 0.3 -0.3 to 6 -0.3 to 40 VIN - 0.3 to VIN + 6 -0.3 to 40 2.0 2.3 (2) 1000 A W V V Unit
1. Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. Exposure to absolute maximum rated conditions for extended periods may affect device reliability. 2. Power dissipation referred to the device mounted on the demonstration board described in section 5.5
3.2
Thermal data
Table 4.
Symbol RthJA TSTG TJ
Thermal data
Parameter Thermal resistance junction to ambient Storage temperature range Junction operating temperature range Value 42 -20 to 125 -50 to 150 Unit C/W C C
7/48
Electrical data
LED7707
3.3
Recommended operating conditions
Table 5.
Symbol VIN VBST fSW Irowx TDIM_ON,min Input voltage range Boost section output voltage Switching frequency ROWs output current Minimum dimming on-time, RRILIM = 30 k (IROWx, NOM = 60 mA) 200 20 10
Recommended operating conditions
Value Parameter Min 4.5 Max 36 36 1000 85 kHz mA s V Unit
8/48
LED7707
Electrical characteristics
4
Table 6.
Symbol
Electrical characteristics
Electrical characteristics (VIN = 12 V; TA = 0 C to 85 C and LDO5 connected to AVCC if not otherwise specified)
Parameter Test condition Min Typ Max Unit
Supply section VLDO5 VAVCC IIN,Q IIN,SHDN VUVLO,ON VUVLO,OFF LDO output and IC supply voltage EN high ILDO5 = 0 mA RRILIM = 51 k, RBILIM = 220 k, RSLOPE = 680 k DIM tied to SGND. EN low 4.0 3.7 4.4 5 5.5 V
Operating quiescent current Operating current in shutdown LDO5 under voltage lock out upper threshold LDO5 under voltage lock out lower threshold
1 20 4.2 3.8 30 4.4
mA A
V 3.9
LDO linear regulator Line regulation LDO dropout voltage LDO maximum output current Boost section tON,min fSW Minimum switching on-time Default switching frequency Adjustable switching frequency Minimum FSW sync frequency FSW sync input threshold FSW sync input hysteresis FSW sync input duty-cycle FSW sync min ON time SYNC output duty-cycle SYNC output high level SYNC output low level Power switch KB RDS(on) LX current coefficient
Internal MOSFET on-resistance
6 V VIN 28 V, ILDO5 = 30 mA ILDO5 = 10 mA (-10 % drop) VLDO5 > VUVLO,ON VLDO5 < VUVLO,OFF 25 80 40 20
25 120 60 30
mV
mA
200 FSW connected to AVCC 570 250 220 240 300 60 FSW connected to AVCC (Internal oscillator selected) 40 270 FSW connected to AVCC (Internal oscillator selected) ISYNC = 10 A ISYNC = -10 A VAVCC -20V 20 34 40 660 750 1000
ns kHz
mV
% % %
mV
RBILIM = 300 k
1E6
1.2E6 280
1.4E6 500
V m
9/48
Electrical characteristics Table 6.
Symbol
LED7707
Electrical characteristics (continued) (VIN = 12 V; TA = 0 C to 85 C and LDO5 connected to AVCC if not otherwise specified)
Parameter Test condition Min Typ Max Unit
OC and OV protections VTH,OVP Over-voltage protection reference threshold 1.145 V
Soft-start and power management EN, Turn-on level threshold EN, Turn-off level threshold DIM, high level threshold DIM, low level threshold EN, pull-up current SS, charge current SS, end-of-startup threshold SS, reduced switching frequency release threshold Current generators section KR KR(1) VIFB Irowx,
FAULT
1.6 0.8 1.3 0.8 2.5 4 2.0 5 2.4 0.8 6 2.8 V A V
Current generators gain Current generators gain accuracy Feedback regulation voltage Rows output maximum current RRILIM = 21 k LED short circuit detection threshold LED short circuit detection masking time FAULT pin low-level voltage IFAULT,SINK = 4 mA MODE tied to SGND
1850 2.0 700 85 4.0 100 200 350 750
V % mV mA V s mV
Vrowx,
FAULT
TMASK VFAULT,
LOW
Thermal shutdown TSHDN Thermal shutdown turn-off temperature Thermal shutdown hysteresis
1. IROW = KR / RRILIM, IROW KR + RRILIM
150 30
C
10/48
LED7707
Operation description
5
Operation description
The device can be divided into two sections: the boost section and the backlight driver section. These sections are described in the next paragraphs. Figure 3 provides an overview of the internal blocks of the device. Figure 3. Simplified block diagram
VIN
SLOPE
Current Sense
LDO5
+5V LDO
Ramp Generator ++
ZCD
LX
BILIM SS
Current Limit _ OVP + 1.172V
Soft Start
Min Voltage Selector VROW6
CTRL6
Prot_EN
SYNC
Ext Sync Detector
/2
OSC
FSW
Prot_EN
AVCC EN MODE FAULT DIM
Thermal Shutdown 1.2V
Boost_EN UVLO CTRL6 CTRL5 CTRL4 CTRL3 CTRL2 4V
CONTROL LOGIC
CTRL1 VROW1
OVP
I to V Current Generator 1
_
COMP
gm
+
UVLO
RILIM
_ 0.7V
UVLO Detector
+
Boost Control Logic
PGND
Boost_EN
OVSEL
Current Generator 6 Current Generator 5 Current Generator 4 Current Generator 3 Current Generator 2
ROW6 ROW5 ROW4 ROW3 ROW2
VROW5
CTRL5
VROW4
CTRL4
VROW3
CTRL3
VROW2
CTRL2
VTH,FLT
MODE
LOGIC
ROW1
I to V + _
SGND
AM00580v1
11/48
Operation description
LED7707
5.1
5.1.1
Boost section
Functional description
The LED7707 is a monolithic LEDs driver for the backlight of LCD panels and it consists of a boost converter and six PWM-dimmable current generators. The boost section is based on a constant switching frequency, peak current-mode architecture. The boost output voltage is controlled such that the lowest row's voltage, referred to SGND, is equal to an internal reference voltage (700 mV typ.). The input voltage range is from 4.5 V up to 36 V. In addition, the LED7707 has an internal LDO that supplies the internal circuitry of the device and is capable to deliver up to 40 mA. The input of the LDO is the VIN pin. The LDO5 pin is the LDO output and the supply for the power MOSFET driver at the same time. The AVCC pin is the supply for the analog circuitry and should be connected to the LDO output through a simple RC filter in order to improve the noise rejection. Figure 4. AVCC filtering
VIN
LDO5 Rfilt 4R AVCC CLDO5 1u CAVCC 100n
LDO
LED7707
SGND
AM00581v1
Two loops are involved in regulating the current sunk by the generators. The main loop is related to the boost regulator and uses a constant frequency peak currentmode architecture to regulate the power rail that supplies the LEDs (Figure 5), while an internal current loop regulates the same current (flowing through the LEDs) at each row according to the set value (RILIM pin). Figure 5. Main loop and current loop diagram
VIN
LX
ROWx
Slope PWM
Minimum voltage drop selector
SGND
RILIM
COMP
E/A
Error amplifier
0.7V
AM00582v1
12/48
LED7707
Operation description A dedicated circuit automatically selects the lowest voltage drop among all the rows and provides this voltage the main loop that, in turn, regulates the output voltage. In fact, once the reference generator has been detected, the error amplifier compares its voltage drop to the internal reference voltage and varies the COMP output. The voltage at the COMP pin determines the inductor peak current at each switching cycle. The output voltage of the boost regulator is thus determined by the total forward voltage of the LEDs strings (see Figure 6): Equation 1
VBST = max (
i=1
NROWS mLEDS
VF,j ) + 700mV
j =1
where the first term represents the highest total forward voltage drop over N active rows and the second is the voltage drop across the leading generator (700 mV typ.). The device continues to monitor the voltage drop across all the rows and automatically switches to the current generator having the lowest voltage drop. Figure 6. Calculation of the output voltage of the boost regulator
Row with the highest voltage drop across LEDs VIN
max Boost controller ILED
VF
VBOOST
Current generators section
700 mV Leading generator
AM00583v1
5.1.2
Enable function
The LED7707 is enabled by the EN pin. This pin is active high and, when forced to SGND, the device is turned off. This pin is connected to a permanently active 2.5 A current source; when sudden device turn-on at power-up is required, this pin must be left floating or connected to a delay capacitor. Starting from an ON state, when the LED7707 is turned off, it quickly discharges the Soft-Start capacitor and turns off the power-MOSFET, the current generators and the LDO. The power consumption is thus reduced to 20 A only.
13/48
Operation description
LED7707
In applications where the dimming signal is used to turn on and off the device, the EN pin can be connected to the DIM pin as shown in Figure 7.
Figure 7. External sync waveforms
DIM BAS69 EN 220k 100n SGND
AM00584v1
LED7707
5.1.3
Soft-start
The soft-start function is required to perform a correct start-up of the system, controlling the inrush current required to charge the output capacitor and to avoid output voltage overshoot. The soft-start duration is set connecting an external capacitor between the SS pin and ground. This capacitor is charged with a 5 A (typ.) constant current, forcing the voltage on the SS pin to ramp up. When this voltage increases from zero to nearly 1.2 V, the current limit of the power MOSFET is proportionally released from zero to its final value. However, because of the limited minimum on-time of the switching section, the inductor might saturate due to current runaway. To solve this problem the switching frequency is reduced to one half of the nominal value at the beginning of the soft-start phase. The nominal switching frequency is restored after the SS pin voltage has crossed 0.8 V.
Figure 8. Soft-start sequence waveforms in case of floating rows
OVP
Floating ROWs detection 95% of OVP
Output voltage
AVCC 2.4V 1.2V 0.8V
SS pin voltage
Protections turn active
tss
100%
Nominal switching frequency release Current limit
EN pin voltage
t
AM00585v1
14/48
LED7707
Operation description
During the soft-start phase the floating rows detection is also performed. In presence of one or more floating rows, the voltage across the involved current generator drops to zero. This voltage becomes the inverting input of the error amplifier through the minimum voltage drop selector (see Figure 5). As a consequence the error amplifier is unbalanced and the loop reacts by increasing the output voltage. When it reaches the floating row detection (FRD) threshold (which coincides with the OVP threshold, see Section 5.1.4), the floating rows are managed according to Table 7 (see Section 5.3 on page 22). After the SS voltage reaches a 2.4 V threshold, the start-up finishes and all the protections turn active. The soft-start capacitor CSS can be calculated according to equation 2.
Equation 2
C SS
ISS t SS 2 .4
Where ISS = 5 A and tSS is the desired soft-start duration.
5.1.4
Over-voltage protection
An adjustable over-voltage protection is available. It can be set feeding the OVSEL pin with a partition of the output voltage. The voltage of the central tap of the divider is thus compared to a fixed 1.145 V threshold. When the voltage of the OVSEL pin exceeds the OV threshold, the switching activity is suspended. It is resumed as OVSEL returns below the OV threshold. A 10 mV hysteresis is provided. No device turn-off is performed. Normally, the value of the high-side resistors of the divider is in the order of 100 k to reduce the output capacitor discharge when the boost converter is off (during the off phase of the dimming cycle), whereas the low-side resistor can be calculated as:
Equation 3
R 2 = R1
1.145 V VOUT,MAX + 4V - 1.145 V
An additional filtering capacitor CF (typically in the 100 pF-330 pF range) may be required to improve noise rejection at the OVSEL pin (see Figure 9).
Figure 9. OVP threshold setting
VIN VOUT
LX
C13 R1
COUT
OVSEL
LED7707
SGND
R2 C10
AM00586v1
15/48
Operation description
LED7707
5.1.5
Switching frequency selection and synchronization
The switching frequency of the boost converter can be set in the 250 kHz-1 MHz range by connecting the FSW pin to ground through a resistor. Calculation of the setting resistor is made using equation 4 and should not exceed the 100 k-400 k range.
Equation 4
RFSW =
FSW 2.5
In addition, when the FSW pin is tied to AVCC, the LED7707 uses a default 660 kHz fixed switching frequency, allowing to save a resistor in minimum component-count applications.
Figure 10. Multiple device synchronization
MASTER AVCC
SLAVE
Sync Out FSW SYNC FSW SYNC SYNC
LED7707
RFSW SGND
LED7707
SGND
AM00587v1
The FSW pin can also be used as synchronization input, allowing the LED7707 to operate both as master or slave device. If a clock signal with a 210 kHz minimum frequency is applied to this pin, the device locks synchronized. The signal provided to the FSW pin must cross the 270 mV threshold in order to be recognized. The minimum pulse width which allows the synchronizing pulses to be detected is 270 ns. An Internal time-out allows synchronization as long as the external clock frequency is greater than 220 kHz. Keeping the FSW pin voltage lower than 270 mV for more than 4.5 s results in device turned off. Normal operation is resumed as soon as FSW rises above the mentioned threshold and the soft-start sequence is repeated. The SYNC pin is a synchronization output and provides a 35 % (typ.) duty-cycle clock when the LED7707 is used as master or a replica of the FSW pin when used as slave. It is used to connect multiple devices in a daisy-chain configuration or to synchronize other switching converters running in the system with the LED7707 (master operation). When an external synchronization clock is applied to the FSW pin, the internal oscillator is over-driven: each switching cycle begins at the rising edge of clock, while the slope compensation (Figure 11) ramp starts at the falling edge of the same signal. Thus, to prevent sub-harmonic instability (see Section 5.1.6), the external synchronization clock is required to have a 40 % maximum duty-cycle when the boost converter is working in continuous-conduction mode (CCM) in order to assure that the slope compensation is effective (starts with duty-cycle lower than 40%)
16/48
LED7707 Figure 11. External sync waveforms
Operation description
FSW pin voltage (ext. sync) 270mV threshold
270ns minimum
Slave SYNC pin voltage
Slave LX pin voltage
AM00588v1
5.1.6
Slope compensation
The constant frequency, peak current-mode topology has the advantage of very easy loop compensation with output ceramic caps (reduced cost and size of the application) and fast transient response. In addition, the intrinsic peak-current measurement simplifies the current limit protection, avoiding undesired saturation of the inductor. On the other side, this topology has a drawback: there is an inherent open loop instability when operating with a duty-ratio greater than 0.5. This phenomenon is known as "SubHarmonic Instability" and can be avoided by adding an external ramp to the one coming from the sensed current. This compensating technique, based on the additional ramp, is called "slope compensation". In Figure 12, where the switching duty-cycle is higher than 0.5, the small perturbation IL dies away in subsequent cycles thanks to the slope compensation and the system reverts to a stable situation. The SLOPE pin allows to properly set the amount of slope compensation connecting a simple resistor RSLOPE between the SLOPE pin and the output. The compensation ramp starts at 35% (typ.) of each switching period and its slope is given by the following equation:
Equation 5
V - VIN - VBE SE = K S OUT R SLOPE

Where KS = 5.8 1010 s-1, VBE = 2 V (typ.) and SE is the slope ramp in [A/s]. To avoid sub-harmonic instability, the compensating slope should be at least half the slope of the inductor current during the off-phase when the duty-cycle is greater than 50%. The value of RSLOPE can be calculated according to equation 6.
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Operation description Equation 6
LED7707
RSLOPE
2 K S L (VOUT - VIN - VBE ) (VOUT - VIN )
Figure 12. Effect of slope compensation on small inductor current perturbation (D > 0.5)
Inductor current (CCM) 0.35*TSW
Programmed inductor peak current with slope compensation (SE)
IBOOST, PEAK
IL
Inductor current perturbation
TSW
t
AM00589v1
18/48
LED7707
Operation description
5.1.7
Boost current limit
The design of the external components, especially the inductor and the flywheel diode, must be optimized in terms of size relying on the programmable peak current limit. The LED7707 improves the reliability of the final application giving the way to limit the maximum current flowing into the critical components. A simple resistor connected between the BILIM pin and ground sets the desired value. The voltage at the BILIM pin is internally fixed to 1.23 V and the current limit is proportional to the current flowing through the setting resistor, according to the following equation:
Equation 7
IBOOST,PEAK = where
KB R BILIM
K B = 1.2 10 V
6
The maximum allowed current limit is 5 A, resulting in a minimum setting resistor RBILIM > 240 k. The maximum guaranteed RMS current in the power switch is 2 A. In a boost converter the RMS current through the internal MOSFET depends on both the input and output voltages, according to equations 8a (DCM) and 8b (CCM). The current limitation works by clamping the COMP pin voltage proportionally to RBILIM. Peak inductor current is limited to the above threshold decreased by the slope compensation contribution.
Equation 8 a
IMOS,rms =
VIN D D FSW L 3
Equation 8 b
2 D VOUT 1 (D(1 - D))3 IMOS,rms = IOUT + (1 - D)2 12 I OUT fSW L
5.1.8
Thermal protection
In order to avoid damage due to high junction temperature, a thermal shutdown protection is implemented. When the junction temperature rises above 150 C (typ.), the device turns off both the control logic and the boost converter and holds the FAULT pin low. The LDO is kept alive and normal operation is automatically resumed after the junction temperature has been reduced by 30 C.
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Operation description
LED7707
5.2
5.2.1
Backlight driver section
Current generators
The LED7707 is a LEDs driver with six channels (rows); each row is able to drive multiple LEDs in series (max. 36 V) and to sink up to 85 mA maximum current, allowing to manage different kinds of LEDs. The LEDs current can be set by connecting an external resistor (RRILIM) between the RILIM pin and ground. The voltage across the RILIM pin is internally set to 1.23 V and the rows current is proportional to the RILIM current according to the following equation:
Equation 9
IROWx Where KR = 1850 V.
=
KR R RILIM
The graph in Figure 13 better shows the relationship between IROW and RRILIM and helps to choose the correct value of the resistor to set the desired row current.
Figure 13. Row current vs RRILIM
AM00590v1
The maximum current mismatch between the rows is 2 % @ Irowx = 60 mA. The LED7707 allows parallelism different rows if required by the application. If the maximum current provided by a single row (85 mA) is not enough for the load, two or more current generators can be connected together, as shown in Figure 14. To keep the parallelism generators stable, the row current should be higher than 40 mA.The connection between channels in parallel must be done as close as possible to the device in order to minimize parasitic inductance.
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LED7707 Figure 14. Rows parallelism for higher current
Operation description
VIN
AVCC
VIN
OVSEL
VCC
SLOPE
SWF
ROW0 ROW1 ROW2 ROW3 ROW4 ROW5
BILIM RILIM
LED7707
SS COMP SGND
FAULT
LX
MODE
Dimming Fault Enable Faults Management Selection Sync Output
SYNC
DIM
EN
PGND
High Current WLEDs
AM00591v1
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Operation description
LED7707
5.2.2
PWM dimming
The brightness control of the LEDs is performed by a pulse-width modulation of the rows current. When a PWM signal is applied to the DIM pin, the current generators are turned on and off mirroring the DIM pin behavior. Actually, the minimum dimming duty-cycle depends on the dimming frequency. The real limit to the PWM dimming is the minimum on-time that can be managed for the current generators; this minimum on-time is approximately 10 s. Thus, the minimum dimming duty-cycle depends on the dimming frequency according to the following formula:
Equation 10
DDIM,min = 10s fDIM
For example, at a dimming frequency of 1 kHz, 1% of dimming duty-cycle can be managed. During the off-phase of the PWM signal the boost converter is paused and the current generators are turned off. The output voltage can be considered almost constant because of the relatively slow discharge of the output capacitor. During the start-up sequence (see Section 5.1.3 on page 14) the dimming duty-cycle is forced to 100% to detect floating rows regardless of the applied dimming signal.
Figure 15. PWM dimming waveforms
10s minimum on-time
5.3
Fault management
The main loop keeps the row having the lowest voltage drop regulated to about 700 mV. This value slightly depends on the voltage across the remaining active rows. After the softstart sequence, all protections turn active and the voltage across the active current generators is monitored to detect shorted LEDs.
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LED7707
Operation description
5.3.1
FAULT pin
The FAULT pin is an open-collector output, (with 4 mA current capability) active low, which gives information regarding faulty conditions eventually detected. This pin can be used either to drive a status LED or to warn the host system. The FAULT pin status is strictly related to the MODE pin setting (see Table 7 for details).
5.3.2
MODE pin
The MODE pin is a digital input and can be connected to AVCC or SGND in order to choose the desired fault detection and management. The LED7707 can manage a faulty condition in two different ways, according to the application needs. Table 7 summarizes how the device detects and handles the internal protections related to the boost section (overcurrent, over-temperature and over-voltage) and to the current generators section (open and shorted LEDs).
Table 7. Faults management summary
FAULT Internal MOSFET over-current Output over-voltage Thermal shutdown MODE to GND MODE to VCC
FAULT pin HIGH Power MOS turned OFF FAULT pin LOW Device turned OFF, latched condition FAULT pin LOW. Device turned OFF. Automatic restart after 30 C temperature drop. "FAULT pin LOW, Device turned OFF (100s masking time), latched condition (Vth = 4.0 V)" FAULT pin LOW Device turned OFF at first occurrence, latched condition -
LED short circuit
Open row(s)
FAULT pin HIGH faulty row(s) disconnected.
5.3.3
Open LED fault
In case a row is not connected or a LED fails open, the device has two different behaviors according to the MODE pin status. If the MODE pin is high (i.e. connected to AVCC), the FAULT pin is set high as soon as the device recognizes the event; the open row is excluded from the control loop and the device continues to work properly with the remaining rows. Thus, if less than six rows are used in the application, the MODE pin must be set high. Connecting the MODE pin to SGND, the LED7707 behaves in a different manner: as soon as an open row is detected the FAULT pin is tied low and the device is turned off. The internal logic latches this status: to restore the normal operation, the device must be restarted by toggling the EN pin or performing a Power-On Reset (POR occurs when the voltage at the LDO5 pin falls below the lower UVLO threshold and subsequently rises above the upper one).
Figure 16 shows an example of open channel detection in case of MODE connected to AVCC.
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Operation description
LED7707
At the point marked as "1" in Figure 16, the row opens (row current drops to zero). From this point on the output voltage is increased as long as the output voltage reaches the floating row detection threshold (see Section 5.1.3 on page 14). Then (point marked as "2") the faulty row is disconnected and the device keeps on working only with the remaining rows.
Figure 16. Open channel detection (MODE to AVCC)
2
1
5.3.4
Shorted LED fault
When a LED is shorted, the voltage across the related current generator increases of an amount equal to the missing voltage drop of the faulty LED. Since the feedback voltage on each active generator is constantly compared with a fault threshold VTH,FAULT, the device detects the faulty condition and acts according to the MODE pin status. A 100 s masking time is introduced to support ESD capacitors eventually connected across the LEDs strings. If the MODE pin is low, the fault threshold is VTH,FAULT = 4.0 V. When the voltage across a row is higher than this threshold for more than 100 s, the FAULT pin is set low and the device is turned off. The internal logic latches this status until the EN pin is toggled or a POR is performed. In case the MODE pin is connected to AVCC, the LED short-circuit protection is disabled. The LED7707 simply keeps on regulating the set current without affecting the FAULT pin. Despite the higher power dissipation, this option is useful to avoid undesired triggering of the shorted-LED protection simply due to the high voltage drop spread across the LEDs.
Figure 17 shows an example of shorted LED detection in case MODE is connected to GND.
At the point marked as "1" in Figure 17 one LED fails becoming a short-circuit. The voltage across the current generator of the channel where the failed LED is connected increases by an amount equal to the forward voltage of the faulty LED. Since the voltage across the current generator is above the threshold (4 V), the device is turned off and the fault pin is set
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LED7707
Operation description
low (point "2"). Note that, once a new dimming cycle starts (point "3"), the device waits the masking time (approximately 100 s) and then sets the FAULT pin low and turns off.
Figure 17. Shorted LED detection (MODE to GND)
1
masking time
2 3
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Application information
LED7707
6
6.1
Application information
System stability
The boost section of the LED7707 is a fixed frequency, current-mode converter. During normal operation, a minimum voltage selection circuit compares all the voltage drops across the active current generators and provides the minimum one to the error amplifier. The output voltage of the error amplifier determines the inductor peak current in order to keep its inverting input equal to the reference voltage (700 mV typ). The compensation network consists of a simple RC series (RCOMP - CCOMP) between the COMP pin and ground. The calculation of RCOMP and CCOMP is fundamental to achieve optimal loop stability and dynamic performance of the boost converter and is strictly related to the operating conditions.
6.1.1
Loop compensation
The compensation network can be quickly calculated using equations 11 to 16. Once both RCOMP and CCOMP have been determined, a fine-tuning phase may be required in order to get the optimal dynamic performance from the application. The first parameter to be fixed is the switching frequency. Normally, a high switching frequency allows reducing the size of the inductor and positively affects the dynamic response of the converter (wider bandwidth) but increases the switching losses. For most of applications, the fixed value (660 kHz) represents a good trade-off between power dissipation and dynamic response, allowing to save an external resistor at the same time. In low-profile applications, the inductor value is often kept low to reduce the number of turns; an inductor value in the 4.7H-15H range is a good starting choice. In order to avoid instability due to interaction between the DC-DC converter's loop and the current generators' loop, the bandwidth of the boost should not exceed the bandwidth of the current generators. A unity-gain frequency (fU) in the order of 30-40 kHz is acceptable. Also, take care not to exceed the CCM-mode right half-plane zero (RHPZ).
Equation 11
fU 0.2 FSW
Equation 12
VIN,min VOUT V 2 MR OUT IOUT fU 0.2 = 0. 2 2 L 2 L
Equation 13 a
2

M=
VIN,min VOUT
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LED7707 Equation 13b
Application information
R=
VOUT IOUT
Where VIN,min is the minimum input voltage and IOUT is the overall output current. Note that, the lower the inductor value (and the higher the switching frequency), the higher the bandwidth can be achieved. The output capacitor is directly involved in the loop of the boost converter and must be large enough to avoid excessive output voltage drop in case of a sudden line transition from the maximum to the minimum input voltages. However a more significant requirement concerns the output voltage ripple. The output capacitor should be chosen in accordance with the following expression:
Equation 14
COUT >
(IL,peak - IOUT ) TOFF
2 VOUT,max
where VOUT, max is the maximum acceptable output voltage ripple, IL, peak is the peak inductor current, TOFF is the off-time of the switching cycle (for an extensive explanation see Section 6.4.4 on page 34). Once the output capacitor has been chosen, the RCOMP can be calculated as:
Equation 15
R COMP = Where GM = 2.7 S and gEA = 375 S
2 fU C GM gEA M
Equation 15 places the loop bandwidth at fU. Then, the CCOMP capacitor is determined to place the frequency of the compensation zero 5 times lower than the loop bandwidth:
Equation 16
C COMP = Where fZ = fU/5.
1 2 fZ R COMP
In most of the applications an experimental approach is also very valid to compensate the circuit. A simple technique to optimize different applications is to choose CCOMP = 4.7 nF and to replace RCOMP with a 10 k trimmer adjusting its value to properly damp the output transient response. Insufficient damping will result in excessive ringing at the output and poor phase margin.
Figure 18 (a and b) give an example of compensation adjustment for a typical application.
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Application information Figure 18. Poor phase margin (a) and properly damped (b) load transient responses a) b)
LED7707
Figure 19. Load transient response measurement set-up
VIN= 12V 6.8H VBOOST
C
IN
2 x 4.7F MLCC
+5V
AVCC
SLOPE
OVSEL
VIN
LX
VCC
FSW
ROW1 ROW2
BILIM RILIM
RL =
VBST 100mA
LED7707
ROW3
SS COMP SGND
ROW4 ROW5
ROW6
FAULT
MODE
SYNC
DIM
PGND
EN
Up to 10 WLEDs per row
500Hz
AM00592v1
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LED7707
Application information
6.2
Thermal considerations
In order to prevent the device from exceeding the thermal shutdown threshold (150 C), it is important to estimate the junction temperature through the following equation:
Equation 17
TJ = TAmb + R th,JA PD,tot
where TA is the ambient temperature, Rth,JA is the equivalent thermal resistance junction to ambient and PD,tot is the power dissipated by the device. The Rth,JA measured on the application demonstration board (described in Section 6.5) is 42 C/W. The PD,tot has several contributions, listed below. a) Conduction losses due to the RDS(on) of the internal power switch, equal to:
Equation 18
2 PD,cond = RDSon IIN D DDIM
where D is defined as:
Equation 19
D = 1-
VIN VOUT
and DDIM is the duty cycle of the PWM dimming signal. b) Switching losses due to the power MOSFET turn on and off, calculated as:
Equation 20
PD,sw = VOUT IIN fsw
(tr + t f ) DDIM 2
where tr and tf are the power MOSFET rise time and fall time respectively. c) Current generators losses. This contribution is strictly related to the LEDs used in the application. Only the contribution of the leading current generator ("master" current generator) can be predicted, regardless of the LEDs forward voltage:
Equation 21
PGEN,Master = IROW VIFB DDIM
where IROW is the current flowing through the row, whereas VIFB is the voltage across the master current generator (typically 700 mV). The voltages across the other current generators depend on the spread of the LEDs forward voltage. The worst case for power dissipation (maximum forward voltage LEDs in the master row, minimum forward voltage LEDs in all other rows) can be estimated as:
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Application information Equation 22
LED7707
PGEN = IROW (nROWs - 1) (VIFB + Vf,LEDs nLEDs ) DDIM where nROWs is the number of active rows, Vf,LEDs is the spread of the LEDs forward voltage and nLEDs is the number of LEDs per row. d) LDO losses, due to the dissipation of the 5 V linear regulator:
Equation 23
PD,LDO = (VIN - VLDO ) ILDO The LED7707 is housed in a 24 leads 4x4-VFQFPN package with exposed pad that allows good thermal performance. However it is also important to design properly the demonstration board layout in order to assure correct heat dissipation.
Figure 20 shows a picture of the LED7707 application demonstration board taken using an infrared camera. The chip temperature, in those application conditions, is kept below 50 C. Figure 20. Demonstration board thermographic analysis
VIN = 12V IROW = 60mA VOUT = 30V F SW = 660kHz DDIM = 100% T amb = 25C
64C
50C
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LED7707
Application information
6.3
6.3.1
Component selection
Inductor selection
Being the LED7707 mostly dedicated to backlighting, real-estate applications dictate severe constrain in selecting the optimal inductor. The inductor choice must take into account different parameters like conduction losses (DCR), core losses (ferrite or iron-powder), saturation current and magnetic-flux shielding (core shape and technology). The switching frequency of the LED7707 can be set in the 200 kHz-1 MHz range, allowing a wide selecting room for the inductance value. Low switching frequencies takes to high inductance value, resulting in significant DCR and size. On the other hand, high switching frequencies result in significant core losses. The suggested range is 4.7-22 H, even if the best trade off between the different loss contributions varies from manufacturer to manufacturer. A 6.8 H inductor has been experimentally found as the most suitable for applications running at a 660 kHz switching frequency.
Table 8. Recommended inductors
Part number LPS6235-682MLC XPL7030-682ML 7440650068 Description 6.8 H, 75 m, 2.7 A 6.8 H, 60 m, 5.8 A 6.8 H, 33 m, 3.6 A Size 6x6 mm 7x7 mm 10x10 mm
Manufacturer Coilcraft Coilcraft Wurth
6.3.2
Capacitors selection
The input and output capacitors should have very low ESR (ceramic capacitors) in order to minimize the ripple voltage. The boost converter of the LED7707 has been designed to support ceramic capacitors. The required capacitance depends on the programmed LED current and the minimum dimming frequency (the boost converter is off when the DIM pin is low and the output capacitor is slowly discharged). Considering the worst case (i.e. 200 Hz dimming frequency and 85 mA/channel), two 4.4 F MLCCs are suitable for almost all applications. Particular care must be taken when selecting the rated voltage and the dielectric type of the output capacitors: 50 V rated MLCC may show a significant capacitance drop when biased, especially in case of Y5V dielectric. As in most of boost converters, the input capacitor is less critical, although it is necessary to reduce the switching noise on the supply rail. The input capacitor is also important for the internal LDO of the LED7707 and must be kept as close as possible to the chip. The rated voltage of the input capacitor can be chosen according to the supply voltage range; a 10 F X5R MLCC is recommended.
Table 9. Recommended capacitors
Part number UMK325BJ106KM-T GRM31CR71H225KA88B Description Ceramic, 35V, X5R, 20 % Ceramic, 50V, X7R, 20 % Package SMD 1210 SMD 1206 Notes CIN COUT
Manufacturer Taiyo Yuden Murata
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Application information
LED7707
6.3.3
Flywheel diode selection
The flywheel diode must be a Schottky type to minimize the losses. This component is subject to an average current equal to the output one and must sustain a reverse voltage equal to the maximum output rail voltage. Considering all the channels sinking 75 mA each (i.e. 450 mA output current) and the maximum output voltage (36 V), the STP1L40M (If,ave = 1 A, Vr = 40 V) diode is a good choice. Smaller diodes can be used in applications involving lower output voltage and/or lower output current.
6.4
Design example
In order to help the design of an application using the LED7707, in this section a simple step-by-step design example is provided. A possible application could be the LED backlight in a 17" LCD panel using the LED7707. Here below the possible application conditions are listed:

VIN = 12 10 % 4 strings of 42 white LEDs (60 mA) each (arranged in 6 rows, 7LEDs per row) VF, LEDs = 3.5 V 200 mV
6.4.1
Switching frequency setting
To reduce the number of the external components, the default switching frequency is selected (660 kHz typ.) by connecting the FSW pin to AVCC pin. However, in case a different switching frequency is required, a resistor from FSW pin and ground can be connected, according to the equation (5) in section 4.1.5.
6.4.2
Row current setting
Considering the equation 9 in Section 5.2.1, the RRILIM resistor can be calculated as:
Equation 24
RRILIM =
KR IROW
=
1850 V = 30.83k 60 mA
The closest standard commercial value is 30 k. The actual value of the row current will be a little lower (61.7 mA).
6.4.3
Inductor choice
The boost section, as all DC-DC converters, can work in CCM (continuous conduction mode) or in DCM (discontinuous conduction mode) depending on load current, input and output voltage and other parameters, among which the inductor value. In a boost converter it is usually preferable to work in DCM. Once the load, the input and output voltage, and the switching frequency are fixed, the inductor value defining the boundary between DCM and CCM operation can be calculated as:
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LED7707 Equation 25
Application information
LB = where D is the duty-cycle defined as:
Equation 26
R 0 D (1 - D) 2 FSW
2
D = 1-
0.59 @ VIN,min = 10.8V VIN = VOUT 0.50 @ VIN,max = 13.2V
whereas R0 is:
Equation 27
R0 = and
Equation 28
VOUT = 74 IOUT
IOUT = 6 IROW = 360mA The output voltage in the above calculations is considered as the maximum value (LED with the maximum forward voltage connected to the leading generator):
Equation 29
VOUT,max = 7 VF,LEDs,max + 700mV = 26.6V Considering the input voltage range, the lower LB will be at the lower input voltage. Hence the condition to assure the DCM operation becomes:
Equation 30
L < L B (VIN,min ) = 5.6H An inductor value of 4.7 H could be a suitable value, considering also a margin from the boundary condition. It is important to highlight that the inductor choice involves not only the value itself but the saturation current (higher than the current limit, see Section 6.4.4), the rated RMS current (the compliance with the saturation current might be not enough; also the thermal performances must be taken into account), the DCR (which affects the efficiency) and the size (in some application might be a strict requirement). However the DCR can't be reduced keeping the size small. Hence a trade off between these two requirements must be achieved according to the application.
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Application information
LED7707
6.4.4
Output capacitor choice
The choice of the output capacitor is mainly affected by the desired output voltage ripple. Since the voltage across the LEDs can be considered almost constant, this ripple is transferred across the current generators, affecting their dynamic response. The output ripple can be estimated as (neglecting the contribution of ESR of COUT, very low in case of MLCC):
Equation 31
VOUT =
(IL,peak - IOUT ) TOFF
2 C OUT
where IL, peak is the inductor peak current (see Figure 21) calculated as:
Equation 32
IL,peak =
VIN D 1.915 A @ VIN,min = 10.8V = Fsw L 1.762A @ VIN,max = 13.2V
whereas D, working in DCM, is:
Equation 33
D=
2 Fsw L M(M - 1) 0.55 @ VIN,min = 10.8V = R0 0.414 @ VIN,max = 13.2V
defining M as:
Equation 34
M=
VOUT VIN
2.463 @ VIN,min = 10.8V 2.015 @ VIN,max = 13.2V
Figure 21. Inductor current in DCM operation
IL
IL, peak
TON
TOFF TSW = 1/FSW
t
AM00593v1
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LED7707
Application information
TOFF can be calculated as:
Equation 35
569.7ns @ VIN,min = 10.8V TOFF = TSW D 2 = 618.2ns @ VIN,max = 13.2V defining D2 as:
Equation 36
D2 =
2 FSW L M 0.376 @ VIN,min = 10.8V = R 0 (M - 1) 0.408 @ VIN,max = 13.2V
The worst case for the output voltage ripple is when input voltage is lower (VIN,min = 10.8 V). A simple way to select the COUT value is fixing a maximum voltage ripple. In order to affect as less as possible the current generators, it would be better to fix the maximum ripple lower than the typical voltage across the generators. For example considering VOUT lower than 70 mV (i.e. the 10 % of the voltage across the leading generator), the required capacitance is:
Equation 37
COUT >
(IL,peak - IOUT ) TOFF
2 VOUT,max
= 6.33F
A margin from the calculated value should be taken into account because of the capacitance drop due to the applied voltage when MLCCs are used. One 10 F MLCC (or two 4.7 F MLCCs) can be a good choice for this application. In case a dimming duty cycle different from 100% is used, a further contribution to the capacitor discharge (during the off time of the dimming cycle) should be considered.
6.4.5
Input capacitor choice
The input capacitor of a boost converter is less critical than the output capacitor, due to the fact that the inductor is in series with the input, and hence, the input current waveform is continuous. A low ESR capacitor is always recommended. A capacitor of 10 F is tentatively a good choice for most of the applications.
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Application information
LED7707
6.4.6
Over-voltage protection divider setting
The over-voltage protection (OVP) divider provides a partition of the output voltage to the OVSEL pin. The OVP divider setting not only fixes the OVP threshold, but also the openchannel detection threshold. The proper OVP divider setting can be calculated by the equation (3):
Equation 38
R 2 = R1
1.145 V VOUT,MAX + 4V - 1.145 V
where VOUT, MAX is the maximum output voltage considering the worst case (all LEDs with the maximum VF = VF,max = 3.7 V on the same row):
Equation 39
VOUT,OVP = nLED VF,max + 700mV = 26.6V R1 can be chosen is in the order of hundreds of kilo-ohms to reduce the leakage current in the resistor divider. For example, setting R1 = 510 k leads to R2 = 21.89 k. The closest standard commercial value is R2 = 22 k.
6.4.7
Compensation network
For the compensation network, the suggestions provided in Section 6.1 are always valid. In this condition, tentatively the following value of R3 and C8 (see Figure 24) are usually a good choice for the loop stability: R3 = 2.4 k C8 = 4.7 nF
6.4.8
Boost current limit
The boost current limit is set to protect the internal power switch against excessive current. The slope compensation may reduce the programmed current limit. Hence, to take into account this effect, as a rule of thumb, the current limit can be set as twice as much the maximum inductor peak current (see Section 6.4.4): IBOOST, PEAK > 3.83 A Therefore, using equation (7) and choosing IBOOST, PEAK = 4 A, RBILIM will be:
Equation 40
RBILIM =
KB = 300k IBOOST,PEAK
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LED7707
Application information
6.4.9
Power dissipation estimate
As explained in section 5.2, there are several contributions to the total power dissipation. Neglecting the power dissipated by the LDO (surely less significant compared with the other contributions), equation (18), (20), (21) and (22) help to estimate the overall power dissipation. Before starting the power dissipation estimate it is important to highlight that the following calculations are considering the worst case (the actual value of the dissipated power would require measurements). Therefore the power dissipation is estimated according to the following assumptions: 1. 2. 3. 4. Minimum input voltage (10.8 V), which leads to maximum input current (and also D will have the higher value, see Section 6.4.4); Maximum RDS(on) of the internal power MOSFET; LEDs in the row of the leading generator will have the maximum forward voltage, whereas all other LEDs in the other rows will have the minimum forward voltage. 100 % dimming signal duty cycle is considered.
The conduction and switching losses on the internal power switch can be calculated as:
Equation 41
2 PD,cond = RDSon IIN D DDIM = 216mW
Equation 42
PD,sw = VOUT IIN fsw
(tr + t f ) DDIM = 233mW 2
where tr = tf = 15 ns The power dissipation related to the current generators is given by:
Equation 43
PGEN,Master = IROW VIFB DDIM = 42mW
Equation 44
PGEN = IROW (nROWs - 1) (VIFB + Vf,LEDs nLEDs ) DDIM = 630mW
Equation 45
PD,tot PD,cond + PD,sw + PGEN,Master + PGEN = 1.12W
The junction temperature can be estimated by equation (18) considering TA = 25 C:
Equation 46
TJ = TAmb + R th,JA PD,tot = 72C
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Application information
LED7707
In order to estimate also the efficiency, other contributions to the power dissipation must be added to PD, tot (which represents only the power dissipated by the device), that is:
Equation 47
PDISS,Diode = VF,Diode IIN D 2 = 133mW where VF, Diode = 0.4 V
Equation 48
2 2 PDISS,Ind = DCR IInd,RMS DCR IIN = 63mW
where DCR = 80 m (typical DCR of the recommended inductors). Therefore the total dissipated power is:
Equation 49
PDISS,TOTAL = PD,tot + PDISS,Diode + PDISS,Ind = 1.316 W Considering the input power as the result of input voltage multiplied by the input current, the estimated efficiency is:
Equation 50
=
Note:
PIN - PDISS,TOT PIN
= 0.862
It is important to remind that the previous calculations consider the worst case, especially for the power dissipated on the current generators. Statistical analysis (confirmed by bench measurements) shows that the series connection of more LEDs on each channel leads to compensation effects. The hypothesis 3 above mentioned is thus rather unlikely. Therefore PGEN is significantly lower and the overall efficiency is typically around 90 %.
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LED7707
Application information
6.5
Layout consideration
1. 2. A careful PCB layout is important for proper operation. In this section some guidelines are provided in order to achieve a good layout. The device has two different ground pins: signal ground (SGND) and power ground (PGND). The PGND pin handles the switching current related to the boost section; for this reason the PCB traces should be kept as short as possible and with adequate width. The signal ground is the return for the device supply and the current generators and can be connected to the thermal pad. The heat dissipation area (adequate to the application conditions) should be placed backside respect to the device and with the lowest thermal impedance possible (i.e. PCB traces in the backside should be avoided). The dissipation area is thermally and electrically connected to the thermal pad by several vias (nine vias are recommended). The signal and power grounds must be connected together in a single point as close as possible to the PGND pin to reduce ground loops. The R-C components of the compensation network should be placed as close as possible to the COMP pin in order to avoid noise issue and instability of the compensation. Noise sensitive signals (i.e. feedbacks and compensation) should be routed as short as possible to minimize noise collection. The LED7707 pinout makes it easy to separate power components (e.g. inductor, diode) from signal ones. The LX switching node should have and adequate width for high efficiency. The critical power path inductor-LX-PGND must be as short as possible by mounting the inductor, the diode and COUT as close as possible each other.
3. 4.
5. 6.
7.
8. 9.
10. The capacitors of the compensated divider connected to the OVSEL pin should be placed as close as possible to the OVSEL pin. 11. In order to assure good performance in terms of row current accuracy/mismatch, the PCB traces from the rows pins to the LEDs should have similar length and width. 12. The capacitors of the filter connected to LDO5 and VIN pins should be mounted as close as possible to the mentioned pins
Figure 22 and Figure 23 shows the demonstration board layout (top view and bottom view respectively).
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Application information Figure 22. Demonstration board layout (top view)
LED7707
Figure 23. Demonstration board layout (bottom view)
Figure 24 shows the LED7707 demonstration board application circuit, whereas Table 10 lists the used components and their value.
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LED7707 Figure 24. LED7707 demonstration board schematic
Application information
Table 10.
LED7707 demonstration board component list
Description Ceramic, 35 V X5R, 20 % Ceramic, 50 V X7R, 20 % Package SMD 1210 SMD 1206 SMD 1206 Part number UMK325BJ106KM-T GRM31CR71H475KA88B Murata GRM31CR71H225KA88B N.M. 1u 100n 3n3 4n7 Ceramic, 25 V X5R, 20 % SMD 0603 Standard N.M. 220p 4n7 N.M. 15p 510k 16k Chip resistor 0.1 W, 1 % 2k4 SMD 0603 Standard 4R7 330k 24k Chip resistor 0.1 W, 1 % SMD 0603 Standard 360k MFR Taiyo Yuden Value 10u 4u7
Component C1 C2,C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 R1 R2 R3 R4 R5 R6 R7
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Application information Table 10. LED7707 demonstration board component list (continued)
Description Package Part number MFR
LED7707
Component R8 R9, R10 R11 R12 R13 L1 D1 D2 D3 U1 J2 J8 SW1, SW2 SW3
Value 680k 100k 1k2 N.M. N.M.
6u8, 60 m, 5.8 A Schottky, 40 V, 1 A Red LED, 3mA Signal Schottky Integrated circuit PCB pad jumper Header 8 Jumper 3 Push button
7x7 mm DO216-AA SMD 0603 SOD-523 QFN4x4
XPL7030-682ML STPS1L40M
Coilcraft ST Standard BAS69
6.8 u STPS1L40M
N.M. LED7707
LED7707
ST
SIL 8 SIL 3 6x6 mm FSM4JSMAT
Standard Standard TYCO
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LED7707
Electrical characteristics
7
Electrical characteristics
Figure 26. Efficiency versus DIM duty cycle, VIN = 18 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 660 kHz
Figure 25. Efficiency versus DIM duty cycle, VIN = 12 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 660 kHz
Figure 27. Efficiency versus DIM duty cycle, VIN = 24 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 825 kHz
Figure 28. Efficiency versus DIM duty cycle, VIN = 24 V, 6 rows, 10 white LEDs (60 mA) in series, FSW = 825 kHz
Figure 29. Soft-start waveforms (EN, SS, and VOUT monitored)
Figure 30. Boost section switching signals (LX, SYNC and inductor current monitored), VIN = 12 V, 10 LEDs
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Electrical characteristics
LED7707
Figure 31. Dimming waveforms (FDIM = 200 Hz)
Figure 32. Dimming waveforms (FDIM = 1 kHz)
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LED7707
Package mechanical data
8
Package mechanical data
In order to meet environmental requirements, ST offers these devices in ECOPACK(R) packages. These packages have a lead-free second level interconnect. The category of second Level Interconnect is marked on the package and on the inner box label, in compliance with JEDEC standard JESD97. The maximum ratings related to soldering conditions are also marked on the inner box label. ECOPACK is an ST trademark. ECOPACK specifications are available at: www.st.com.
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Package mechanical data
LED7707
Table 11.
VFQFPN-24 4 mm x 4 mm mechanical data
mm
Dim. Min A A1 A3 b D D2 E E2 e L ddd 0.30 0.18 3.85 2.40 3.85 2.40 0.80 0.00 Typ 0.90 0.02 0.20 0.25 4.00 2.50 4.00 2.50 0.50 0.40 0.50 0.08 0.30 4.15 2.60 4.15 2.60 Max 1.00 0.05
Figure 33. Package dimensions
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LED7707
Revision history
9
Revision history
Table 12.
Date 18-Sep-2008
Document revision history
Revision 1 Initial release Changes
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LED7707
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