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FEATURES
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LTC3642 High Efficiency, High Voltage 50mA Synchronous Step-Down Converter DESCRIPTION
The LTC(R)3642 is a high efficiency step-down DC/DC converter with internal high side and synchronous power switches that draws only 12A typical DC supply current at no load while maintaining output voltage regulation. The LTC3642 can supply up to 50mA load current and features a programmable peak current limit that provides a simple method for optimizing efficiency in lower current applications. The LTC3642's combination of Burst Mode(R) operation, integrated power switches, low quiescent current, and programmable peak current limit provides high efficiency over a broad range of load currents. With its wide 4.5V to 45V input range and internal overvoltage monitor capable of protecting the part through 60V surges, the LTC3642 is a robust converter suited for regulating a wide variety of power sources. Additionally, the LTC3642 includes a precise run threshold and soft-start feature to guarantee that the power system start-up is well-controlled in any environment. The LTC3642 is available in the thermally enhanced 3mm x 3mm DFN and MS8E packages.
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Wide Input Voltage Range: 4.5V to 45V Tolerant of 60V Input Transients Internal High Side and Low Side Power Switches No Compensation Required 50mA Output Current Low Dropout Operation: 100% Duty Cycle Low Quiescent Current: 12A 0.8V Feedback Voltage Reference Adjustable Peak Current Limit Internal and External Soft-Start Precise RUN Pin Threshold with Adjustable Hysteresis 3.3V, 5V and Adjustable Output Versions Only Three External Components Required for Fixed Output Versions Low Profile (0.75mm) 3mm x 3mm DFN and Thermally-Enhanced MS8E Packages
APPLICATIONS
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4mA to 20mA Current Loops Industrial Control Supplies Distributed Power Systems Portable Instruments Battery-Operated Devices Automotive Power Systems
TYPICAL APPLICATION
5V, 50mA Step-Down Converter
VIN 5V TO 45V 1F VIN SW LTC3642-5 RUN VOUT SS HYST ISET GND
3642 TA01a
Efficiency and Power Loss vs Load Current
100 95 EFFICIENCY 90 VOUT 5V 10F 50mA EFFICIENCY (%) POWER LOSS (mW) VIN = 10V 100
150H
85 80 75 70 65 0.1 POWER LOSS
10
1
1 10 LOAD CURRENT (mA)
0.1 100
3642 TA01b
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LTC3642 ABSOLUTE MAXIMUM RATINGS (Note 1)
VIN Supply Voltage ..................................... -0.3V to 60V SW Voltage (DC) ............................-0.3V to (VIN + 0.3V) RUN Voltage .............................................. -0.3V to 60V VFB/VOUT, HYST, ISET, SS Voltages ................ -0.3V to 6V Operating Junction Temperature Range (Note 2).................................................. -40C to 125C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 sec) MS8E ................................................................ 300C
PIN CONFIGURATION
TOP VIEW TOP VIEW SW VIN ISET SS 1 2 3 4 8 7 6 5 GND HYST VOUT/VFB RUN SW 1 VIN 2 ISET 3 SS 4 9 8 7 6 5 GND HYST VOUT/VFB RUN
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MS8E PACKAGE 8-LEAD PLASTIC MSOP TJMAX = 125C, JA = 40C/W, JC = 5-10C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB DD PACKAGE 8-LEAD (3mm 3mm) PLASTIC DFN TJMAX = 125C, JA = 43C/W, JC = 3C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3642EMS8E#PBF LTC3642EMS8E-3.3#PBF LTC3642EMS8E-5#PBF LTC3642IMS8E#PBF LTC3642IMS8E-3.3E#PBF LTC3642IMS8E-5#PBF LTC3642EDD#PBF LTC3642EDD-3.3#PBF LTC3642EDD-5#PBF LTC3642IDD#PBF LTC3642IDD-3.3#PBF LTC3642IDD-5#PBF TAPE AND REEL LTC3642EMS8E#TRPBF LTC3642EMS8E-3.3#TRPBF LTC3642EMS8E-5#TRPBF LTC3642IMS8E#TRPBF LTC3642IMS8E-3.3#TRPBF LTC3642IMS8E-5#TRPBF LTC3642EDD#TRPBF LTC3642EDD-3.3#TRPBF LTC3642EDD-5#TRPBF LTC3642IDD#TRPBF LTC3642IDD-3.3#TRPBF LTC3642IDD-5#TRPBF PART MARKING* PACKAGE DESCRIPTION 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead Plastic MSOP 8-Lead (3mm x 3mm) Plastic DFN 8-Lead (3mm x 3mm) Plastic DFN 8-Lead (3mm x 3mm) Plastic DFN 8-Lead (3mm x 3mm) Plastic DFN 8-Lead (3mm x 3mm) Plastic DFN 8-Lead (3mm x 3mm) Plastic DFN TEMPERATURE RANGE -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C -40C to 125C
LTDTH LTDYN LTDYQ LTDTH LTDYN LTDYQ LDTJ LDYM LDYP LDTJ LDYM LDYP
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LTC3642 ELECTRICAL CHARACTERISTICS
SYMBOL VIN UVLO PARAMETER Input Voltage Operating Range VIN Undervoltage Lockout VIN Rising VIN Falling Hysteresis VIN Rising VIN Falling Hysteresis
l l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 10V, unless otherwise noted.
CONDITIONS MIN 4.5 3.80 3.75 47 45 4.15 4.00 150 50 48 2 125 12 3 3.277 3.257 4.965 4.935 0.792 3.5 -10 3.310 3.290 5.015 4.985 0.800 5 0 0.001 TYP MAX 45 4.40 4.25 52 50 UNITS V V V mV V V V A A A V V V V V mV nA %/V
Input Supply (VIN)
OVLO
VIN Overvoltage Lockout
IQ
DC Supply Current (Note 3) Active Mode Sleep Mode Shutdown Mode Output Voltage Trip Thresholds
VRUN = 0V LTC3642-3.3V, VOUT Rising LTC3642-3.3V, VOUT Falling LTC3642-5V, VOUT Rising LTC3642-5V, VOUT Falling
l l l l l l l l
220 22 6 3.343 3.323 5.065 5.035 0.808 6.5 10
Output Supply (VOUT/VFB) VOUT
VFB VHYST IFB VLINEREG Operation VRUN
Feedback Comparator Trip Voltage Feedback Comparator Hysteresis Feedback Pin Current Feedback Voltage Line Regulation
VFB Rising Adjustable Output Version, VFB = 1V VIN = 4.5V to 45V LTC3642-5, VIN = 6V to 45V RUN Rising RUN Falling Hysteresis RUN = 1.3V RUN < 1V, IHYST = 1mA VHYST = 1.3V VSS < 1.5V SS Pin Floating ISET Floating 500k Resistor from ISET to GND ISET Shorted to GND ISW = -25mA ISW = 25mA
Run Pin Threshold
1.17 1.06 -10 -10 4.5
l
1.21 1.10 110 0 0.07 0 5.5 0.75 115 55 25 3.0 1.5
1.25 1.14 10 0.1 10 6.5 130 32
V V mV nA V nA A ms mA mA mA
IRUN VHYSTL IHYST ISS tINTSS IPEAK
Run Pin Leakage Current Hysteresis Pin Voltage Low Hysteresis Pin Leakage Current Soft-Start Pin Pull-Up Current Internal Soft-Start Time Peak Current Trip Threshold
100 20
RON
Power Switch On-Resistance Top Switch Bottom Switch
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3642E is guaranteed to meet specifications from 0C to 85C with specifications over the -40C to 125C operating junction temperature range assured by design, characterization and correlation with statistical process controls. The LTC3642I is guaranteed to meet specifications from -40C to 125C.
TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * JAC/W) Note 3: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information.
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LTC3642 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100 FIGURE 10 CIRCUIT 95 ISET OPEN VOUT = 5V 90 EFFICIENCY (%) 85 80 75 70 65 60 0.1 1 10 LOAD CURRENT (mA) 100
3642 G01
Line Regulation
0.30 ILOAD = 25mA FIGURE 10 CIRCUIT OUTPUT VOLTAGE (V) 5.05 5.04 5.03 5.02 5.01 5.00 4.99 4.98 4.97 4.96 -0.30 5 10 15 20 25 30 35 VIN VOLTAGE (V) 40 45 4.95 0.20 VOUT/VOUT (%) 0.10 0
Load Regulation
VIN = 10V FIGURE 10 CIRCUIT ISET OPEN
VIN = 10V
VIN = 15V
VIN = 45V VIN = 24V VIN = 36V
-0.10
-0.20
0
10
30 40 20 LOAD CURRENT (mA)
50
3642 G03
3642 G02
Feedback Comparator Voltage vs Temperature
FEEDBACK COMPARATOR TRIP VOLTAGE (V) VIN = 10V FEEDBACK COMPARATOR HYSTERESIS (mV) 0.801 5.6 5.4 5.2 5.0 4.8 4.6
Feedback Comparator Hysteresis vs Temperature
PEAK CURRENT TRIP THRESHOLD (mA) VIN = 10V
Peak Current Trip Theshold vs Temperature
130 V = 10V 120 IN 110 100 90 80 70 60 50 40 30 20 10 0 -40 -10 ISET OPEN
0.800
RISET = 500k
0.799
ISET = GND
0.798 -40
-10
20 50 80 TEMPERATURE (C)
110
3642 G04
4.4 -40
-10
20 50 80 TEMPERATURE (C)
110
3642 G05
20 50 80 TEMPERATURE (C)
110
3642 G06
Peak Current Trip Threshold vs RISET
120 PEAK CURRENT TRIP THRESHOLD (mA) 110 100 90 80 70 60 50 40 30 20 10 0 200 400 600 800 RSET (k) 1000 1200 0 VIN = 10V VIN SUPPLY CURRENT (A) 14 12 10 8 6 4 2
Quiescent Supply Current vs Input Voltage
14 12 VIN SUPPLY CURRENT (A) SLEEP
Quiescent Supply Current vs Temperature
VIN = 10V SLEEP 10 8 6 4 2 0 -40 SHUTDOWN
SHUTDOWN
5
15
25 35 VIN VOLTAGE (V)
45
3642 G08
-10
20 50 80 TEMPERATURE (C)
110
3642 G09
3642 G07
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LTC3642 TYPICAL PERFORMANCE CHARACTERISTICS
Switch On-Resistance vs Input Voltage
4.5 4.0 SWITCH ON-RESISTANCE () SWITCH ON-RESISTANCE () 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 10 30 20 VIN VOLTAGE (V) 40 50
3642 G10
Switch On-Resistance vs Temperature
5 VIN = 10V SWITCH LEAKAGE CURRENT (A) 0.6 0.5 0.4
Switch Leakage Current vs Temperature
VIN = 45V
4 TOP 3 BOTTOM
TOP
SW = 0V 0.3 0.2 SW = 45V 0.1 0 -40
BOTTOM
2
1
0 -40
-10
50 80 20 TEMPERATURE (C)
110
3642 G11
-10
20 50 80 TEMPERATURE (C)
110
3642 G12
Efficiency vs Input Voltage
95 90 EFFICIENCY (%) 85 ILOAD = 10mA 80 ILOAD = 1mA 75 70 65 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 45 RUN COMPARATOR THRESHOLD (V) FIGURE 10 CIRCUIT ISET OPEN ILOAD = 50mA 1.30 1.25 1.20 1.15
Run Comparator Thresholds vs Temperature
1.20 INTERNAL SOFT-START TIME (ms) 110
3642 G14
Internal Soft-Start Time vs Temperature
RISING
1.15 1.10 1.05 1.00 0.95 0.90 -40
FALLING 1.10 1.05 1.00 -40
-10
20 50 80 TEMPERATURE (C)
-10
20 50 80 TEMPERATURE (C)
110
3642 G15
3642 G13
Soft-Start Waveforms
OUTPUT VOLTAGE 1V/DIV OUTPUT VOLTAGE 50mV/DIV SWITCH VOLTAGE 5V/DIV INDUCTOR CURRENT 50mA/DIV VIN = 10V 500s/DIV CSS = 0.047F ISET OPEN FIGURE 10 CIRCUIT
4632 G16
Operating Waveforms
OUTPUT VOLTAGE 25mV/DIV
Load Step Transient Response
INDUCTOR CURRENT 50mA/DIV
LOAD CURRENT 25mA/DIV
VIN = 10V 10s/DIV ISET OPEN FIGURE 10 CIRCUIT
4632 G17
VIN = 10V 1ms/DIV ISET OPEN FIGURE 10 CIRCUIT
4632 G18
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LTC3642 PIN FUNCTIONS
SW (Pin 1): Switch Node Connection to Inductor. This pin connects to the drains of the internal power MOSFET switches. VIN (Pin 2): Main Supply Pin. A ceramic bypass capacitor should be tied between this pin and GND (Pin 8). ISET (Pin 3): Peak Current Set Input. A resistor from this pin to ground sets the peak current trip threshold. Leave floating for the maximum peak current (115mA). Short this pin to ground for the minimum peak current (25mA). A 1A current is sourced out of this pin. SS (Pin 4): Soft-Start Control Input. A capacitor to ground at this pin sets the ramp time to full current output during start-up. A 5A current is sourced out of this pin. If left floating, the ramp time defaults to an internal 0.75ms soft-start. RUN (Pin 5): Run Control Input. A voltage on this pin above 1.2V enables normal operation. Forcing this pin below 0.7V shuts down the LTC3642, reducing quiescent current to approximately 3A. VOUT/VFB (Pin 6): Output Voltage Feedback. For the fixed output versions, connect this pin to the output supply. For the adjustable version, an external resistive divider should be used to divide the output voltage down for comparison to the 0.8V reference. HYST (Pin 7): Run Hysteresis Open-Drain Logic Output. This pin is pulled to ground when RUN (Pin 5) is below 1.2V. This pin can be used to adjust the RUN pin hysteresis. See Applications Information. GND (Pin 8): Chip Ground. Exposed Pad (Pin 9): Ground. Must be soldered to PCB ground for optimal electrical and thermal performance.
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LTC3642 BLOCK DIAGRAM
VIN ISET 3 1A 2 C2
PEAK CURRENT COMPARATOR
RUN 5
+
1.2V
-
LOGIC AND SHOOTTHROUGH PREVENTION
HYST 7
+
REVERSE CURRENT COMPARATOR VOLTAGE REFERENCE
FEEDBACK COMPARATOR
GND 8
GND 9
+ + -
0.800V
PART NUMBER LTC3642 LTC3642-3.3 LTC3642-5
R1 0 2.5M 4.2M
- +
SW 1 C1 L1 VOUT
-
5A
SS 4
R1 R2 R2 800k 800k
VOUT/VFB 6 IMPLEMENT DIVIDER EXTERNALLY FOR ADJUSTABLE VERSION
3642 BD
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LTC3642 OPERATION (Refer to Block Diagram)
The LTC3642 is a step-down DC/DC converter with internal power switches that uses Burst Mode control, combining low quiescent current with high switching frequency, which results in high efficiency across a wide range of load currents. Burst Mode operation functions by using short "burst" cycles to ramp the inductor current through the internal power switches, followed by a sleep cycle where the power switches are off and the load current is supplied by the output capacitor. During the sleep cycle, the LTC3642 draws only 12A of supply current. At light loads, the burst cycles are a small percentage of the total cycle time which minimizes the average supply current, greatly improving efficiency. Main Control Loop The feedback comparator monitors the voltage on the VFB pin and compares it to an internal 800mV reference. If this voltage is greater than the reference, the comparator activates a sleep mode in which the power switches and current comparators are disabled, reducing the VIN pin supply current to only 12A. As the load current discharges the output capacitor, the voltage on the VFB pin decreases. When this voltage falls 5mV below the 800mV reference, the feedback comparator trips and enables burst cycles. At the beginning of the burst cycle, the internal high side power switch (P-channel MOSFET) is turned on and the inductor current begins to ramp up. The inductor current increases until either the current exceeds the peak current comparator threshold or the voltage on the VFB pin exceeds 800mV, at which time the high side power switch is turned off. The low side power switch (N-channel MOSFET) is then turned on. The inductor current ramps down until the reverse current comparator trips, signaling that the current is close to zero. If the voltage on the VFB pin is still less than the 800mV reference, the high side power switch is turned on again and another cycle commences. The average current during a burst cycle will normally be greater than the average load current. For this architecture, the maximum average output current is equal to half of the peak current. The hysteretic nature of this control architecture results in a switching frequency that is a function of the input voltage, output voltage and inductor value. This behavior provides inherent short-circuit protection. If the output is shorted to ground, the inductor current will decay very slowly during a single switching cycle. Since the high side switch turns on only when the inductor current is near zero, the LTC3642 inherently switches at a lower frequency during start-up or short-circuit conditions. Start-Up and Shutdown If the voltage on the RUN pin is less than 0.7V, the LTC3642 enters a shutdown mode in which all internal circuitry is disabled, reducing the DC supply current to 3A. When the voltage on the RUN pin exceeds 1.21V, normal operation of the main control loop is enabled. The RUN pin comparator has 110mV of internal hysteresis, and therefore must fall below 1.1V to disable the main control loop. The HYST pin provides an added degree of flexibility for the RUN pin operation. This open-drain output is pulled to ground whenever the RUN comparator is not tripped, signaling that the LTC3642 is not in normal operation. In applications where the RUN pin is used to monitor the VIN voltage through an external resistive divider, the HYST pin can be used to increase the effective RUN comparator hysteresis. An internal 1ms soft-start function limits the ramp rate of the output voltage on start-up to prevent excessive input supply droop. If a longer ramp time and consequently less supply droop is desired, a capacitor can be placed from the SS pin to ground. The 5A current that is sourced out of this pin will create a smooth voltage ramp on the capacitor. If this ramp rate is slower than the internal 1ms soft-start, then the output voltage will be limited by the ramp rate
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LTC3642 OPERATION (Refer to Block Diagram)
on the SS pin instead. The internal and external soft-start functions are reset on start-up and after undervoltage or overvoltage event on the input supply. In order to ensure a smooth start-up transition in any application, the internal soft-start also ramps the peak inductor current from 25mA during its 1ms ramp time to the set peak current threshold. The external ramp on the SS pin does not limit the peak inductor current during start-up; however, placing a capacitor from the ISET pin to ground does provide this capability. Peak Inductor Current Programming The offset of the peak current comparator nominally provides a peak inductor current of 115mA. This peak inductor current can be adjusted by placing a resistor from the ISET pin to ground. The 1A current sourced out of this pin through the resistor generates a voltage that is translated into an offset in the peak current comparator, which limits the peak inductor current.
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LTC3642 APPLICATIONS INFORMATION
The basic LTC3642 application circuit is shown on the front page of this data sheet. External component selection is determined by the maximum load current requirement and begins with the selection of the peak current programming resistor, RISET. The inductor value L can then be determined, followed by capacitors CIN and COUT. Peak Current Resistor Selection The peak current comparator has a maximum current limit of 115mA nominally, which results in a maximum average current of 55mA. For applications that demand less current, the peak current threshold can be reduced to as little as 25mA. This lower peak current allows the use of lower value, smaller components (input capacitor, output capacitor and inductor), resulting in lower input supply ripple and a smaller overall DC/DC converter. The threshold can be easily programmed with an appropriately chosen resistor (RISET) between the ISET pin and ground. The value of resistor for a particular peak current can be computed by using Figure 1 or the following equation: RISET = IPEAK * 9.09 * 106 where 25mA < IPEAK < 115mA. The peak current is internally limited to be within the range of 25mA to 115mA. Shorting the ISET pin to ground programs the current limit to 25mA, and leaving it floating sets the current limit to the maximum value of 115mA. When selecting this resistor value, be aware that the
1100 1000 900 800 700 RISET (k) 600 500 400 300 200 100 0 10 15 20 25 30 35 40 45 MAXIMUM LOAD CURRENT (mA) 50
maximum average output current for this architecture is limited to half of the peak current. Therefore, be sure to select a value that sets the peak current with enough margin to provide adequate load current under all foreseeable operating conditions. Inductor Selection The inductor, input voltage, output voltage and peak current determine the switching frequency of the LTC3642. For a given input voltage, output voltage and peak current, the inductor value sets the switching frequency when the output is in regulation. A good first choice for the inductor value can be determined by the following equation: V L = OUT f * IPEAK V * 1 - OUT VIN
The variation in switching frequency with input voltage and inductance is shown in the following two figures for typical values of VOUT. For lower values of IPEAK, multiply the frequency in Figure 2 and Figure 3 by 115mA/IPEAK. An additional constraint on the inductor value is the LTC3642's 100ns minimum on-time of the high side switch. Therefore, in order to keep the current in the inductor well controlled, the inductor value must be chosen so that it is larger than LMIN, which can be computed as follows: L MIN = VIN(MAX ) * tON(MIN) IPEAK(MAX )
3642 F01
Figure 1. RISET Selection
where VIN(MAX) is the maximum input supply voltage for the application, tON(MIN) is 100ns, and IPEAK(MAX) is the maximum allowed peak inductor current. Although the above equation provides the minimum inductor value, higher efficiency is generally achieved with a larger inductor value, which produces a lower switching frequency. For a given inductor type, however, as inductance is increased DC resistance (DCR) also increases. Higher DCR translates into higher copper losses and lower current rating, both of which place an upper limit on the inductance. The recommended range of inductor values for small surface mount inductors as a function of peak current is shown in Figure 4. The values in this range are a good compromise between the tradeoffs discussed above. For applications
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LTC3642 APPLICATIONS INFORMATION
700 600 500 400 300 200 100 0 5 10 15 20 25 30 35 VIN INPUT VOLTAGE (V) 40 45 VOUT = 5V ISET OPEN L = 47H SWITCHING FREQUENCY (kHz)
where board area is not a limiting factor, inductors with larger cores can be used, which extends the recommended range of Figure 4 to larger values. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of the more expensive ferrite cores. Actual core loss is independent of core size for a fixed inductor value but is very dependent of the inductance selected. As the inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequently output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate energy but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price vs size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, Toko, Sumida and Vishay. CIN and COUT Selection
L = 68H L = 100H L = 150H L = 220H L = 470H
3642 F02
Figure 2. Switching Frequency for VOUT = 5V
500 450 SWITCHING FREQUENCY (kHz) 400 350 300 250 200 150 100 50 0 5 10 15 20 25 30 35 VIN INPUT VOLTAGE (V) L = 470H L = 150H L = 220H L = 68H L = 100H
VOUT = 3.3V ISET OPEN
L = 47H
40
45
3642 F03
Figure 3. Switching Frequency for VOUT = 3.3V
10000
INDUCTOR VALUE (H)
1000
100 10 PEAK INDUCTOR CURRENT (mA)
3642 F04
100
The input capacitor, CIN, is needed to filter the trapezoidal current at the source of the top high side MOSFET. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used. Approximate RMS current is given by: IRMS = IOUT(MAX ) * VOUT VIN * -1 VIN VOUT
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Figure 4. Recommended Inductor Values for Maximum Efficiency
11
LTC3642 APPLICATIONS INFORMATION
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based only on 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. The output capacitor, COUT, filters the inductor's ripple current and stores energy to satisfy the load current when the LTC3642 is in sleep. The output voltage ripple during a burst cycle is dominated by the output capacitor equivalent series resistance (ESR) and can be estimated by the following equation: VOUT < VOUT IPEAK * ESR 160 where the lower limit of VOUT/160 is due to the 5mV feedback comparator hysteresis. The value of the output capacitor must be large enough to accept the energy stored in the inductor without a large change in output voltage. Setting this voltage step equal to 1% of the output voltage, the output capacitor must be: COUT I > 50 * L * PEAK V
OUT 2
electrolytic capacitors have significantly higher ESR but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and longterm reliability. Ceramic capacitors have excellent low ESR characteristics but can have high voltage coefficient and audible piezoelectric effects. The high quality factor (Q) of ceramic capacitors in series with trace inductance can also lead to significant ringing. Using Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. For applications with inductive source impedance, such as a long wire, a series RC network may be required in parallel with CIN to dampen the ringing of the input supply. Figure 5 shows this circuit and the typical values required to dampen the ringing.
LIN LIN CIN CIN LTC3642 VIN R=
Typically, a capacitor that satisfies the ESR requirement is adequate to filter the inductor ripple. To avoid overheating, the output capacitor must also be sized to handle the ripple current generated by the inductor. The worst-case ripple current in the output capacitor is given by IRMS = IPEAK/2. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic, and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important only to use types that have been surge tested for use in switching power supplies. Aluminum
3642 F05
4 * CIN
Figure 5. Series RC to Reduce VIN Ringing
Output Voltage Programming For the adjustable version, the output voltage is set by an external resistive divider according to the following equation: R1 VOUT = 0.8 V * 1+ R2
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LTC3642 APPLICATIONS INFORMATION
The resistive divider allows the VFB pin to sense a fraction of the output voltage as shown in Figure 6.
VOUT R1 VFB LTC3642 GND R2 R3 VIN R1 RUN R2 LTC3642 HYST
3642 F08
Figure 8. Adjustable Undervoltage Lockout Figure 6. Setting the Output Voltage
To minimize the no-load supply current, resistor values in the megohm range should be used. The increase in supply current due to the feedback resistors can be calculated from: V IVIN = OUT R1+ R2 V * OUT VIN
Specific values for these UVLO thresholds can be computed from the following equations: R1 Ri sin g VIN UVLO Threshold = 1.21V * 1+ R2 R1 Fallin g VIN UVLO Threshold = 1.10 V * 1+ R2 + R3 The minimum value of these thresholds is limited to the internal VIN UVLO thresholds that are shown in the Electrical Characteristics table. The current that flows through this divider will directly add to the shutdown, sleep and active current of the LTC3642, and care should be taken to minimize the impact of this current on the overall efficiency of the application circuit. Resistor values in the megohm range may be required to keep the impact on quiescent shutdown and sleep currents low. Be aware that the HYST pin cannot be allowed to exceed its absolute maximum rating of 6V. To keep the voltage on the HYST pin from exceeding 6V, the following relation should be satisfied: R3 < 6V VIN(MAX ) * R1+ R2 + R3 The RUN pin may also be directly tied to the VIN supply for applications that do not require the programmable undervoltage lockout feature. In this configuration, switching is enabled when VIN surpasses the internal undervoltage lockout threshold. Soft-Start The internal 0.75ms soft-start is implemented by ramping both the effective reference voltage from 0V to 0.8V and the peak current limit set by the ISET pin (25mA to 115mA).
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Run Pin with Programmable Hysteresis The LTC3642 has a low power shutdown mode controlled by the RUN pin. Pulling the RUN pin below 0.7V puts the LTC3642 into a low quiescent current shutdown mode (IQ ~ 3A). When the RUN pin is greater than 1.2V, the controller is enabled. Figure 7 shows examples of configurations for driving the RUN pin from logic.
VSUPPLY LTC3642 RUN VIN 4.7M LTC3642 RUN
3642 F07
Figure 7. RUN Pin Interface to Logic
The RUN pin can alternatively be configured as a precise undervoltage lockout (UVLO) on the VIN supply with a resistive divider from VIN to ground. The RUN pin comparator nominally provides 10% hysteresis when used in this method; however, additional hysteresis may be added with the use of the HYST pin. The HYST pin is an opendrain output that is pulled to ground whenever the RUN comparator is not tripped. A simple resistive divider can be used as shown in Figure 8 to meet specific VIN voltage requirements.
13
LTC3642 APPLICATIONS INFORMATION
To increase the duration of the reference voltage soft-start, place a capacitor from the SS pin to ground. An internal 5A pull-up current will charge this capacitor, resulting in a soft-start ramp time given by: tSS = CSS * 0.8 V 5A Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses: VIN operating current and I2R losses. The VIN operating current dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. 1. The VIN operating current comprises two components: The DC supply current as given in the electrical characteristics and the internal MOSFET gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. When switching, the average output current flowing through the inductor is "chopped" between the high side PMOS switch and the low side NMOS switch. Thus, the series resistance looking back into the switch pin is a function of the top and bottom switch RDS(ON) values and the duty cycle (DC = VOUT/VIN) as follows: RSW = (RDS(ON)TOP)DC + (RDS(ON)BOT)(1 - DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain the I2R losses, simply add RSW to RL and multiply the result by the square of the average output current: I2R Loss = IO2(RSW + RL)
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When the LTC3642 detects a fault condition (input supply undervoltage or overvoltage) or when the RUN pin falls below 1.1V, the SS pin is quickly pulled to ground and the internal soft-start timer is reset. This ensures an orderly restart when using an external soft-start capacitor. The duration of the 1ms internal peak current soft-start may be increased by placing a capacitor from the ISET pin to ground. The peak current soft-start will ramp from 25mA to the final peak current value determined by a resistor from ISET to ground. A 1A current is sourced out of the ISET pin. With only a capacitor connected between ISET and ground, the peak current ramps linearly from 25mA to 115mA, and the peak current soft-start time can be expressed as: tSS(ISET) = CISET * 0.8 V 1A
A linear ramp of peak current appears as a quadratic waveform on the output voltage. For the case where the peak current is reduced by placing a resistor from ISET to ground, the peak current offset ramps as a decaying exponential with a time constant of RISET * CISET. For this case, the peak current soft-start time is approximately 3 * RISET * CISET. Unlike the SS pin, the ISET pin does not get pulled to ground during an abnormal event; however, if the ISET pin is floating (programmed to 115mA peak current), the SS and ISET pins may be tied together and connected to a capacitor to ground. For this special case, both the peak current and the reference voltage will soft-start on power-up and after fault conditions. The ramp time for this combination is CSS(ISET) * (0.8V/6A).
14
LTC3642 APPLICATIONS INFORMATION
Other losses, including CIN and COUT ESR dissipative losses and inductor core losses, generally account for less than 2% of the total power loss. Thermal Considerations The LTC3642 does not dissipate much heat due to its high efficiency and low peak current level. Even in worst-case conditions (high ambient temperature, maximum peak current and high duty cycle), the junction temperature will exceed ambient temperature by only a few degrees. Design Example As a design example, consider using the LTC3642 in an application with the following specifications: VIN = 24V, VOUT = 3.3V, IOUT = 50mA, f = 250kHz. Furthermore, assume for this example that switching should start when VIN is greater than 12V and should stop when VIN is less than 8V. First, calculate the inductor value that gives the required switching frequency: 3.3V L= 250kHz * 115mA 3.3V * 1- 100H 24V COUT will be selected based on the ESR that is required to satisfy the output voltage ripple requirement. For a 1% output ripple, the value of the output capacitor ESR can be calculated from: VOUT = 0.01 115mA * ESR A capacitor with a 90m ESR satisfies this requirement. A 10F ceramic capacitor has significantly less ESR than 90m. The output voltage can now be programmed by choosing the values of R1 and R2. Choose R2 = 240k and calculate R1 as: V R1 = OUT - 1 * R2 = 750k 0.8 V The undervoltage lockout requirement on VIN can be satisfied with a resistive divider from VIN to the RUN and HYST pins. Choose R1 = 2M and calculate R2 and R3 as follows: 1.21V R2 = * R1 = 224k VIN(RISING) - 1.21V 1.1V R3 = * R1 - R2 = 90.8k VIN(FALLING) - 1.1V Choose standard values for R2 = 226k and R3 = 91k. The ISET pin should be left open in this example to select maximum peak current (115mA). Figure 9 shows a complete schematic for this design example.
VIN 24V 1F 2M 226k 100H VIN RUN SW LTC3642 ISET SS VFB HYST GND 750k 240k
3642 F09
Next, verify that this value meets the LMIN requirement. For this input voltage and peak current, the minimum inductor value is: 24V * 100ns L MIN = 22H 115mA Therefore, the minimum inductor requirement is satisfied, and the 100H inductor value may be used. Next, CIN and COUT are selected. For this design, CIN should be size for a current rating of at least: 3.3V 24V IRMS = 50mA * * - 1 18mA RMS 24V 3.3V Due to the low peak current of the LTC3642, decoupling the VIN supply with a 1F capacitor is adequate for most applications.
VOUT 3.3V 50mA 10F
91k
Figure 9. 24V to 3.3V, 50mA Regulator at 250kHz
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15
LTC3642 APPLICATIONS INFORMATION
PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3642. Check the following in your layout: 1. Large switched currents flow in the power switches and input capacitor. The loop formed by these components should be as small as possible. A ground plane is recommended to minimize ground impedance. 2. Connect the (+) terminal of the input capacitor, CIN, as close as possible to the VIN pin. This capacitor provides the AC current into the internal power MOSFETs. 3. Keep the switching node, SW, away from all sensitive small signal nodes. The rapid transitions on the switching node can couple to high impedance nodes, in particular VFB, and create increased output ripple. 4. Flood all unused area on all layers with copper. Flooding with copper will reduce the temperature rise of power components. You can connect the copper areas to any DC net (VIN, VOUT, GND or any other DC rail in your system).
TYPICAL APPLICATIONS
Efficiency vs Load Current
VIN 5V TO 45V L1 220H VIN CIN 4.7F SW LTC3642 VFB SS GND CSS 47nF
3642 F10a
94 R1 4.2M R2 800k COUT 100F VOUT 5V 93
VIN = 10V RSET = 750k RSET = 500k
EFFICIENCY (%)
RUN HYST ISET RSET
92 ISET OPEN 91 90 89 88 1 10 LOAD CURRENT (mA) 100
3642 F10b
CIN: TDK C5750X7R2A475MT COUT: AVX 1812D107MAT L1: TDK SLF7045T-221MR33-PF
Figure 10. High Efficiency 5V Regulator
3.3V, 50mA Regulator with Peak Current Soft-Start, Small Size
L1 47H VIN CIN 1F RUN ISET SS CSS 0.1F SW LTC3642 VFB HYST GND R1 294k R2 93.1k
3642 TA02a
Soft-Start Waveforms
VIN 4.5V TO 24V
COUT 10F
VOUT 3.3V 50mA
OUTPUT VOLTAGE 1V/DIV
INDUCTOR CURRENT 20mA/DIV
CIN: TDK C3216X7R1E105KT COUT: AVX 08056D106KAT2A L1: TAIYO YUDEN CBC2518T470K
2ms/DIV
3642 TA03b
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16
LTC3642 TYPICAL APPLICATIONS
Positive-to-Negative Converter
L1 100H MAXIMUM OUTPUT CURRENT (mA) VIN CIN 1F RUN ISET SW LTC3642 VFB R2 71.5k VOUT -12V
3642 TA04a
Maximum Load Current vs Input Voltage
50 VOUT = -3V 45 40 35 30 25 20 15 10 5 10 15 20 25 30 35 VIN VOLTAGE (V) 40 45 VOUT = -12V VOUT = -5V
VIN 4.5V TO 33V
R1 1M COUT 10F
SS HYST GND
CIN: TDK C3225X7R1H105KT COUT: MURATA GRM32DR71C106KA01 L1: TYCO/COEV DQ6530-101M
3642 TA04b
Small Size, Limited Peak Current, 10mA Regulator
VIN 7V TO 45V L1 470H VIN CIN 1F R3 470k R4 100k R5 33k RUN SW LTC3642 VFB R1 470k R2 88.7k
3642 TA05a
COUT 10F
VOUT 5V 10mA
HYST SS ISET GND
CIN: TDK C3225X7R1H105KT COUT: AVX 08056D106KAT2A L1: MURATA LQH32CN471K23
High Efficiency 15V, 10mA Regulator
L1 4700H VIN CIN 1F SW LTC3642 VFB HYST GND R1 3M R2 169k
3642 TA07a
Efficiency vs Load Current
100 VOUT 15V 10mA EFFICIENCY (%) 95 90 85 80 75 70 VIN = 36V VIN = 24V
VIN 15V TO 45V
COUT 4.7F
RUN ISET SS
VIN = 45V
CIN: AVX 18125C105KAT2A COUT: TDK C3216X7R1E475KT L1: COILCRAFT DS1608C-475
65 60 0.1 1 LOAD CURRENT (mA) 10
3642 TA07b
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17
LTC3642 PACKAGE DESCRIPTION
DD Package 8-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1698)
0.675 0.05
3.5 0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 5
0.38 8
0.10
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
1.65 0.10 (2 SIDES)
(DD) DFN 1203
0.200 REF
0.75 0.05
0.25
4 0.05 2.38 0.10 (2 SIDES)
1 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON TOP AND BOTTOM OF PACKAGE
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18
LTC3642 PACKAGE DESCRIPTION
MS8E Package 8-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1662 Rev D)
BOTTOM VIEW OF EXPOSED PAD OPTION 1 2.794 0.102 (.110 .004) 0.889 0.127 (.035 .005) 2.06 0.102 (.081 .004) 1.83 0.102 (.072 .004)
5.23 (.206) MIN
2.083 0.102 3.20 - 3.45 (.082 .004) (.126 - .136) 8
0.42 0.038 (.0165 .0015) TYP
0.65 (.0256) BSC
3.00 0.102 (.118 .004) (NOTE 3)
8
7 65
0.52 (.0205) REF
RECOMMENDED SOLDER PAD LAYOUT DETAIL "A" 0 - 6 TYP 4.90 0.152 (.193 .006) 3.00 0.102 (.118 .004) (NOTE 4)
0.254 (.010) GAUGE PLANE
1 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 0.22 - 0.38 (.009 - .015) TYP 1.10 (.043) MAX
23
4 0.86 (.034) REF
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.65 (.0256) BSC
0.1016 0.0508 (.004 .002)
MSOP (MS8E) 0307 REV D
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LTC3642 TYPICAL APPLICATION
5V, 50mA Regulator for Automotive Applications
VBATT 12V L1 220H VIN CIN 1F RUN ISET SS SW LTC3642 VFB HYST GND R1 470k R2 88.7k
3642 TA06a
COUT 10F
VOUT 5V 50mA
CIN: TDK C3225X7R2A105M COUT: KEMET C1210C106K4RAC L1: COILTRONICS DRA73-221-R
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20 Linear Technology Corporation
(408) 432-1900
LT 1108 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
FAX: (408) 434-0507 www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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