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LTC3458L 1.7A, 1.5MHz Synchronous Step-Up DC/DC Converter with Output Disconnect DESCRIPTIO
The LTC(R)3458L is a high efficiency, current mode, fixed frequency, step up DC/DC converter with true output disconnect and inrush current limiting. The LTC3458L is rated for up to 6V output and includes a 0.2 N-channel MOSFET switch and a 0.3 P-channel MOSFET synchronous rectifier. The LTC3458L is well suited for battery powered applications and includes programmable output voltage, switching frequency and loop compensation. The oscillator frequency can be set up to 1.5MHz or synchronized to an external clock. Quiescent current is only 12A during Burst Mode operation maximizing battery life in portable applications. The Burst Mode current threshold, peak current limit, and softstart are externally programmable. Other features include <1A shutdown current, antiringing control, and thermal limit. The LTC3458L is available in a low profile (0.75mm), 4mm x 3mm 12-pin DFN package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5481178, 6304066, 6580258, 5511131, 6498466
FEATURES

High Efficiency: Up to 95% Inrush Current Limiting and Output Disconnect Programmable Output Voltages up to 6V 1.5V to 6V Input Range Programmable/Synchronizable Fixed Frequency Operation up to 1.5MHz Programmable Automatic Burst Mode(R) Operation Current Mode Control with Programmable Soft-Start Period and Peak Current Limit 0.2 N-Channel and 0.3 P-Channel Switches Ultralow Quiescent Currents: 12A Sleep, <1A in Shutdown 12-Pin 4mm x 3mm Thermally Enhanced DFN Package
APPLICATIO S

Point-of-Load Regulators USB VBUS Power Portable GPS MP3 Players
TYPICAL APPLICATIO
7.4H
Li-Ion to 5V at 1MHz Li-Ion to 5VOUT
Li Ion 3.1V TO 4.2V VIN 2.2F LTC3458L GND/PGND SHDN SYNC RT ILIM 200k 84.5k VOUT 10pF ON OFF FB COMP 0.01F SS 0.01F BURST 133k 560pF
3458L TA01a
SW VOUT 5V 750mA 1M
EFFICIENCY (%)
100 95 90 85 80 75 70
22F X5R
324k 33k 10pF
65 60 0.1 1 100 10 LOAD CURRENT (mA) 0.1 1000
3458L TA01b
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1000 4.2VIN
3.6 VIN POWER LOSS (mW)
3.1VIN 3.6VIN
100
10 POWER LOSS 1
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LTC3458L
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW VIN SYNC SHDN ILIM RT 1 2 3 13 4 5 6 9 8 7 GND COMP FB 12 VOUT 11 BURST 10 SS
VIN, SS, SYNC Voltages ................................. -0.3 to 7V BURST, SHDN, VOUT Voltages ....................... -0.3 to 7V Operating Temperature Range (Notes 2, 3) .........................................-40C to 85C Storage Temperature Range ..................-65C to 125C SW Voltage DC ........................................................... -0.3V to 7V Pulsed <100ns ........................................ -0.3V to 8V
ORDER PART NUMBER LTC3458LEDE
DE PART MARKING 3458L
DE PACKAGE 12-LEAD (4mm x 3mm) PLASTIC DFN EXPOSED PAD IS PGND (PIN 13), MUST BE SOLDERED TO PCB TJMAX = 125C, JA = 45C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VOUT = 5V, RT = 200k, unless otherwise noted.
PARAMETER Minimum VIN Operating Voltage Output Voltage Adjust Range Feedback Voltage Undervoltage (Exit Burst Mode Operation) Feedback Input Current Quiescent Current - Burst Mode Operation Quiescent Current - Shutdown Quiescent Current - Active NMOS Switch Leakage PMOS Switch Leakage NMOS Switch On Resistance PMOS Switch On Resistance Fixed NMOS Current Limit Maximum Duty Cycle Minimum Duty Cycle Frequency Accuracy Error Amplifier Transconductance Error Amplifier Source Current Error Amplifier Sink Current SYNC Input High SYNC Input Low

ELECTRICAL CHARACTERISTICS
CONDITIONS TA = 0C to 85C TA = -40C to 0C
MIN
TYP 1.4 1.4
MAX 1.5 1.7 6 1.25 1.25 50 30 10 1 3 3 5 5
UNITS V V V V V % nA A A A A mA A A A %
2 1.21 1.20 1.225 -4 1 12 5 0.5 1 1
0C to 85C, VOUT = 3.3V -40C to 0C Below Feedback Voltage VFB = 1.23V VIN Current at 3.3V VOUT Current at 5V VIN Current at 3.3V VOUT Current at 0V VIN Current Switching

0.05 0.05 0.2 0.3
VOUT = 5V VOUT = 5V RILIM = 84.5k VIN = 3.3V, fOSC = 1MHz RT = 200k

1.7 80 0.85
2.1 90 0 1 60 7 7 1.15
1.5 0.35
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% MHz A/V A A V V
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The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VOUT = 5V, RT = 200k, unless otherwise noted.
PARAMETER SHDN Input High SHDN Input Low BURST Mode Peak Current (IBURSTPEAK) BURST Threshold Voltage Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. RILIM = 84.5k CONDITIONS

ELECTRICAL CHARACTERISTICS
MIN 1.25
TYP
MAX 0.3
UNITS V V A V
0.55 1.10
Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 3: The LTC3458LE is guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
TYPICAL PERFOR A CE CHARACTERISTICS
ILIMIT, IBURSTPEAK, IZERO Currents
2200 2000 1800 1600 2.15 ILIMIT 2.25 RILIMIT = 84.5k
CURRENT (mA)
1200 1000 800 600 400 200 0 -200 2.0 2.5 3.0 3.5 4.0 VIN (V) IZERO IBURSTPEAK
CURRENT (A)
CURRENT (A)
1400
VOUT = 5V L = 7.4H RLIM = 84.5k
4.5
Typical Burst Mode Threshold and Hysteresis vs RBURST
160 140 120 OUT OF BURST 300 250
CURRENT (mA)
RT (k)
100 80 60 40 INTO BURST
CURRENT (mA)
20 0 50 100 150 200 RBURST (k) 250 300
3458L G04
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5.0
3458L G01
(TA = 25C unless otherwise specified) Burst Mode Quiescent Current
20 VIN = 3.3V VOUT = 5V 15 IVIN 10
Current Limit Accuracy
2.05
1.95
5 IVOUT 0 -40
5.5
1.85 -45 -30 -15
0 15 30 45 60 TEMPERATURE (C)
75
90
-15
35 10 TEMPERATURE (C)
60
85
3458L G03
3458L G02
Maximum Load Current in Burst
600 RILIM = 84.5k 3.3VOUT 200 150 100 50 0 1.5 5VOUT 550 500 450 400 350 300 250 200 150 2.0 2.5 3.0 VIN (V) 3.5 4.0 4.5
3458L G05
Oscillator Programming Resistor
100 400
600 1000 1200 1400 800 OSCILLATOR FREQUENCY (kHz)
3458L G06
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LTC3458L TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Accuracy
1.05 95 RT = 200k 93
1.03
FREQUENCY (MHz)
EFFICIENCY (%)
1.01
91
RDS(ON) ()
0.99
0.97
0.95 -45 -30 -15
0 15 30 45 60 TEMPERATURE (C)
Maximum Load Current
1600 1400 1.8 to 5.5VIN at 1MHz RILIM = 84.5k 5VOUT 0.9 1.0
MAX LOAD CURRENT (mA)
1200
VOLTAGE (V)
0.8
VOLTAGE (V)
1000 800 600 400
3.3VOUT
200 0 1.5 2.0 2.5 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5 0.5 -45 -30 -15
VOLTAGE (V)
4
UW
75 90
3458L G07 3458L G10
(TA = 25C unless otherwise specified) N-Channel and P-Channel RDS(ON)
0.5 VIN = 3.3V VOUT = 5V 0.3 P-CHANNEL
Efficiency vs Frequency
0.2 N-CHANNEL
89
0.1 87 VIN = 3.3V VOUT = 5V at 200mA L = 7.4H 700 1100 1300 900 FREQUENCY (kHz) 1500
3458L G08
85 500
0 -40
-15
35 10 TEMPERATURE (C)
60
85
3458L G09
SHDN Pin Threshold and Hysteresis
1.5 1.4 1.3 OPERATING 1.2 1.1 1.0 0.6 0.9
SYNC Pin Threshold
0.7
SHUTDOWN
0 15 30 45 60 TEMPERATURE (C)
75
90
0.8 -45 -30 -15
0 15 30 45 60 TEMPERATURE (C)
75
90
3458L G11
3458L G12
FB Voltage
1.25
1.24
1.23
1.22
1.21
1.20 -45 -30 -15
0 15 30 45 60 TEMPERATURE (C)
75
90
3458L G13
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LTC3458L TYPICAL PERFOR A CE CHARACTERISTICS
Fixed Frequency (FF) Discontinuous Current
SW 1V/DIV IL 200mA/DIV IL 100mA/DIV GND 0mA VIN = 3.3V VOUT = 5V L = 7.4H 500ns/DIV VIN = 3.3V VOUT = 5V L = 7.4H 500ns/DIV 0mA
Over-Current with 2A ILIMIT
VOUT 100mV/DIV IL 0.5A/DIV
SW 2V/DIV 0mA VIN = 2.25V VOUT = 5V L = 7.4H RILIM = 84.5k 1s/DIV IL 200mA/DIV VIN = 3.3V VOUT = 5V L = 7.4H COUT = 22F CFF = 22pF RILIM = 84.5k 50s/DIV
3458L G17
Burst Mode Operation Close-Up
VOUT 100mV/DIV
SW 5V/DIV
IL 200mA/DIV
VIN = 3.3V 3458L G18 2s/DIV VOUT = 5V L = 7.4H COUT = 22F CFF = 22pF* RILIM = 84.5k *CFF: FEED FORWARD CAPACITOR CONECTED BETWEEN VOUT AND FB.
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(TA = 25C unless otherwise specified) Fixed Frequency (FF) Continuous Current
SW 1V/DIV
3458L G14
3458L G15
Burst Mode Operation
SW 5V/DIV
0mA
3458L G16
Soft-Start into 20 Load
VOUT 2V/DIV VIN 2V/DIV
0mA
SS 200mV/DIV IL 200mA/DIV VIN = 3.3V VOUT = 5V L = 7.4H CSS = 0.1F 2ms/DIV
3458L G19
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LTC3458L TYPICAL PERFOR A CE CHARACTERISTICS
Sync Operation at 1.33MHz
SW 5V/DIV SYNC 2V/DIV IL 200mA/DIV COMP 500mV/DIV IL 0.5A/DIV 0mA 500ns/DIV VIN = 3.3V VOUT = 5V RZ = 33K CC1 = 270pF CC2 = 10pF COUT = 22F 200s/DIV 0mA
VIN = 3.3V VOUT = 5V RT = 200k
Burst Mode Operation 10mA to 50mA Load Step
VOUT 200mV/DIV BURST 500mV/DIV LOAD 50mA VOUT 200mV/DIV
IL 200mA/DIV
VIN = 3.3V VOUT = 5V L = 10H COUT = 22F RILIM = 110k
10mA to 200mA Load Step Showing UV Trip
VOUT 200mV/DIV - 4%
IL 200mA/DIV 200s/DIV
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(TA = 25C unless otherwise specified) FF Mode 100-300mA Load Step
VOUT 200mV/DIV
3458L G20
3458L G21
L = 10H F = 1MHz
Auto Mode 10mA to 100mA Load Step
BURST 1V/DIV LOAD
100mA 10mA
10mA
IL 200mA/DIV
200s/DIV
3458L G22
500s/DIV VIN = 3.3V VOUT = 5V L = 10H CBURST = 0.015F RBURST = 133k RILIM = 110k
3458L G23
Forced BURST to FF Mode Switch with 50mA Load
VOUT 200mV/DIV FIXED FREQUENCY BURST VIN = 3.3V VOUT = 5V L = 10H CBURST = 0.015F RBURST = 133k RILIM = 110k 3458L G24 VIN = 3.3V VOUT = 5V L = 10H RZ = 33k CC1 = 270pF CC2 = 10pF COUT = 22F RILIM = 110k
IL 500mA/DIV 200s/DIV
3458L G25
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PI FU CTIO S
SW (Pin 1): Switch Pin for Inductor Connection. During discontinuous conduction mode an antiring resistor connects SW to VIN to reduce noise. VIN (Pin 2): Input Supply Pin. Connect this to the input supply and decouple with 1F minimum. SYNC (Pin 3): Oscillator Synchronization Pin. A clock pulse width of 100ns to 2s is required to synchronize the internal oscillator. This pin is disabled when grounded. SHDN (Pin 4): Shutdown Pin. Grounding this pin shuts down the IC. Connect to >1.25V to enable. ILIM (Pin 5): Adjustable Peak Current Limit. Connect a resistor from ILIM to GND to program the peak inductor current according to the following formula: compensate the loop. See the section "Compensating the Feedback Loop" for guidelines. GND (Pin 9): Signal Ground Pin. SS (Pin 10): Connect a capacitor between this pin and ground to set soft-start period. 5A of current is sourced from SS during soft-start. t(msec) = CSS (F )* 200 BURST (Pin 11): Burst Mode Threshold Adjust Pin. A resistor/capacitor combination from this pin to ground programs the average load current at which automatic Burst Mode operation is entered, according to the formula:
180 ILIMIT = RILIM where ILIMIT is in amps and RT is in k.
RT (Pin 6): Connect a resistor to ground to program the oscillator frequency, according to the formula:
1 0.2 + 0.004 * RT where fOSC is in MHz and RT is in k. fOSC =
Oscillator frequency can be programmed between 400kHz and 1.5MHz. FB (Pin 7): Connect Resistor Divider Tap Here. The output voltage can be adjusted from 2V to 6V. Feedback reference voltage is typically 1.225V. COMP (Pin 8): gm Error Amp Output. A frequency compensation network is connected from this pin to ground to
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RBURST =
10 IBURST
where RBURST is in k and IBURST is in amps.
C BURST =
C OUT * VOUT 10, 000
where CBURST(MIN) and COUT are in F. To force fixed frequency PWM mode, connect BURST to VOUT through a 51k resistor. VOUT (Pin 12): Output of the Synchronous Rectifier and Internal Gate Drive Source for the Power Switches. R1 VOUT = 1.225 1 + R2 Exposed Pad (PGND) (Pin 13): Must be soldered to PCB ground, for electrical contact and optimum thermal performance.
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LTC3458L
BLOCK DIAGRA
VIN SYNC RT 3 6 OSC/SYNC MAX DUTY
CLOCK
SLOPE UNDER MODE IZERO P-DRIVE IZERO DETECT BURST MODE CONTROL N-DRIVE
PWM AND BURST MODE DRIVE LOGIC
N-DRIVE
SLEEP
IPEAK
MODE FIXED FREQUENCY BURST MODE MUX
9 GND
APPLICATIO S I FOR ATIO
Detailed Description
The LTC3458L provides high efficiency, low noise power for boost applications with output voltages up to 6V. The true output disconnect feature eliminates inrush current, and allows VOUT to go to zero during shutdown. The current mode architecture with adaptive slope compensation provides ease of loop compensation with excellent transient load response. The low RDS(ON), low gate charge synchronous switches eliminate the need for an external Schottky rectifier, and provide efficient high frequency pulse width modulation (PWM) control. High efficiency is achieved at light loads when Burst Mode operation is entered, where the IC's quiescent current is a low 12A typical on VIN. The LTC3458L is designed to provide custom performance in a variety of applications with programmable feedback, current limit, oscillator frequency, soft-start, and Burst Mode threshold.
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BURST 11 SW 1 VIN 2 ANTIRING P-DRIVE VBEST VSELECT - + SW1 12 VOUT P-DRIVE UNDER SLEEP TO ALL BLOCKS PGND BURST ACTIVE (DISABLED IN BURST MODE) I_SENSE TSD SOFT-START N-DRIVE THERMAL SD SD 4% UNDERVOLTAGE SLEEP CONTROL ICOMP/LIMIT_PEAK IBURST_PEAK SLOPE VBEST ERROR AMPLIFIER/ BURST COMPARATOR - + MODE BIAS CURRENTS UVLO VIN 7 FB PEAK CURRENT COMPARATOR ICOMP, ILIMIT, IBURST_PEAK, SLOPE COMP REFERENCE/ BIAS SDB TO ALL BLOCKS PGND 5 ILIM 13 PGND 10 SS 8 COMP 4 SHDN
3458L BD
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LTC3458L Programmable Functions Current Limit (ILIMIT)/Peak Burst Current (IBURSTPEAK). The programmable current limit circuit sets the maximum peak current in the internal N-channel MOSFET switch. This clamp level is programmed using a resistor to ground on ILIM. In Burst Mode operation, the current limit is automatically set to ~1/4 of the programmed current limit for optimal efficiency. An 84.5k RILIM resistor is recommended in most applications unless a lower limit is needed to prevent the external inductor from saturating. ILIMIT = 180 R
I is in amps and R is in k. IBURSTPEAK 1 * ILIM 4
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APPLICATIO S I FOR ATIO
Error Amp. The error amplifier is a transconductance type, with its positive input internally connected to the 1.225V reference, and its negative input connected to FB. A simple compensation network is placed from COMP to ground. Internal clamps limit the minimum and maximum error amp output voltage for improved large signal transient response. During sleep (in Burst Mode), the compensation pin is high impedance, however clamps limit the voltage on the external compensation network, preventing the compensation capacitor from discharging to zero during the sleep time. Oscillator. The frequency of operation is set through a resistor from RT to ground. An internally trimmed timing capacitor resides inside the IC. The oscillator frequency is calculated using the following formula:
fOSC =
1 0.2 + 0.004 * RT
where fOSC is in MHz and RT is in k The oscillator can be synchronized with an external clock applied to the SYNC pin. When synchronizing the oscillator, the free running frequency must be set to approximately 30% lower than the desired synchronized frequency. Soft-Start. The soft-start time is programmed with an external capacitor to ground on SS. An internal current source charges it with a nominal 5A. The voltage on the SS pin (in conjunction with the external resistor on ILIM) is used to control the peak current limit until the voltage on the capacitor exceeds ~1V, at which point the external resistor sets the peak current. In the event of a commanded shutdown, severe short-circuit, or a thermal shutdown, the capacitor is discharged automatically. t(msec) = CSS (F) * 200 Other LTC3458L Features and Functions Antiringing Control. The antiringing control places a resistor across the inductor to damp the ringing on SW pin discontinuous conduction mode. The LC ringing (L = inductor, CSW = Capacitance on SW pin) is low energy, but can cause EMI radiation.
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Current Sensing. Lossless current sensing converts the peak current signal to a voltage to sum in with the internal slope compensation. This summed signal is compared to the error amplifier output to provide a peak current control command for the PWM. The slope compensation in the IC is adaptive to the input and output voltage, therefore the converter provides the proper amount of slope compensation to ensure stability, but not an excess to cause a loss of phase margin in the converter. Output Disconnect and Inrush Limiting. The LTC3458L is designed to allow true output disconnect by eliminating body diode conduction of the internal P-channel MOSFET rectifier. This allows V0UT to go to zero volts during shutdown, drawing no current from the input source. It also allows for inrush current limiting at turn-on, minimizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, there must not be any external Schottky diodes connected between SW and VOUT. Shutdown. The part is shut down by pulling SHDN below 0.3V, and made active by pulling the pin above 1.25V. Note that SHDN can be driven above VIN or VOUT, as long as it is limited to less than 6V. Synchronous Rectifier. To prevent the inductor current from running away, the P-channel MOSFET synchronous rectifier is only enabled when VOUT > (VIN + 0.25V). Thermal Shutdown. If the die temperature reaches approximately 150C, the part will go into thermal shutdown and all switches will be turned off and the soft-start capacitor will be reset. The part will be enabled again when the die temperature has dropped by 10C (nominal). Zero Current Amplifier (IZERO). The zero current amplifier monitors the inductor current to the output and shuts off the synchronous rectifier once the current is below 50mA typical, preventing negative inductor current. Burst Mode Operation Burst Mode operation can be automatic or user controlled. In automatic operation, the IC will automatically enter Burst Mode operation at light load and return to fixed frequency PWM mode for heavier loads. The user can program the average load current at which the mode
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LTC3458L
APPLICATIO S I FOR ATIO
transition occurs using a single resistor. During Burst Mode operation, the oscillator is shut down, since the on time is determined by the time it takes the inductor current to reach a fixed peak current, and the off time is determined by the time it takes for the inductor current to return to zero. In Burst Mode operation, the IC delivers energy to the output until it is regulated and then goes into a sleep mode where the outputs are off and the IC is consuming only 12A of quiescent current. In this mode the output ripple voltage has a variable frequency component with load current and will be typically 2% peak-to-peak. This maximizes efficiency at very light loads by minimizing switching and quiescent losses. Burst Mode ripple can be reduced slightly by using more output capacitance (22F or greater). This capacitor does not need to be a low ESR type if low ESR ceramics are also used. Another method of reducing Burst Mode ripple is to place a small feedforward capacitor across the upper resistor in the VOUT feedback divider network. During Burst Mode operation, COMP is disconnected from the error amplifier in an effort to hold the voltage on the external compensation network where it was before entering Burst Mode operation. To minimize the effects of leakage current and stray resistance, voltage clamps limit the minimum and maximum voltage on COMP during Burst Mode operation. This minimizes the transient experienced when a heavy load is suddenly applied to the converter after being in Burst Mode operation for an extended period of time. For automatic operation, an RC network should be connected from BURST to ground. The value of the resistor will control the average load current (IBURST) at which Burst Mode operation will be entered and exited (there is hysteresis to prevent oscillation between modes). The equation given for the capacitor on BURST is for the minimum value, to prevent ripple on the BURST pin from causing the part to oscillate in and out of Burst Mode operation at the current where the mode transition occurs.
RBURST =
10 IBURST
where RBURST is in k and IBURST is in amps.
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C BURST = C OUT * VOUT 10, 000
where CBURST(MIN) and COUT are in F. Note: the BURST pin only sources current based on current delivered to VOUT through the P-channel MOSFET. If current in the inductor is allowed to go negative (this can occur at very light loads and high step-up ratios), the burst threshold may become inaccurate, preventing the IC from entering Burst Mode operation. For RBURST values greater than 200k, a larger than recommended inductor value may be needed to ensure positive inductor current and automatic Burst Mode operation. In the event that a sudden load transient causes the voltage level on FB to drop by more than 4% from the regulation value, an internal pull-up is applied to BURST, forcing the part quickly out of Burst Mode operation. For optimum transient response when going between Burst Mode operation and PWM mode, Burst can be controlled manually by the host. This way PWM mode can be commanded before the load step occurs, minimizing output voltage drop. Note that Burst Mode operation is inhibited during start-up and soft-start. Manual Control For applications requiring fixed frequency operation at all load currents, connect the BURST pin to VOUT through a 51k resistor. To force Burst Mode operation, ground the BURST pin. For applications where a large load step can be anticipated, the circuit below can be used to reduce the voltage transient on VOUT. Automatic operation is achieved when the external P-channel MOSFET is off and fixed frequency operation is commanded when the external P-channel MOSFET is on. In shutdown, the P-channel MOSFET should be off.
VIN HIGH: AUTO MODE LOW: FIXED FREQUENCY P-CHANNEL MOSFET BURST 133k 0.01F
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Figure 1
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APPLICATIO S I FOR ATIO
COMPONENT SELECTION Inductor Selection
The high frequency operation of the LTC3458L allows for the use of small surface mount inductors. Since the internal slope compensation circuit relies on the inductor's current slope and frequency, Table 1 should be used to select an inductor value for a given frequency of operation ( 25%). The recommended value will yield optimal transient performance while maintaining stable operation. Inductor values larger than listed in Table 1 are permissible to reduce the current ripple.
Table 1. Recommended Inductor Values
Frequency 1.5MHz 1.25MHz 1MHz 750kHz 500kHz Inductor Value(H) 3.3 to 4.7 4.7 to 6.8 6.8 to 10 10 to 15 15 to 22
For high efficiency, choose an inductor with high frequency core material, such as ferrite, to reduce core losses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support peak inductor currents in the 1A to 3A region. To minimize radiated noise, use a toroidal or shielded inductor. (Note that the inductance of shielded types will drop more as current increases, and will saturate more easily). See Table 2 for a list of inductor manufacturers.
Table 2. Inductor Vendor Information
Supplier Coilcraft TDK Murata Sumida COEV Toko .. Wurth Phone (847) 639-6400 (847) 803-6100 USA: (814) 237-1431 (800) 831-9172 USA: (847) 956-0666 Japan: 81-3-3607-5111 (800) 227-7040 (847) 297-0070 (202) 785-8800 Website www.coilcraft.com www.component.tdk.com www.murata.com www.japanlink.com/sumida www.coev.net www.tokoam.com www.we-online.com
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Some example inductor part types are: Coilcraft: MSS7341 and MSS5131 Series TDK: RLF5018T and SLF7045 Series Murata: LQH4C and LQN6C Series Sumida: CDRH4D28 and CDRH6D28 Series COEV: DQ7545 Series TOKO: D62CB and D63LCB Series .. WURTH: WE-PD2 Series Output Capacitor Selection The output voltage ripple has three components to it. The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The max ripple due to charge is given by:
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VRBULK =
IP * VIN C OUT * VOUT * f
where IP = peak inductor current and f = switching frequency. The ESR (equivalent series resistance) is usually the most dominant factor for ripple in most power converters. The ripple due to capacitor ESR is given by: VRCESR = IP * CESR where CESR = Capacitor Series Resistance. The ESL (equivalent series inductance) is also an important factor for high frequency converters. Using small, surface mount ceramic capacitors, placed as close as possible to the VOUT pins, will minimize ESL. Low ESR/ESL capacitors should be used to minimize output voltage ripple. For surface mount applications, AVX TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo Yuden X5R type ceramic capacitors are recommended. For through-hole applications, Sanyo OS-CON capacitors offer low ESR in a small package size. In all applications, a minimum of 4.7F (generally 22F is recommended), low ESR ceramic capacitor should be placed as close to the VOUT pin as possible, and grounded to a local ground plane.
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APPLICATIO S I FOR ATIO
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn from the input source and reduces input switching noise. In most applications >1F per amp of peak input current is recommended. See Table 3 for a list of capacitor manufacturers for input and output capacitor selection.
Table 3. Capacitor Vendor Information
Supplier AVX Sanyo TDK Murata Taiyo Yuden Phone (803) 448 - 9411 (619) 661 - 6322 (847) 803 - 6100 USA: (814) 237-1431 (800) 831-9172 (408) 573 - 4150 Website www.avxcorp.com www.sanyovideo.com www.component.tdk.com www.murata.com www.t-yuden.com
Operating Frequency Selection There are several considerations in selecting the operating frequency of the converter. The first is staying clear of sensitive frequency bands, which cannot tolerate any spectral noise. For example in products incorporating RF communications the 455kHz IF frequency is sensitive to any noise, therefore switching above 600kHz is desired. Some communications have sensitivity to 1.1MHz and in that case a 1.5MHz switching converter frequency may be employed. The second consideration is the physical size of the converter. As the operating frequency goes up, the inductor and filter capacitors go down in value and size. The trade off is in efficiency, since the switching losses due to gate charge increase proportional with frequency. Thermal Considerations For the LTC3458L to deliver its full output power, it is imperative that a good thermal path be provided to dissipate the heat generated within the package. This can be accomplished by taking advantage of the large thermal pad on the underside of the IC. It is recommended that multiple vias in the printed circuit board be used to conduct heat away from the IC and into a copper plane with as much area as possible. If the junction temperature rises above ~150C, the part will go into thermal shutdown, and all switching will stop until the temperature drops.
12
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Compensating the Feedback Loop The LTC3458L uses current mode control, with internal adaptive slope compensation. Current mode control eliminates the 2nd order filter due to the inductor and output capacitor exhibited in voltage mode controllers, and simplifies the power loop to a single pole filter response. The product of the modulator control to output DC gain, and the error amp open-loop gain gives the DC gain of the system:
W
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G DC = G CONTROL * G EA * G CONTROL = G EA 1000 , 2 * VIN , IOUT
VREF * G CURRENT _ SENSE VOUT
G CURRENT _ SENSE =
1 RDS(ON)
The output filter pole is given by:
IOUT , * VOUT * C OUT where COUT is the output filter capacitor. fFILTER _ POLE =
The output filter zero is given by:
fFILTER _ ZERO =
where RESR resistance.
, 2 * RESR * C OUT is the output capacitor equivalent series
1
A troublesome feature of the boost regulator topology is the right half plane zero (RHP), and is given by:
fRHPZ
VIN = 2 * IOUT * VOUT * L
2
At heavy loads this gain increase with phase lag can occur at a relatively low frequency. The loop gain is typically rolled off before the RHP zero frequency.
3458lf
LTC3458L
APPLICATIO S I FOR ATIO
The typical error amp compensation is shown in Figure 2. The equations for the loop dynamics are as follows:
2 * 10e 6 * CC1 1 fZERO1 = 2 * RZ * CC1 1 fPOLE2 2 * RZ * CC 2
fPOLE1
1
which is close to DC
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1.225V VOUT
W
UU
+
ERROR AMP R1 FB 7 R2 COMP 8 CC1 RZ CC2
-
3458L F01
Figure 2
3458lf
13
LTC3458L
TYPICAL APPLICATIO S
Lithium-Ion to 5V, 750mA at 750kHz
COILCRAFT 10H MSS7341
Li-Ion 3.1V to 4.2V
VIN 2.2F LTC3458L GND/PGND SHDN SYNC RT ILIM 274k 84.5k
10pF ON OFF FB COMP 0.01F SS 0.01F BURST 133k 560pF 33k 10pF
EFFICIENCY (%)
Two Cell to 5VOUT, 250mA at 750kHz
COILCRAFT 10H MSS7341
100
2 ALKALINE 1.8V to 3.3V
VIN 2.2F LTC3458L GND/PGND SHDN SYNC RT ILIM 274k 84.5k
EFFICIENCY (%)
ON OFF
COMP 0.01F SS 0.01F BURST 133k 560pF
3458L TA04a
14
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Li-Ion to 5VOUT
100
SW VOUT 1M VOUT 5V 750mA
95 90
4.2VIN
3.1VIN 85 80 75 70 0.1
3.6VIN
324k
22F X5R
1
10 100 LOAD CURRENT (mA)
1000
3458L TA03b
3458L TA03a
Two Alkaline to 5VOUT
SW VOUT 10pF FB 1M VOUT 5V 250mA
95 3.3VIN 90 85 80 75 2.5VIN 1.8VIN
324k 33k 10pF 22F X5R
70 0.1
1
10 100 LOAD CURRENT (mA)
1000
3458L TA04b
3458lf
LTC3458L
PACKAGE DESCRIPTIO
3.40 0.05 1.70 0.05 2.24 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 3.30 0.05 (2 SIDES) 0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 0.10 (2 SIDES) R = 0.20 TYP 3.00 0.10 (2 SIDES) PIN 1 TOP MARK 6 0.25 0.05 3.30 0.10 (2 SIDES) 1 0.50 BSC 1.70 0.10 (2 SIDES) PIN 1 NOTCH
(UE12/DE12) DFN 0802
0.200 REF
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE A VARIATION OF VERSION (WGED) IN JEDEC PACKAGE OUTLINE M0-229 2. ALL DIMENSIONS ARE IN MILLIMETERS 3. 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 4. EXPOSED PAD SHALL BE SOLDER PLATED
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.
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DE/UE Package 12-Lead Plastic DFN (4mm x 3mm)
(Reference LTC DWG # 05-08-1695)
0.58 0.05 7 R = 0.115 TYP 0.38 0.10 12 0.75 0.05 0.00 - 0.05
3458lf
15
LTC3458L
TYPICAL APPLICATIO
L1 2 ALKALINE 1.8V TO 3.3V
High Current LED Driver with BURST Pin Current Regulation
+ -
VIN CIN 2.2F LTC3458L GND/PGND SHDN SYNC RT ILIM 274k 84.5k VOUT Z1 ON OFF FB
EFFICIENCY (%)
COMP 0.01F SS 0.01F BURST RBURST 33k
CIN, COUT: TAIYO YUDEN JMK107BJ225MA D1: LUXEON EMITTER LUMILED WHITE LXHLMW1D L1: Coilcraft 10H MSS7341
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3458lf LT/TP 0505 500 * PRINTED IN USA
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16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
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SW fOSC = 750kHz VOUT 3V TO 3.5V
Two Alkaline Cells Powering a 1-Watt Lumiled
100 50mA, 3V 90 150mA, 3.2V
80 300mA, 3.5V 70
D1 COUT 2.2F
60
0.01F
NOTE: LUMILED CURRENT REGULATION ~20% OVER VIN RANGE 50 1.8 2.3 2.8 INPUT VOLTAGE (V) 3.3
3458L TA06b
RBURST: 45.3k FOR 300mA, 3458L TA06a 82.5k FOR 150mA, 232k FOR 50mA Z1: CENTRAL SEMI 6.8V ZENER DIODE SOT-23 CMPZ5235B
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005

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