LTM4656 datasheet(14/28 Pages) AD

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LTM4656/LTM4656-1

Rev. 0

For more information www.analog.com

Theoutputcapacitorrecommendationsandinternalcontrol

loop compensation assure stability over the operating

ranges. The Analog Devices LTpowerCAD® design tool

is available to download online for RMS current, output

ripple, stability and transient response analysis.

LOW VOLTAGE OPERATION

The LTM4656 is designed to allow start-up from input

voltages as low as 4.5V. The limiting factors for the low

voltage applications become the availability of the power

source to supply sufficient power to the output at the

low input voltage, and the maximum duty cycle, which is

clamped at 96%. Note that at low input voltages, small

voltage drops due to series resistance become critical and

greatly limit the power delivery capability of the converter.

The input current can get large at high boost ratios. The

input is limited to a minimum of 9A. The below equation

can be used to calculate the input current need to support

a particular design.

IIN =

VOUT •IOUT

VIN

For example, 5V to 12V at 3.5A output current would

equate to an input current of ~8.5A.

SHDN AND RUN PINS

The SHDN pin controls the complete shutdown of the

LTM4656. When this pin is below 0.4V, the LTM4656

will be in complete shutdown drawing only 40µA. When

SHDN goes above 2.1V then the LTM4656 will enable.

The LTM4656 boost converter can be shut down using

the RUN pin while the inline overcurrent protection is

still powered. Pulling this pin below 1.28V shuts down

the main boost control loop. Pulling this pin below 0.7V

disables the boost controller and most internal circuits,

including the INTVCC LDOs. In this state, total current

draw is about 50µA.

SOFT-START (SS PIN)

The start-up of the VOUT is controlled by the voltage on the

SS pin. When the voltage on the SS pin is less than the

APPLICATIONS INFORMATION

internal1.2Vreference,theLTM4656regulatestheVFBpin

voltage to the voltage on the SS pin instead of 1.2V. Soft-

start is enabled by simply connecting a capacitor from the

SS pin to ground. An internal 10µA current source charges

the capacitor, providing a linear ramping voltage at the SS

pin. The LTM4656 will regulate the VFB pin (and hence,

VOUT) according to the voltage on the SS pin, allowing

VOUT to rise smoothly from VIN to its final regulated value.

The total soft-start time will be approximately:

TSS = CSS •

1.2V

10µA

A 0.1µF is a good value to use for CSS.Thisprovidesaslow

12ms ramp that will slowly turn on the boost regulator into

load, and eliminate false overcurrent tripping at start-up.

INTVCC Power

The LTM4656 boost control section features an internal

P-channellowdropoutlinearregulator(LDO)thatsupplies

power at the INTVCC pin from the VBIAS supply pin. INTVCC

powers the gate drivers and much of the boost control’s

internal circuitry. The VBIAS LDO regulates INTVCC to 5.4V.

It can supply at least 50mA and is bypassed to ground

with an internal 4.7µF ceramic capacitor.

Power Good

The PGOOD pin is connected to an open-drain of an

N-channel MOSFET. The MOSFET turns on and pulls the

PGOODpinlowwhentheVFBpinvoltageisnotwithin ±10%

ofthe1.2Vreferencevoltage.ThePGOODpinisalsopulled

low when the corresponding RUN pin is low (shutdown).

When the VFB pin voltage is within the ±10% require-

ment, the MOSFET is turned off and the pin is allowed to

be pulled up by an external resistor to a source of up to

6V (ABS max).

Loop Compensation

The LTM4656 has internal compensation while the

LTM4656-1 provides for optimized external compen-

sation. LTpowerCAD can be used to optimize External

Compensation.

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