LTC1430 datasheet(7/16 Pages) LINER

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LTC1430

MOSFET Gate Drive

Gate drive for the top N-channel MOSFET M1 is supplied

from PVCC1. This supply must be above PVCC ( the main

power supply input) by at least one power MOSFET

VGS(ON) for efficient operation. An internal level shifter

allows PVCC1 to operate at voltages above VCC and PVCC,

up to 13V maximum. This higher voltage can be supplied

with a separate supply, or it can be generated using a

simple charge pump as shown in Figure 4. When using a

separate PVCC1 supply, the PVCC input may exhibit a large

inrush current if PVCC1 is present during power up. The

90% maximum duty cycle ensures that the charge pump

will always provide sufficient gate drive to M1. Gate drive

for the bottom MOSFET M2 is provided through PVCC2 for

16-lead devices or VCC/PVCC2 for 8-lead devices. PVCC2

can usually be driven directly from PVCC with 16-lead

parts, although it can also be charge pumped or connected

to an alternate supply if desired. The 8-lead parts require

an RC filter from PVCC to ensure proper operation; see

Input Supply Considerations.

EXTERNAL COMPONENT SELECTION

Power MOSFETs

Two N-channel power MOSFETs are required for most

LTC1430 circuits. These should be selected based prima-

rily on threshold and on-resistance considerations; ther-

mal dissipation is often a secondary concern in high

efficiency designs. Required MOSFET threshold should be

determined based on the available power supply voltages

and/or the complexity of the gate drive charge pump

scheme. In 5V input designs where an auxiliary 12V supply

is available to power PVCC1 and PVCC2, standard MOSFETs

with RDS(ON) specified at VGS = 5V or 6V can be used with

good results. The current drawn from this supply varies

with the MOSFETs used and the LTC1430’s operating

frequency, but is generally less than 50mA.

LTC1430 designs that use a doubler charge pump to

generate gate drive for M1 and run from PVCC voltages

below 7V cannot provide enough gate drive voltage to fully

enhance standard power MOSFETs. When run from 5V, a

doubler circuit may work with standard MOSFETs, but the

MOSFET RON may be quite high, raising the dissipation in

the FETs and costing efficiency. Logic level FETs are a

better choice for 5V PVCC systems; they can be fully

enhanced with a doubler charge pump and will operate at

maximum efficiency. Doubler designs running from PVCC

voltages near 4V will begin to run into efficiency problems

even with logic level FETs; such designs should be built

with tripler charge pumps (see Figure 5) or with newer,

super low threshold MOSFETs. Note that doubler charge

pump designs running from more than 7V and all tripler

charge pump designs should include a zener clamp diode

DZ at PVCC1 to prevent transients from exceeding the

absolute maximum rating at that pin.

APPLICATIO S I FOR ATIO

Figure 4. Doubling Charge Pump

12V

1N5242

1N5817

LTC1430

PVCC1

PVCC2

0.1

µF

PVCC

COUT

VOUT

LTC1430 • F05

0.1

µF

1N5817

Figure 5. Tripling Charge Pump

12V

1N5242

OPTIONAL

USE FOR PVCC ≥ 7V

LTC1430

PVCC1

PVCC2

1N4148

PVCC

COUT

VOUT

LTC1430 • F04

0.1

µF

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