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LTC1430A

APPLICATIONS INFORMATION

MOSFET Gate Drive

Gate drive for the top N-channel MOSFET Q1 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 5. When using a

separate PVCC1 supply, the PVCC input may exhibit a large

inrush current if PVCC1 is present during power up. The

93.5% maximum duty cycle ensures that the charge pump

will always provide sufficient gate drive to Q1. Gate drive

for the bottom MOSFET Q2 is provided through PVCC2 for

16-lead devices or VCC/PVCC2 for the 8-lead device. 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. 3.3V input applications

use 3.3V at PVCC and 5V at VCC and PVCC1. See 3.3V Input

Supply Operation for more details. The 8-lead part

requires an RC filter from PVCC to VCC to ensure proper

operation; see Input Supply Considerations.

12V

1N5242

OPTIONAL

USE FOR PVCC ≥ 7V

LTC1430A

PVCC1

PVCC2

MBR0530T1

PVCC

COUT

VOUT

1430 F05

0.1

µF

Figure 5. Doubling Charge Pump

VIN

VOUT

CONTROLLER

1430 F06a

Figure 6a. Classical Buck Architecture

VIN

VOUT

CONTROLLER

1430 F06b

Figure 6b. Synchronous Buck Architecture

much lower than the VF of the diode in the classical circuit.

This more than offsets the additional gate drive required

by the second MOSFET, allowing the LTC1430A to achieve

efficiencies in the mid-90% range for a wide range of load

currents.

Another feature of the synchronous architecture is that

unlike a diode, Q2 can conduct current in either direction.

This allows the output of a typical LTC1430A circuit to sink

current as well as sourcing it while remaining in regula-

tion. The ability to sink current at the output allows the

LTC1430A to be used with reactive or other nonconventional

loads that may supply current to the regulator as well as

drawing current from it. An example is a high current logic

termination supply, such as the GTL terminator shown in

the Typical Applications section.

EXTERNAL COMPONENT SELECTION

Power MOSFETs

Two N-channel power MOSFETs are required for most

LTC1430A circuits. These should be selected based pri-

marily on threshold and on-resistance considerations;

thermal 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

Synchronous Operation

The LTC1430A uses a synchronous switching architec-

ture, with MOSFET Q2 taking the place of the diode in a

classical buck circuit (Figure 6). This improves efficiency

by reducing the voltage drop and the resultant power

dissipation across Q2 to VON = (I)(RDSON(Q2)), usually

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