LTC1430AI datasheet(13/24 Pages) LINER

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LTC1430A

APPLICATI

I FOR ATIO

by using a larger RC filter from the PVCC pin; 22Ω and 10µF

work well here. The 10

µF capacitor must be VERY close to

the part (preferably right underneath the unit) or output

regulation may suffer.

For both versions of the LTC1430A, PVCC1 must be higher

than PVCC by at least one external MOSFET VGS(ON) to fully

enhance the gate of Q1. This higher voltage can be

provided with a separate supply (typically 12V) which

should power up after PVCC, or it can be generated with a

simple charge pump (Figure 5). The charge pump consists

of a Schottky diode from PVCC to PVCC1 and a 0.1µF

capacitor from PVCC1 to the switching node at the drain of

Q2. This circuit provides 2PVCC – VF to PVCC1 while Q1 is

ON and PVCC – VF while Q1 is OFF where VF is the ON

voltage of the Schottky diode. Ringing at the drain of Q2

can cause transients above 2PVCC at PVCC1; if PVCC is

higher than 7V, a 12V zener diode should be included from

PVCC1 to PGND to prevent transients from damaging the

circuitry at PVCC2 or the gate of Q1.

More complex charge pumps can be constructed with the

16-lead versions of the LTC1430A to provide additional

voltages for use with standard threshold MOSFETs or very

low PVCC voltages. A tripling charge pump (Figure 7) can

provide 2PVCC and 3PVCC voltages. These can be con-

nected to PVCC2 and PVCC1 respectively, allowing stan-

dard threshold MOSFETs to be used with 5V at PVCC or 5V

logic level threshold MOSFETs to be used with 3.3V at

PVCC. VCC can be driven from the same potential as PVCC2,

allowing the entire system to run from a single 3.3V

supply. Tripling charge pumps require the use of Schottky

diodes to minimize forward drop across the diodes at

start-up. The tripling charge pump circuit will tend to

rectify any ringing at the drain of Q2 and can provide well

more than 3PVCC at PVCC1; all tripling (or higher multiply-

ing factor) circuits should include a 12V zener clamp diode

DZ to prevent overvoltage at PVCC1.

3.3V Input Supply Operation

The LTC1430A can be used with input supply voltages

lower than 5V as long as a low power 5V supply is available

to power the LTC1430A itself and to provide gate drive to

the external MOSFETs. A typical 3.3V to 2.5V application

is shown in Figure 10. The circuit can supply up to 10A at

2.5V output, and draws this power from the 3.3V supply.

The 5V supply typically needs to supply about 20mA to

provide gate drive to the external MOSFETs and keep the

LTC1430A control circuits powered. For applications where

there is no 5V supply available, see the LTC1649 data

sheet.

Compensation and Transient Response

The LTC1430A voltage feedback loop is compensated at

the COMP pin; this is the output node of the internal gm

error amplifier. The loop can generally be compensated

Figure 10. 3.3V to 2.5V, 10A Application

CIN

220

µF

×4

COUT

330

µF

×6

2.7

µH/15A

PVCC1

VCC

FREQSET

SHDN

COMP

PVCC2

PGND

GND

IFB

IMAX

1430 F10

SHUTDOWN

Q1A, Q1B, Q2: INTERNATIONAL RECTIFIER IRF7801

CIN: AVX-TPSE227M010R0100

COUT: AVX-TPSE337M006R0100

SENSE+

LTC1430A

SENSE–

16k

100

Ω

976

Ω

7.5k

4700pF

220pF

MBR0530T1

0.1

µF

0.01

µF

0.1

µF

4.7

µF

2.5V

10A

Q1A, Q1B

2 IN PARALLEL

3.3V

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