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LTC1553CG

5-Bit Programmable Synchronous Switching Regulator Controller for Pentium® II Processor

Linear Technology 

APPLICATIONS INFORMATION

MOSFET Gate Drive

Power for the internal MOSFET drivers is supplied by

PVCC. This supply must be above the input supply voltage

by at least one power MOSFET VGS(ON) for efficient opera-

tion. This higher voltage can be supplied with a separate

supply, or it can be generated using a simple charge pump

as shown in Figure 7. The 84% typical maximum duty

cycle ensures sufficient off-time to refresh the charge

pump during each cycle. Figure 8 shows a tripling charge

pump, which provides additional VGS overdrive to the

external MOSFETs. This circuit can be useful for standard

threshold MOSFETs which demand a higher turn-on volt-

age. An 18V Zener diode (1N5248B) is recommended with

tripler charge pump designs to ensure that PVCC never

exceeds the LTC1553’s 20V absolute maximum PVCC

voltage. This becomes more critical as VIN rises. With VIN

= 12V, the doubler circuit of Figure 7 will also exceed the

20V limit. Figure 9 shows an alternate 17V charge pump

derived from both the 5V and 12V supplies.

If the OUTEN pin is low, G1 and G2 are both held low to

prevent output voltage undershoot. As VCC and PVCC

power up from a 0V condition, an internal undervoltage

lockup circuit prevents G1 and G2 from going high until

VCC reaches about 3.5V. If VCC powers up while PVCC is at

ground potential, the SS is forced to ground potential

internally. SS clamps the COMP pin low and prevents the

drivers from turning on. On power-up or recovery from

thermal shutdown, the drivers are designed such that G2

is held low until G1 first goes high.

Power MOSFETs

Two N-channel power MOSFETs are required for most

LTC1553 circuits. They should be selected based prima-

rily on threshold and on-resistance considerations. The

required MOSFET threshold should be determined based

on the available power supply voltages and/or the com-

plexity of the gate driver charge pump scheme. In 5V input

designs where a 12V supply is used to power PVCC,

standard MOSFETs with RDS(ON) specified at VGS = 5V or

6V can be used with good results. However, logic level

devices will improve efficiency. The current drawn from

the 12V supply varies with the MOSFETs used and the

LTC1553 operating frequency, but is generally less than

50mA.

LTC1553

OPTIONAL FOR VIN > 5V

1N5248B

18V

2 PVCC

LTC1553

G1

20

G2

1

1N5817 VIN

0.1µF

+

CIN

Q1

LO

VOUT

+

Q2

COUT

1553 F07

Figure 7. Doubling Charge Pump

1N5248B

18V

VIN

1N5817 1N5817 1N5817

2 PVCC

0.1µF + 10µF

LTC1553

G1

20

G2

1

0.1µF

+

CIN

Q1

LO

+

Q2

VOUT

COUT

1553 F08

Figure 8. Tripling Charge Pump

1N5817

VCC

5V

10Ω

1N5248B

18V

CVCC

5 VCC 2 PVCC

LTC1553

G1

20

G2

1

VIN

12V

0.1µF

+

CIN

Q1

LO

Q2

VOUT

+

COUT

1553 F09

Figure 9. 17V Charge Pump for VIN = 12V

13

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