US3018

4-5

Rev. 1.4

12/8/00

Pin Description

This pin is internally connected to the undervoltage and overvoltage comparators sensing

the Vcore status. It must be connected directly to the Vcore supply.

This pin is the input that provides power for the internal LDO regulator. It is also monitored

for the under voltage and over voltage conditions.

This pin is connected to the Drain of the power MOSFET of the Core supply and it

provides the positive sensing for the internal current sensing circuitry. An external resis-

tor programs the C.S threshold depending on the Rds of the power MOSFET. An external

capacitor is placed in parallel with the programming resistor to provide high frequency

noise filtering.

This pin is connected to the Source of the power MOSFET for the Core supply and it

provides the negative sensing for the internal current sensing circuitry.

This pin provides the soft start for the switching regulator. An internal resistor charges an

external capacitor that is conected from 5V supply to this pin which ramps up the outputs

of the switching regulators, preventing the outputs from overshooting as wellas limiting

the input current. The second function of the Soft Start cap is to provide long off time

(HICCUP) for the synchronous MOSFET during current limiting.

This pin has dual function. It acts as an output of the OVP circuitry or it can be used to

program the frequency using an external resistor . When used as a fault detector, if the

switcher output exceed the OVP trip point, the FAULT pin switches to 12V and the soft

start cap is discharged. If the FAULT pin is to be connected to any external circuitry, it

needs to be buffered as shown in the application circuit.

This pin controls the gate of an external transistor for the 1.5V GTL+ linear regulator.

This pin provides the feedback for the linear regulator that its output drive is GATE3.

This pin is the output of the internal LDO regulator.

This pin provides the feedback for the internal LDO regulator that its output is Vout4.

This pin serves as the ground pin and must be conected directly to the ground plane.

This pin serves as the Power ground pin and must be conected directly to the GND plane

close to the source of the synchronous MOSFET. A high frequency capacitor (typically 1

uF) must be connected from V12 pin to this pin for noise free operation.

Output driver for the synchronous power MOSFET for the Core supply.

Output driver for the high side power MOSFET for the Core supply.

This pin is connected to the 12 V supply and serves as the power Vcc pin for the output

drivers. A high frequency capacitor (typically 1 uF) must be placed close to this pin and

PGND pin and be connected directly from this pin to the GND plane for the noise free

operation.

5V supply voltage. A high frequency capacitor (0.1 to 1 uF) must be placed close to this

pin and connected from this pin to the GND plane for noise free operation.

This pin is a TTL compatible Enable pin. When this pin is left open or pulled high, the

device is enabled and when is pulled low, it will disable the switcher and the LDO

controller (Vout 3) leaving the internal 200mA regulator operational. When signal is

given to enable the device, both switcher and Vout 3 will go through soft start, the

same as during start up.

PIN# PIN SYMBOL

19

VSEN1

12

VIN2

20

OCSET1

23

PHASE1

9

SS

10

FAULT/Rt

15

GATE3

16

FB3

13

VOUT2

11

FB2

14

GND

21

PGND

22

LGATE1

24

UGATE1

1

V12

8

V5

17

En