Applications Information: continued

10

Figure 13: OVP response to an input-to-output short circuit by pulling

the input voltage to ground.

External Output Enable Circuit

On/off control of the regulator can be implemented

through the addition of two additional discrete compo-

nents (see Figure 14). This circuit operates by pulling the

circuit condition.

Figure 14: Implementing shutdown with the CS5157.

External Power Good Circuit

An optional Power Good signal can be generated through

the use of four additional external components (see Figure

15). The threshold voltage of the Power Good signal can be

adjusted per the following equation:

This circuit provides an open collector output that drives

the Power Good output to ground for regulator voltages

Figure 15: Implementing Power Good with the CS5157.

Figure 16: CS5157 demonstration board during power up. Power Good

signal is activated when output voltage reaches 1.70V.

Selecting External Components

The CS5157 can be used with a wide range of external

power components to optimize the cost and performance of

a particular design. The following information can be used

as general guidelines to assist in their selection.

NFET Power Transistors

Both logic level and standard MOSFETs can be used. The

reference designs derive gate drive from the 12V supply

which is generally available in most computer systems and

utilize logic level MOSFETs. Multiple MOSFETs may be

paralleled to reduce losses and improve efficiency and ther-

mal management.

Voltage applied to the MOSFET gates depends on the

application circuit used. Both upper and lower gate driver

outputs are specified to drive to within 1.5V of ground

when in the low state and to within 2V of their respective

bias supplies when in the high state. In practice, the MOS-

FET gates will be driven rail to rail due to overshoot caused

by the capacitive load they present to the controller IC. For

Trace 4 = 5V Input (2V/div.)

Trace 1 = Regulator Output Voltage (1V/div.)

Trace 2 = Power Good Signal (2V/div.)

Ch 1 High

2.80V

CS5157

R1

10k

R2

6.2k

R3

10k

Power Good

5V

PN3904

PN3904

(R1 + R2) × 0.65V

R2

Shutdown

Input

5V

CS5157

SS

5

8

IN4148

MMUN2111T1 (SOT-23)

Trace 4 = 5V from PC Power Supply (2V/div.)

Trace 1 = Regulator Output Voltage (1V/div.)