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CS5151GN16

CPU 4−Bit Nonsynchronous Buck Controller

ON Semiconductor 

CS5151

This circuit operates by pulling the Soft Start pin high, and

the VFFB pin low, emulating a short circuit condition.

5.0 V

Shutdown

Input

MMUN2111T1 (SOT−23)

IN4148

5 SS

CS5151

8 VFFB

Figure 14. Implementing Shutdown with the CS5151

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:

VPower

Good

+

(R1

)

R2)

R2

0.65 V

This circuit provides an open collector output that drives

the Power Good output to ground for regulator voltages less

than VPower Good.

5.0 V

R1

10 k

VOUT

CS5151

R3

10 k

PN3904

R2

6.2 k

Power Good

PN3904

Figure 15. Implementing Power Good with the CS5151

M 2.50 ms

Trace 3 − 12 V Input (VCC1) and (VCC2) (10 V/div.)

Trace 4− 5.0 V Input (2.0 V/div.)

Trace 1− Regulator Output Voltage (1.0 V/div.)

Trace 2− Power Good Signal (2.0 V/div.)

Figure 16. CS5151 Demonstration Board During Power

Up. Power Good Signal is Activated when Output

Voltage Reaches 1.70 V.

Selecting External Components

The CS5151 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 12 V supply

which is generally available in most computer systems and

use logic level MOSFETs. A charge pump may be easily

implemented to support 5.0 V only systems. Multiple

MOSFETs may be paralleled to reduce losses and improve

efficiency and thermal management.

Voltage applied to the MOSFET gates depends on the

application circuit used. The gate driver output is specified

to drive to within 1.5 V of ground when in the low state and

to within 2.0 V of its bias supply when in the high state. In

practice, the MOSFET gate will be driven rail to rail due to

overshoot caused by the capacitive load it presents to the

controller IC. For the typical application where VCC1 =

VCC2 = 12 V and 5.0 V is used as the source for the regulator

output current, the following gate drive is provided;

VGATE(H) + 12 V * 5.0 V + 7.0 V

(see Figure 17.)

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