CS8120

http://onsemi.com

7

functionally independent of the rest of the IC, thereby

as 1.0 V.

Figure 12. RC Network for RESET Delay Circuitry

CS8120

5.0 V to

mP

and System

Power

RESET

to

mP

RESET

Port

22

mF

An external RC network on the RESET lead (Figure 12)

provides a sufficiently long delay for most microprocessor

based applications. RC values can be chosen using the

following formula:

RTOT

CRST +

−tDelay

ln

VT*VOUT

VRST*VOUT

where:

mP port impedance,

mP logic threshold voltage.

APPLICATION NOTES

The circuit depicted in Figure 13 lets the microprocessor

control its power source, the CS8120 regulator. An I/O port

on the

mP and the SWITCH port are used to drive the base

of Q1. When Q1 is driven into saturation, the voltage on the

ENABLE lead falls below its lower threshold. The

regulator’s output is switched out. When the drive current is

removed, the voltage on the ENABLE lead rises, the output

is switched off and the IC moves into Sleep mode where it

draws 250

mA.

By coupling these two controls with the ENABLE, the

system has added flexibility. Once the system is running, the

state of the SWITCH is irrelevant as long as the I/O port

continues to drive Q1. The microprocessor can turn off its

own power by withdrawing drive current, once the

SWITCH is open. This software control at the I/O port

allows the microprocessor to finish key housekeeping

functions before power is removed.

The logic options are summarized in Table 1.

Table 1. Logic Control of CS8120 Output

Microprocessor

I/O Drive

SWITCH

ENABLE

Output

ON

Closed

LOW

ON

Open

LOW

ON

OFF

Closed

LOW

ON

Open

HIGH

OFF

The I/O port of the microprocessor typically provides

50

mA to Q1. In automotive applications the SWITCH is

connected to the ignition switch.

SWITCH

Q1

500 k

W

0.1

mF

500 k

W

100 k

W

100 k

W

22

mF

RESET

I/O Port

mP

RESET

ENABLE

Figure 13. Microprocessor Control of CS8120 Using External Switching Transistor Q1

CS8120

GND

C1

C2