If the input voltage rises above 30V (e.g. load dump), the

output shuts down. This response protects the internal cir-

cuitry and enables the IC to survive unexpected voltage

transients.

Using an emitter sense scheme, the amount of current

through the NPN pass transistor is monitored. Feedback

circuitry insures that the output current never exceeds a

preset limit.

Should the junction temperature of the power device

exceed 180°C (typ) the power transistor is turned off.

Thermal shutdown is an effective means to prevent die

overheating since the power transistor is the principle

heat source in the IC.

The CS8121 contains two microprocessor compatible con-

trol functions:

and

(Figure 3).

Function

The

function switches the output transistor.

When the voltage on the

lead exceeds 2.9V typ,

the output pass transistor turns off, leaving a high

impedance facing the load. The IC will remain in Sleep

mode, drawing only 250µA, until the voltage on the lead

drops below 2.1V typ. Hysteresis (800mV) is built into the

function to provide good noise immunity.

Function

A

signal (low voltage) is generated as the IC pow-

is included in the function to minimize oscillations.

The

output is an open collector NPN transistor,

controlled by a low voltage detection circuit. The circuit is

functionally independent of the rest of the IC, thereby

low as 1V.

An external RC network on the

lead (Figure 4) pro-

vides a sufficiently long delay for most microprocessor

based applications. RC values can be chosen using the

following formula:

where:

= µP port impedance,

delay capacitor,

lead (0.7V @ turn - on), and

= µP logic threshold voltage.

RESET

RESET

]

[

RESET

RESET

RESET

RESET

ENABLE

ENABLE

ENABLE

ENABLE

RESET

ENABLE

5

Regulator Control Functions

Figure 4. RC Network for

Delay

RESET

Circuit Description: continued

ln

)

(

RESET

CS8121

10

µF

tantalum

5V to

µP

and

System

Power

to

µP

RESET

Port

(1) = NO RESET DELAY CAPACITOR

(2) = WITH RESET DELAY CAPACITOR

ENABLE

(1)

(2)

HI

LO

RESET

Figure 3. Circuit Waveforms for the CS8121