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CS8121

5V, 1A Linear Regulator with RESET and ENABLE

Cherry semiconductor 

Circuit Description: continued

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 180ûC (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.

Regulator Control Functions

The CS8121 contains two microprocessor compatible con-

trol functions: ENABLE and RESET (Figure 3).

RESET Function

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

ers up (VOUT > VOUT - 100mV) or when VOUT drops out of

regulation (VOUT < VOUT - 140mV, typ). 40mV of hysteresis

is included in the function to minimize oscillations.

The RESET 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

guaranteeing that the RESET signal is valid for VOUT as

low as 1V.

An external RC network on the RESET lead (Figure 4) pro-

vides a sufficiently long delay for most microprocessor

based applications. RC values can be chosen using the

following formula:

[ ( ) ] RTOT´CRST

ln

ÐtDelay

VT Ð VOUT

VRST Ð VOUT

VIN

ENABLE

VOUT

HI

VIN(HI)

LO

VRT(ON)

VRT(OFF)

FOR 7V < VIN < 26V

where:

RTOT = RRST in parallel with RIN,

RIN = µP port impedance,

CRST = RESET delay capacitor,

tDelay = desired delay time,

VRST = VSAT of RESET lead (0.7V @ turn - on), and

VT = µP logic threshold voltage.

RESET

VR PEAK

(1)

(2)

VR SAT

(1) = NO RESET DELAY CAPACITOR

(2) = WITH RESET DELAY CAPACITOR

Figure 3. Circuit Waveforms for the CS8121

VR PEAK

ENABLE Function

The ENABLE function switches the output transistor.

When the voltage on the ENABLE 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

ENABLE function to provide good noise immunity.

VOUT

CS8121

RESET

RRST

CRST

Figure 4. RC Network for RESET Delay

5V to mP

and

System

10mF Power

tantalum

to mP

RESET

Port

5

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