6

Application Notes

The output or compensation capacitor helps determine

three main characteristics of a linear regulator: start-up

delay, load transient response and loop stability.

The capacitor value and type should be based on cost,

availability, size and temperature constraints. A tantalum

or aluminum electrolytic capacitor is best, since a film or

ceramic capacitor with almost zero ESR, can cause insta-

bility. The aluminum electrolytic capacitor is the least

expensive solution, but, if the circuit operates at low tem-

peratures (-25¡C to -40¡C), both the value and ESR of the

capacitor will vary considerably. The capacitor manufac-

turers data sheet usually provides this information.

applications circuit should work for most applications,

however it is not necessarily the optimized solution.

ular application, start with a tantalum capacitor of the rec-

Stability Considerations

S1

ON/OFF

RESET

FLAG

20k

W

C1*

0.1

mF

RESET/

ENABLE

Gnd

C3**

10

mF

+

+

10

mF

C2 **

CS8135

NOTES:

* C1 required if regulator is located far from power supply filter.

** C2, C3 required for stability.

Test & Application Circuit

Unlike the standby regulated output, which must remain

on whenever possible, the high current regulated output is

fault protected against overvoltage and also incorporates

thermal shutdown. If the input voltage rises above approx-

imately 30V (e.g., load dump), this output will automatical-

ly shutdown. This protects the internal circuitry and

enables the IC to survive higher voltage transients than

would otherwise be expected. Thermal shutdown is effec-

tive against die overheating since the high current output

is the dominant source of power dissipation in the IC.

The

function has the ability to serve a dual purpose

if desired. When controlled in the manner shown in the

test circuit (common in automotive systems where

/ENABLE is connected to the ignition switch), the

lead also serves as an output flag that is active low when-

ever a fault condition is detected with the high current

regulated output. Under normal operating conditions, the

output voltage of this lead is high (5V). This is set by an

internal clamp. If the high current output becomes unreg-

ulated for any reason (line transients, short circuit, thermal

shutdown, low input voltage, etc.) the lead switches to the

active low state, and is capable of sinking several mil-

liamps. This output signal can be used to initiate any reset

or start-up procedure that may be required of the system.

The

lead can also be driven directly from logic cir-

cuits. The only requirement is that the 20k½ pull-up resis-

tor remain in place. This will not affect the logic gate since

the voltage on this lead is limited by the internal clamp to

5V. The

signal is sacrificed in this arrangement

since the maximum sink capability of the lead in the active

low state (approximately 5mA), is usually not sufficient to

pull down the active high logic gate. The flag can be

retained if the driving gate is open collector logic.

Controlling ON/OFF Terminal with a typical CMOS or TTL Logic Gate

Reset Pulse on Power-Up (with approximately 300ms delay)

RESET/

ENABLE

Gnd

CS8135

R1

20k

W

R2

100k

W

4.7

mF

CMOS MM 74CO4

or Equivalent

Delayed

Reset

Out

CS8135

R1

20k

W

RESET/

ENABLE

RESET

RESET

RESET

RESET

RESET Function

High Current Output

2

10k

W

+

2

Circuit Description: continued