TCL1584

TCL1585

TCL1587

7A / 5A / 4.6A / 3A, FAST RESPONSE,

LOW DROPOUT POSITIVE LINEAR

VOLTAGE REGULATORS

PRELIMINARY INFORMATION

6

TCL1584/1585/1587-04 6/6/97

Figure 3. Overload Recovery

Current Limit Curve

Load Line

Start Up

Point

Unrecoverable

I-V Point

parameters will allow it. Typically, increasing the output

capacitance helps reduce the oscillation. NOTE: The

TCL1585 and TCL1584 have fixed current limit over the

entire voltage range and are not susceptible to this phenom-

enon.

Protection Diodes

The TCL158x family of devices do not normally require

any external current limiting circuitry such as protection

diodes, frequently used with older LDO regulators. A diode

is internally present between the output and input which is

capable of handling short-duration surge currents of up to

100A. This capability typically ensures safe operation ex-

cept for the case where output capacitance is exceedingly

large (>1000

µF) and the input is suddenly shorted to ground.

This situation can produce excessive reverse current in the

device - enough to cause damage. An external high current

diode should be used as shown in Figure 2.

The ADJ pin does not normally need protection diodes

either. It can handle

±7V without any performance degrada-

tion or device damage. Current at this pin is internally limited

by a series resistor so the bypass capacitors do not present

any danger. Of course, exceeding 7V differential between

any two pins will cause catastrophic junction breakdowns

and possible damage to the device.

Overload Recovery

The built-in Short Circuit and Safe-Operating-Area (SOA)

protection function of the TCL158x family can cause sec-

ondary effects which must be considered for robust system

design. The behavior of the regulator under heavy loads

(short circuit) at start-up is such that the output voltage will

remain low while sourcing maximum current until the load is

removed or reduced. Normally, the output voltage will rise as

the load is reduced and trace a line I-V relationship accord-

ing to the SOA limit. If the load line intersects this output

curve at two points the output voltage may not recover from

the heavy load/short-circuit condition. This condition is illus-

trated in Figure 3. The current limit constraint does not allow

any load point above it and the load line is defined absolutely

by the I-V characteristics of the load (a resistor, in this case).

Under these conditions it may be necessary to cycle the

power supply off and then on again. This phenomenon is

common for LDO’s with fold-back current limiting schemes.

NOTE: Overload recovery is

always guaranteed on the

TCL1585 and TCL1587 because of the constant current

limit characteristic.

Ripple Rejection

A typical ripple rejection curve for the fixed output

voltage devices is shown in Figure 3. It is possible to obtain

improved performance in ripple rejection by using the ad-

shown in Figure 2. This capacitor should be chosen to have

a value such that its impedance at the ripple frequency is

less than R1 (see Figure 2.). Usually this is on the order of

100

bypass capacitor should be chosen to be 22

µF or greater. If

these conditions are met the ripple rejection will be improved

of the fixed output devices.

+

+

VOUT

C1

10

µF

C2

22

µF

OUT

IN

TCL1584-3.3

D1

1N4002

(OPTIONAL)

GND

VIN

+

+

+

VOUT

R1

R2

C1

10

µF

C2

22

µF

C

ADJ

OUT

IN

TCL1584

D1

1N4002

(OPTIONAL)

ADJ

VIN