8

LT1020

S

APPLICATI

I FOR ATIO

Like most other IC regulators, a minimum load is required

on the output of the LT1020 to maintain regulation. For

most standard regulators this is normally specified at

5mA. Of course, for a micropower regulator this would be

a tremendously large current. The output current must be

large enough to absorb all the leakage current of the pass

transistor at the maximum operating temperature. It also

affects the transient response; low output currents have

long recovery times from load transients. At high operat-

ing temperatures the minimum load current increases and

having too low of a load current may cause the output to

go unregulated. Devices are tested for minimum load

current at high temperature. The output voltage setting

resistors to the feedback terminal can usually be used to

provide the minimum load current.

Frequency Compensation

The LT1020 is frequency compensated by a dominant pole

on the output. An output capacitor of 10

µF is usually large

enough to provide good stability. Increasing the output

capacitor above 10

µF further improves stability. In order

to insure stability, a feedback capacitor is needed between

the output pin and the feedback pin. This is because stray

capacitance can form another pole with the large value of

feedback resistors used with the LT1020. Also, a feedback

capacitor minimizes noise pickup and improves ripple

rejection.

With the large dynamic operating range of the output

current, 10000:1, frequency response changes widely.

Low AC impedance capacitors are needed to insure stabil-

ity. While solid tantalum are best, aluminum electrolytics

can be used but larger capacitor values may be needed.

The CURRENT LIMIT pin allows one of the internal nodes

to be rolled off with a 0.05

µF capacitor to ground. With this

capacitor, lower values of regulator output capacitance

can be used (down to 1

µF) for low (<20mA) output

currents. Values of capacitance greater than 0.05

µF de-

grade the transient response, so are not recommended.

If the CURRENT LIMIT pin is connected to GND, the

current limit is decreased and only a 1

µF output capacitor

is needed.

When bypassing the reference, a 20

Ω resistor must be

connected in series with the capacitor.

other precautions are necessary and the reference is

stable with nominal stray capacitances.

Figure 2. Minimizing Oscillation in Thermal Shutdown

Overload Protection

The main regulator in the LT1020 is current limited at

approximately 350mA. The current limit is stable with both

input voltage and temperature. A current limit pin, when

strapped to ground, decreases the output current. This

allows the output current to be set to a lower value than

250mA. The output current available with the current limit

pin strapped to ground is not well controlled so if precise

current limiting is desired it should be provided externally

as is shown in some of the application circuits.

If the device is overloaded for long periods of time, thermal

shutdown turns the output off. In thermal shutdown, there

may be some oscillations which can disturb external

circuitry. A diode connected between the reference and

feedback terminal provides hysteresis under thermal shut-

down, so that the device turns on and off with about a 5

second period and there are no higher frequency oscilla-

tions. This is shown in Figure 2. This diode is recom-

mended for most applications. Thermal shutdown tem-

perature is set at approximately 145

°C.

Figure 1. Bypassing Reference

+

REF

+

REF

OUTPUT

OUTPUT

20

Ω

20

Ω

10

µF

10

µF

OR

1020 F01

4

4

+

10

µF

0.001

µF

*

2

11

4

FB

REF

LT1020

1020 F02

* DIODE ADDS FEEDBACK