7

LT1120

APPLICATIO S I FOR ATIO

Reference

Internal to the LT1120 is a 2.5V trimmed class B output

reference. The reference was designed to be able to source

or sink current so it could be used in supply splitting

applications as well as a general purpose reference for

external circuitry. The design of the reference allows it to

source typically 4mA or 5mA and sink 2mA. The available

source and sink current decreases as temperature in-

creases. It is sometimes desirable to decrease the AC

output impedance by placing an output capacitor on them.

The reference in the LT1120 becomes unstable with large

capacitive loads placed directly on it. When using an

output capacitor, about 20

Ω should be used to isolate the

capacitor from the reference pin. This 20

Ω resistor can be

placed directly in series with the capacitor or alternatively

the reference line can have 20

Ω placed in series with it and

then a capacitor to ground. This is shown in Figure 1. Other

than placing large capacitive loads on the reference, no

other precautions are necessary and the reference is

stable with nominal stray capacitances.

Overload Protection

The main regulator in the LT1120 is current limited at

approximately 250mA. The current limit is stable with both

input voltage and temperature.

Like most other IC regulators, a minimum load is required

on the output of the LT1120 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 must be large

enough to absorb all the leakage current of the pass

transistor at the maximum operating temperature. It also

effects 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 LT1120 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 ensure 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 LT1120. 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.

+

REF

OUTPUT

OR

20

Ω

10

µF

1120 F01

6

+

REF

OUTPUT

20

Ω

10

µF

6

Figure 1. Bypassing Reference