3

LT1029/LT1029A

1029afa

Reverse Characteristics

APPLICATIO S I FOR ATIO

Output Trimming

Output voltage trimming on the LT1029 is nominally

accomplished with a potentiometer connected from

output to ground with the wiper tied to the trim pin. The

LT1029 was made compatible with existing references, so

the trim range is large; 5%, – 13%. This large trim range

makes precision trimming rather difficult. One solution is

to insert resistors in series with both ends of the

potentiometer. This has the disadvantage of potentially

poor tracking between the fixed resistors and the

potentiometer. A second method of reducing trim range is

to insert a resistor in series with the wiper of the

potentiometer. This works well only for a very small trim

range because of the mismatch in TCs between the series

resistor and the internal thin film resistors. These film

resistors can have a TC as high as 500ppm/°C. That same

TC is then transferred to the change in output voltage;

a 1% shift in output voltage causes a (500ppm) (1%) =

5ppm/°C change in output voltage drift. The worst case

error in initial output voltage for the LT1029A is 0.2% and

the LT1029 is 1%, so a series resistor is satisfactory if the

output is simply trimmed to nominal value. 1ppm/°C TC

shift would be the maximum expected for the LT1029A

and 5ppm/°C for the LT1029.

Shunt Capacitance

The LT1029 is stable with all values of shunt capacitance,

but values between 300pF and 0.01µF are not recom-

mended because they cause longer settling following a

TYPICAL PERFOR A CE CHARACTERISTICS

OUTPUT TO GROUND VOLTAGE (V)

0

0

0.1

0.3

0.4

0.5

1.0

0.7

2

4

5

0.2

0.8

0.9

0.6

13

6

7

8

LT1029 • TPC01

Wide Trim Range (5%, – 13%)

Narrow Trim Range

transient in operating current. A 1µF solid tantalum

capacitor is suggested for most situations where

bypassing is desirable.

Trimming Output to 5.120V

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

25k

I ≥ 900µA

LT1029 • AI01

TRIM RANGE

0.4% – LT1029A, R1 = 750k

1.2% – LT1029, R1 = 250k

LT1029 • TA02

100k

I ≥ 800µA

R1*

R1

41.2k

R2

5k

R3

4.02k

I ≥ 800µA

LT1029 • AI03