Data Sheet

LT83401/LT83402

analog.com

Rev. 0

30 of 44

the vast majority of applications. For more information on offset voltage, SET pin current, and output noise, see the

Electrical Characteristics table. Table 6 lists many common output voltages and their corresponding 1% RSET resistors.

Where the exact resistor value required for the output voltage is not available, two resistors can be paralleled to

achieve the desired value. For example, for a 0.8V output voltage, a resistor value of exactly 8kΩ is required. The

closest value with a single 1% resistor is 8.06kΩ, with two resistors, 8.25kΩ can be paralleled with 267kΩ to achieve

(almost) exactly 8kΩ. 0.1% resistors may be used to achieve higher accuracy.

Table 6. 1% Resistor for Common Output Voltages

VOUT(V)

RSET(kΩ)

0.8

8.06

1

10

1.8

18

2.5

24.9

3.3

33.2

5

49.9

6

60.4

9

90.9

12

120

The benefit of using a current reference compared with a voltage reference, as used in conventional regulators, is

that the regulator always operates in a unity gain configuration, independent of the programmed output voltage.

This allows the LT83401/LT83402 to have loop gain, frequency response, and bandwidth independent of the output

voltage. Moreover, since none of the error amp gain is needed to amplify the SET pin voltage to a higher output

voltage, output load regulation is more tightly specified.

Since the zero TC current source is highly accurate, the SET pin resistor can become the limiting factor in achieving

high accuracy. Hence, it should be a precision resistor. Additionally, any leakage paths to or from the SET pin create

errors in the output voltage. If necessary, use high-quality insulation (for example, Teflon, Kel-F); moreover, cleaning

of all insulating surfaces to remove fluxes and other residues may be required. High-humidity environments may

require a surface coating at the SET pin to provide a moisture barrier.

Since the SET pin is a high-impedance node, unwanted signals may couple into the SET pin and cause erratic

behavior. This is most noticeable when operating with a minimum output capacitor at heavy load currents.

Bypassing the SET pin with a small capacitance to GND resolves this issue—100nF is sufficient. This is the minimum

recommended capacitance. In general, a larger capacitance is typically preferred. For more information, see the SET

Pin Capacitor: Noise and Soft-Start section.

For applications requiring higher accuracy or an adjustable output voltage, the SET pin may be actively driven by an

external voltage source capable of sinking 100μA. Connecting a precision voltage reference to the SET pin eliminates

any errors present in the output voltage due to the reference current SET pin resistor tolerances.