LM3420 datasheet(11/17 Pages) NSC

11 / 17 page

Compensation (Continued)

portant when analyzing closed loop stability. The minimum

and maximum room temperature values of this resistor are

specified in the Electrical Characteristics section of this data

sheet, and a curve showing the temperature coefficient is

shown in the curves section. Minimum values of R

f result in

lower phase margins.

Test Circuit

The test circuit shown in

Figure 2 can be used to measure

and verify various LM3420 parameters. Test conditions are

set by forcing the appropriate voltage at the V

OUT Set test

point and selecting the appropriate R

L or IOUT as specified in

the Electrical Characteristics section. Use a DVM at the

“measure” test points to read the data.

REG External Voltage Trim

The regulation voltage (V

REG) of the LM3420 can be exter-

nally trimmed by adding a single resistor from the COMP pin

to the +IN pin or from the COMP pin to the GND pin, depend-

ing on the desired trim direction. Trim adjustments up to

±10% of V

REG can be realized, with only a small increase in

the temperature coefficient. (See temperature coefficient

curve shown in

Figure 3 below.)

Formulas for selecting trim resistor values are shown below,

based on the percent of increase (%incr) or percent of de-

crease (%decr) of the output voltage from the nominal volt-

age.

For LM3420-4.2

increase = 22x10

5/%incr

decrease = (53x10

5/%decr) − 75x103

For LM3420-8.2

DS012359-7

FIGURE 2. LM3420 Test Circuit

DS012359-8

Normalized Temperature Drift with

Output Externally Trimmed

DS012359-9

Increasing V

REG

DS012359-10

Decreasing V

REG

FIGURE 4. Changing V

REG

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