Datasheet

LM3420
SNVS116D –MAY 1998–REVISED MAY 2013 www.ti.com
where
• R
f
≈ 75 kΩ for the 4.2V part
• R
f
≈ 181 kΩ for the 8.4V part
• R
f
≈ 287 kΩ for the 12.6V part
• R
f
≈ 392 kΩ for the 16.8V part (1)
The resistor (R
f
) in the formula is an internal resistor located on the die. Since this resistor value will affect the
phase margin, the worst case maximum and minimum values are important 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 27 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 I
OUT
as specified in the Electrical Characteristics section. Use a DVM at the “measure” test points to read the
data.
Figure 27. LM3420 Test Circuit
V
REG
External Voltage Trim
The regulation voltage (V
REG
) of the LM3420 can be externally trimmed by adding a single resistor from the
COMP pin to the +IN pin or from the COMP pin to the GND pin, depending 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 28 below.)
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