Data Sheet

LT83401/LT83402

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Rev. 0

32 of 44

The LT83401/LT83402 is an externally compensated part, so even if the recommended layout is not followed

(sometimes it is not possible due to application-specific limitations), it is possible to choose a more conservative

compensation with a lower gain or bandwidth in order to retain stability during operation. However, this is at the

expense of a transient response. A superior layout allows a better tradeoff between transient response, phase

margin, and output noise performance when selecting compensation values.

Frequency Compensation

The loop compensation determines the stability and transient performance and is provided by the components

connected to the VC pin. Generally, a capacitor (CC) and a resistor (RC) in series to ground are used. Designing the

compensation network can be complicated, and the best values depend on the application. A practical approach is

to start with one of the circuits in this data sheet that is similar to the user’s application and tune the compensation

network to optimize the performance. LTpowerCAD simulation can help in this process. Stability should then be

checked across all operating conditions, including load current, input voltage, and temperature. The LT1375 data

sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a

transient load.

Figure 54 shows an equivalent circuit for the LT83401/LT83402 control loop. The error amplifier is a

transconductance amplifier with transconductance gm(EA)= 12mS, with finite output impedance. The power section,

consisting of the modulator, power switches, and inductor, is modeled as a transconductance amplifier with

transconductance GM = 2.3A/V (LT83401)/3.6A/V (LT83402) generating an output current proportional to the voltage

at the VC pin. Note that the output capacitor integrates this current and that the capacitor on the VC pin (CC) integrates

the error amplifier output current, resulting in two poles in the loop. A zero is required and comes from a resistor RC

in series with CC. This simple model works as long as the value of the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency. For more information about the compensation of switching

mode power supplies, refer to Application Note 149: Modeling and Loop Compensation Design of Switching Mode

Power Supplies.

Figure 54. Model for Loop Response

VC

220kΩ

OUTS

SET

CSET

LT83401/LT83402

COUT

RSET

OUTPUT

CF

CC

RC

GM

gm (EA)