LM5171 datasheet(11/19 Pages) HTC

11 / 19 page

1.5A, 280kHz, Boost Regulator

LM5171

Dec. 2010 - Rev. 1.2.1

- 11

HTC

COMPONENT SELECTION

Frequency Compensation

The goal of frequency compensation is to achieve desirable transient response and DC regulation while

ensuring the stability of the system. A typical compensation network, as shown in Figure 23, provides a

frequency response of two poles and one zero. This frequency response is further illustrated in the Bode plot

shown in Figure 24.

AGND

LM5171

Figure 23. A Typical Compensation Network

The high DC gain in Figure 24 is desirable for achieving DC accuracy over line and load variations. The DC

gain of a transconductance error amplifier can be calculated as follows:

Gain

where:

GM = error amplifier transconductance;

RO = error amplifier output resistance ≈ 1 MΩ.

The low frequency pole, fP1, is determined by the error amplifier output resistance and C1 as:

C1R

The first zero generated by C1 and R1 is:

C1R1

fZ1

The phase lead provided by this zero ensures that the loop has at least a 45° phase margin at the crossover

frequency. Therefore, this zero should be placed close to the pole generated in the power stage which can be

identified at frequency:

LOAD

where:

CO = equivalent output capacitance of the error amplifier≈120pF;

RLOAD= load resistance.

LM5171 Datasheet(PDF) 11 Page - HTC Korea TAEJIN Technology Co.