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APPLICATION INFORMATION

Loop Compensation and External Parts Selection

Current Mode Operation

H

H

(6)

R2

(R2 ) R1)

(7)

R

C

C

R

C

1 ) s

C

C

R

O

1 ) s

C

C2

R

C

(8)

H

(1 ) s

C

O

ESR)

1 ) s

C

O

RL

RL

R

S

(9)

TPS51116

SLUS609A – MAY 2004 – REVISED JUNE 2004

A buck converter using TPS51116 current mode operation can be partitioned into three portions, a voltage

divider, an error amplifier and a switching modulator. By linearizing the switching modulator, we can derive the

transfer function of the whole system. Since current mode scheme directly controls the inductor current, the

modulator can be linearized as shown in Figure 2.

Figure 2. Linearizing the Modulator

Here, the inductor is located inside the local feedback loop and its inductance does not appear in the small signal

model. As a result, a modulated current source including the power inductor can be modeled as a current source

is applicable in the frequency space up to approximately a half of the switching frequency. One note is, although

the inductance has no influence to small signal model, it has influence to the large signal model as it limits slew

rate of the current source. This means the buck converter’s load transient response, one of the large signal

behaviors, can be improved by using smaller inductance without affecting the loop stability.

Total open loop transfer function of the whole system is given by Equation 6.

Equation 7.

There are three poles and two zeros in H(s). Each pole and zero is given by the following five equations.

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