TPS54521 datasheet(20/35 Pages) TI

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VOUT

RESR

R L

gm ps

VOUT

RESR

gm ps

Adc

1 +

VOUT

= Adc

1 +

´ ¦

Adc = gm

p =

ESR

z =

TPS54521

SLVS981 – JUNE 2010

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modulator) supplying current to the output capacitor and load resistor. The control to output transfer function is

shown in Equation 10 and consists of a dc gain, one dominant pole and one ESR zero. The quotient of the

change in switch current and the change in COMP pin voltage (node c in Figure 30) is the power stage

transconductance (gmps) which is 12 A/V for the device. The DC gain of the power stage is the product of gmps

and the load resistance (RL), as shown in Equation 11 with resistive loads. As the load current increases, the DC

gain decreases. This variation with load may seem problematic at first glance, but fortunately the dominant pole

moves with load current (see Equation 12). The combined effect is highlighted by the dashed line in Figure 32.

As the load current decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover

frequency the same for the varying load conditions which makes it easier to design the frequency compensation.

Figure 31. Simplified Small Signal Model for Peak Current Mode Control

Figure 32. Simplified Frequency Response for Peak Current Mode Control

(10)

(11)

(12)

(13)

Where

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TPS54521 Datasheet(PDF) 20 Page - Texas Instruments