TPS54821 datasheet(19/42 Pages) TI2

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ESR

z =

p =

Adc = gm

VOUT

= Adc

´ ¦

VOUT

RESR

gm ps

Adc

VOUT

RESR

R L

gm ps

TPS54821

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SLVSB14B – OCTOBER 2011 – REVISED FEBRUARY 2016

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Device Functional Modes (continued)

and the load resistance, R L, 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 27.

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 26. Simplified Small Signal Model for Peak Current Mode Control

Figure 27. Simplified Frequency Response for Peak Current Mode Control

(10)

(11)

(12)

(13)

Where

gmea is the GM amplifier gain ( 1300μA/V)

gmps is the power stage gain (21A/V).

RL is the load resistance

CO is the output capacitance.

RESR is the equivalent series resistance of the output capacitor.

TPS54821 Datasheet(PDF) 19 Page - Texas Instruments