(

)

1

C11

2

R8 fc

=

×

×

×

p

Time = 2 msec / div

Vin = 10 V / div

EN = 2 V / div

Vout = 2 V / div

SS/TR = 1 V / div

Time = 20

sec / div

Vout = 100 mV / div (AC coupled)

Iout = 2 A / div (1 A to 4 A load step)

Vin = 12 V

Time = 2 msec / div

Vin = 10 V / div

EN = 2 V / div

Vout = 2 V / div

SS/TR = 1 V / div

Time = 2 msec / div

Vin = 10 v / div

SS/TR = 1 V / div

EN = 2 V / div

Vout = 2 V / div

TPS54521

www.ti.com

SLVS981 – JUNE 2010

In order to provide a zero around the crossover frequency to boost the phase at crossover, a capacitor (C11) is

added in parallel to R8. The value of this capacitor is given by Equation 40.

(40)

For this design, the closest standard value is 47 pF. Increasing the value of C11 decreases the frequency of the

added zero, which increases the bandwidth and phase of the control loop. While 47 pF produces a stable design,

empirical measurements of the control loop and transient response show that 100 pF gives better performance

and is an optimal value for this design. A 100 pF capacitor is used for C11.

Application Curves from the Design Example

STARTUP with VIN

LOAD TRANSIENT

(0.66

Ω Load)

Figure 35.

Figure 36.

STARTUP with VIN

STARTUP with EN

(No Load)

(0.66

Ω Load)

Figure 37.

Figure 38.

Copyright © 2010, Texas Instruments Incorporated

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