IRF7821 datasheet(19/33 Pages) TI

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Loop Compensation

ESR

Control

logic

Driver

VIN

VFB

DRVH

DRVL

PWM

Switching Modulator

Voltage Divider

Output Capacitor

fSW

ESR C

p ´

(1)

Ramp Signal

TPS51123

www.ti.com ........................................................................................................................................................................................... SLUS890 – DECEMBER 2008

From small-signal loop analysis, a buck converter using D-CAPTM mode can be simplified as below.

Figure 36. Simplifying the Modulator

The output voltage is compared with internal reference voltage after divider resistors, R1 and R2. The PWM

comparator determines the timing to turn on high-side MOSFET. The gain and speed of the comparator is high

enough to keep the voltage at the beginning of each on cycle substantially constant. For the loop stability, the

0dB frequency, f0, defined below need to be lower than 1/4 of the switching frequency.

As f0 is determined solely by the output capacitor's characteristics, loop stability of D-CAP

TM mode is determined

by the capacitor's chemistry. For example, specialty polymer capacitors (SP-CAP) have Co in the order of

several 100

µF and ESR in range of 10 mΩ. These make f

0 on the order of 100 kHz or less and the loop will be

stable. However, ceramic capacitors have f0 at more than 700 kHz, which is not suitable for this operational

mode.

The TPS51123 adds a ramp signal to the 2-V reference in order to improve its jitter performance. As described in

the previous section, the feedback voltage is compared with the reference information to keep the output voltage

in regulation. By adding a small ramp signal to the reference, the S/N ratio at the onset of a new switching cycle

is improved. Therefore the operation becomes less jitter and stable. The ramp signal is controlled to start with

-20mV at the beginning of ON-cycle and to become 0 mV at the end of OFF-cycle in steady state. By using this

scheme, the TPS51123 improve jitter performance without sacrificing the reference accuracy.

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