PWM FREQUENCY CONTROL

5

1 × 10

kHz

=

sw

RF k

f

é

ù

ë

û

é

ù

ë

û

(1)

RF-Resistance-kW

0

200

300

400

500

600

700

800

900

1000

100

200

300

400

500

100

LIGHT LOAD OPERATION

I

LL(DC) + ILL(PEAK) * 0.5

I

IND(RIPPLE)

(2)

IN

OUT

OUT

IND(RIPPLE)

SW

IN

(V

- V

) × V

1

I

=

×

L ×

V

f

(3)

TPS51222

www.ti.com............................................................................................................................................................................................... SLUS908 – JANUARY 2009

The TPS51222 has a fixed frequency control scheme with 180° phase shift. The switching frequency can be

determined by an external resistor which is connected between RF pin and GND, and can be calculated using

Equation 1.

TPS51222 can also synchronize to more than 2.5 V amplitude external clock by applying the signal to the RF

pin. The set timing of channel 1 initiates at the raising edge (1.3 V typ) of the clock and channel 2 initiates at the

falling edge (1.1 V typ). Therefore, the 50% duty signal makes both channels 180° phase shift.

Figure 39. Switching Frequency vs RF

The TPS51222 automatically reduces switching frequency at light load conditions to maintain high efficiency if

Auto Skip or Out-of-Audio™ mode is selected by SKIPSELx. This reduction of frequency is achieved by skipping

pulses. As the output current decreases from heavy load condition, the inductor current is also reduced and

eventually comes to the point that its peak reaches a predetermined current, ILL(PEAK), which indicates the

boundary between heavy-load condditions and light-load conditions. Once the top MOSFET is turned on, the

TPS51222 does not allow it to be turned off until it reaches ILL(PEAK). This eventually causes an overvoltage

condition to the output and pulse skipping. From the next pulse after zero-crossing is detected, ILL(PEAK) is limited

by the ramp-down signal ILL(PEAK)RAMP, which starts from 25% of the overcurrent limit setting (IOCL(PEAK): (see the

Current Protection section) toward 5% of IOCL(PEAK) over one switching cycle to prevent causing large ripple. The

transition load point to the light load operation ILL(DC) can be calculated in Equation 2.

where

• f

SW is the PWM switching frequency which is determined by RF resistor setting or external clock

Copyright © 2009, Texas Instruments Incorporated

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