TPS51221 datasheet(21/36 Pages) TI

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RF-Resistance-kW

LIGHT LOAD OPERATION

LL(DC) + ILL(PEAK) * 0.5

IND(RIPPLE)

(2)

IND(RIPPLE) +

IN * VOUT

OUT

(3)

TPS51221

SLVS786 – NOVEMBER 2007

Figure 41. Switching Frequency vs RF

The TPS51221 automatically reduces switching frequency at light load condition to maintain high efficiency if

Auto Skip or OOA 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 touches a predetermined current, ILL(PEAK), which indicates the boundary between

a heavy load conduction and a light load condition. Once the top MOSFET is turned on, the TPS51221 does not

allow turning it off until it touches ILL(PEAK). This eventually causes an over-voltage 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

which starts from 25% of the over-current limit setting (IOCL(PEAK): see the CURRENT PROTECTION section)

toward 5% of IOCL(PEAK), over one switching cycle to prevent causing a large ripple. The transition load point to

the light load operation ILL(DC) can be calculated as follows;

where fSW is the PWM switching frequency as determined by RF resistor setting or external clock. Switching

frequency versus output current in the light load condition is a function of L, f, Vin and Vout; but it decreases

almost proportional to the output current from the ILL(DC) given above however, as the switching is synchronized

with clock. Due to the synchronization, the switching waveform in boundary load condition (close to ILL(DC)),

appears as a sub-harmonic oscillation; however, it is the intended operation.

If SKIPSELx is tied to GND, TPS51221 works at the constant frequency of fSW, regardless of its load current.

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