Datasheet

1/f

t – Time

LL(PEAK)

Ramp Signal20% of I

OCL

LL(PEAK)

Light Load

7% of I

OCL

LL(peak)

– Inductor Current Limit – A

TPS51220A

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SLUS897E –DECEMBER 2008–REVISED JANUARY 2013

Figure 43. Inductor Current Limit at Pulse Skipping

Table 2. Skip Mode Selection

SKIPSELx GND VREF2 VREG3 VREG5

OOA Skip (maximum 7 OOA Skip (maximum 15 skips, for

OPERATING MODE Continuous Conduction Auto Skip

skips, for <400 kHz) equal to or greater than 400kHz)

OUT OF AUDIO SKIP OPERATION

Out-Of-Audio™ (OOA) light-load mode is a unique control feature that keeps the switching frequency above

acoustic audible frequencies toward virtually no load condition while maintaining state-of-the-art high conversion

efficiency. When OOA is selected, the switching frequency is kept higher than audible frequency range in any

load condition. The TPS51220A automatically reduced switching frequency at light-load conditions. The OOA

control circuit monitors the states of both MOSFETs and forces an ON state if the predetermined number of

pulses are skipped. The high-side MOSFET is turned on before the output voltage declines down to the target

value, so that eventually an overvoltage condition is caused. The OOA control circuit detects this overvoltage

condition and begins modulating the skip-mode on time to keep the output voltage.

The TPS51220A supports a wide-switching frequency range, therefore, the OOA skip mode has two selections.

See Table 2. When the 300-kHz switching frequency is selected, a maximum of seven (7) skips (SKIPSEL=3.3

V) makes the lowest frequency at 37.5 kHz. If a 15-skip maximum is chosen, it becomes 18.8 kHz, hence the

maximum 7 skip is suitable for less than 400 kHz, and the maximum 15 skip is 400 kHz or greater.

99% DUTY CYCLE OPERATION

In a low-dropout condition such as 5-V input to 5-V output, the basic control loop attempts to maintain 100% of

the high-side MOSFET ON. However, with the N-channel MOSFET used for the top switch, it is not possible to

use the 100% on-cycle to charge the boot strap capacitor. When high duty is required, the TPS51220A extends

the ON period (by skipping a maximum of three clock cycles and reducing the switching frequency to 25% of the

steady state value) and asserts the OFF state after extended ON.

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