OUT OF AUDIO SKIP OPERATION

99% DUTY CYCLE OPERATION

HIGH-SIDE DRIVER

LOW-SIDE DRIVER

CURRENT SENSING SCHEME

TPS51222

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

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 TPS51222 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 TPS51222 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.

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. TPS51222 detects the 100% ON condition and

asserts the OFF state at the appropriate time.

The high-side driver is designed to drive high current, low RDS(on) N-channel MOSFET(s). The drive capability is

represented by its internal resistance, which is 1.7

Ω for VBSTx to DRVHx, and 1Ω for DRVHx to SWx. When

configured as a floating driver, 5 V of bias voltage is delivered from VREG5 supply. The instantaneous drive

current is supplied by the flying capacitor between VBSTx and SWx pins. The average drive current is equal to

the gate charge at Vgs = 5V times switching frequency. This gate drive current as well as the low-side gate drive

current times 5 V makes the driving power which needs to be dissipated mainly from TPS51222 package. A

dead time to prevent shoot through is internally generated between high-side MOSFET off to low-side MOSFET

on, and low-side MOSFET off to high-side MOSFET on.

The low-side driver is designed to drive high-current low-RDS(on) N-channel MOSFET(s). The drive capability is

represented by its internal resistance, which are 1.3

Ω for VREG5 to DRVLx and 0.7Ω for DRVLx to GND. The

5-V bias voltage is delivered from VREG5 supply. The instantaneous drive current is supplied by an input

capacitor connected between VREG5 and GND. The average drive current is also calculated by the gate charge

at Vgs = 5 V times switching frequency.

In order to provide both good accuracy and cost effective solution, the TPS51222 supports external resistor

sensing and inductor DCR sensing. An RC network with high quality X5R or X7R ceramic capacitor should be

used to extract voltage drop across DCR. 0.1

µF is a good value to start the design. CSPx and CSNx should be

connected to positive and negative terminal of the sensing device respectively. The output signal of the internal

current amplifier becomes 100 mV at the OCL setting point. This means that the current sensing amplifier

normalize the current information signal based on the OCL setting. Attaching a RC network recommended even

with a resistor sensing scheme to get an accurate current sensing; see the External Components Selection

session for detailed configurations.

Copyright © 2009, Texas Instruments Incorporated

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