8 clock cycles

10ms

Vout

Current limit threshold

TPS65281, TPS65281-1

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SLVSBH7B – JULY 2012 – REVISED DECEMBER 2012

Internal V7V Regulator

The TPS65281/TPS65281-1 features an internal P-channel low dropout linear regulator (LDO) that supplies

power at the V7V pin from the VIN supply. V7V powers the gate drivers and much of the TPS65281/

TPS65281-1’s internal circuitry. The LDO regulates V7V to 6.3 V of over drive voltage on the power MOSFET for

the best efficiency performance. The LDO can supply a peak current of 50 mA and must be bypassed to ground

with a minimum 1-µF ceramic capacitor. The capacitor placed directly adjacent to the V7V and PGND pins is

highly recommended to supply the high transient currents required by the MOSFET gate drivers.

Short Circuit Protection

During the PWM on-time, the current through the internal high side switching MOSFET is sampled. The sampled

current is compared to a nominal 5-A over-current limit. If the sampled current exceeds the over-current limit

reference level, an internal over-current fault counter is set to 1 and an internal flag is set. Both internal high side

and low side power MOSFETs are immediately turned off and will not be turned on again until the next switching

cycle. If the over-current condition persists for eight sequential clock cycles, the over-current fault counter

overflows indicating an over-current fault condition exists. The buck regulator is shut down and stays turned off

for 10 ms. If the over-current condition clears prior to the counter reaching eight consecutive cycles, the internal

flag and counter are reset. The protection circuitry attempts to recover from the over-current condition after

10-ms power down time. The internal over-current flag and counter are reset. A normal soft-start cycle is

attempted and normal operation continues if the over-current fault condition has cleared. If the over-current fault

counter overflows during soft-start, the converter shuts down and this hiccup mode operation repeats.

Figure 30. DC/DC Over-Current Protection

Inductor Selection

The higher operating frequency allows the use of smaller inductor and capacitor values. A higher frequency

generally results in lower efficiency because of MOSFET gate charge losses. In addition to this basic trade-off,

the effect of the inductor value on ripple current and low current operation must also be considered. The ripple

inductances, but results in higher output voltage ripple and greater core losses.

Use Equation 7 to calculate the value of the output inductor. LIR is a coefficient that represents inductor peak-to-

peak ripple to DC load current. It is suggested to use 0.1 ~ 0.3 for most LIR applications.

Copyright © 2012, Texas Instruments Incorporated

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