TPS56921

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SLVSBL4 – OCTOBER 2012

To improve drop out, the device is designed to operate at 100% duty cycle as long as the BOOT to PH pin

voltage is greater than the BOOT-PH UVLO threshold which is typically 2.1V. When the voltage between BOOT

and PH drops below the BOOT-PH UVLO threshold the high-side MOSFET is turned off and the low-side

MOSFET is turned on allowing the boot capacitor to be recharged. In applications with split input voltage rails

100% duty cycle operation can be achieved as long as (VIN – PVIN) > 4V.

Output Overvoltage Protection (OVP)

The device incorporates an output overvoltage protection (OVP) circuit to minimize output voltage overshoot. For

example, when the power supply output is overloaded the error amplifier compares the actual output voltage to

the internal reference voltage. If the VSENSE pin voltage is lower than the internal reference voltage for a

considerable time, the output of the error amplifier demands maximum output current. Once the condition is

removed, the regulator output rises and the error amplifier output transitions to the steady state voltage. In some

applications with small output capacitance, the power supply output voltage can respond faster than the error

amplifier. This leads to the possibility of an output overshoot. The OVP feature minimizes the overshoot by

comparing the VSENSE pin voltage to the OVP threshold. If the VSENSE pin voltage is greater than the OVP

threshold the high-side MOSFET is turned off preventing current from flowing to the output and minimizing output

overshoot. When the VSENSE voltage drops lower than the OVP threshold, the high-side MOSFET is allowed to

turn on at the next clock cycle.

Overcurrent Protection

The device is protected from overcurrent conditions by cycle-by-cycle current limiting on both the high-side

MOSFET and the low-side MOSFET.

High-side MOSFET overcurrent protection

The device implements current mode control which uses the COMP pin voltage to control the turn off of the high-

side MOSFET and the turn on of the low-side MOSFET on a cycle by cycle basis. Each cycle the switch current

and the current reference generated by the COMP pin voltage are compared, when the peak switch current

intersects the current reference the high-side switch is turned off.

Low-side MOSFET overcurrent protection

While the low-side MOSFET is turned on its conduction current is monitored by the internal circuitry. During

normal operation the low-side MOSFET sources current to the load. At the end of every clock cycle, the low-side

MOSFET sourcing current is compared to the internally set low-side sourcing current limit. If the low-side

sourcing current is exceeded the high-side MOSFET is not turned on and the low-side MOSFET stays on for the

next cycle. The high-side MOSFET is turned on again when the low-side current is below the low-side sourcing

current limit at the start of a cycle.

The low-side MOSFET may also sink current from the load. If the low-side sinking current limit is exceeded the

low-side MOSFET is turned off immediately for the rest of that clock cycle. In this scenario both MOSFETs are

off until the start of the next cycle.

Furthermore, if an output overload condition (as measured by the COMP pin voltage) has lasted for more than

the hiccup wait time which is programmed for 512 switching cycles, the device will shut down itself and restart

after the hiccup time of 16384 cycles. The hiccup mode helps to reduce the device power dissipation under

severe overcurrent conditions.

Overheat Protection

The internal thermal shutdown circuitry forces the device to stop switching if the junction temperature exceeds

170°C typically. Once the junction temperature drops below 160°C typically, the internal thermal hiccup timer will

start to count. The device reinitiates the power up sequence after the built-in thermal shutdown hiccup time

(32768 cycles) is over.

Copyright © 2012, Texas Instruments Incorporated

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