TPS542951

SLVSBI4B – JULY 2012 – REVISED OCTOBER 2013

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Current Sensing and Over-Current Protection

The output over-current protection (OCP) is implemented using a cycle-by-cycle valley detection control circuit.

The switch current is monitored by measuring the low-side FET switch voltage between the SWx and PGNDx

pins. This voltage is proportional to the switch current and the on-resistance of the FET. To improve the

measurement accuracy, the voltage sensing is temperature compensated.

During the on-time of the high-side FET switch, the switch current increases at a linear rate determined by VINx,

VOx, the on-time and the output inductor value. During the on-time of the low-side FET switch, this current

low-side FET is above the voltage proportional to the current limit, the converter keeps the low-side switch on

until the measured voltage falls below the voltage corresponding to the current limit and a new switching cycle

begins. In subsequent switching cycles, the on-time is set to the value determined for CCM and the current is

monitored in the same manner.

Important considerations for this type of over-current protection: The load current is one half of the peak-to-peak

inductor current higher than the over-current threshold. Also when the current is being limited, the output voltage

tends to fall as the demanded load current may be higher than the current available from the converter. When

the over current condition is removed, the output voltage returns to the regulated value. This protection is non-

latching.

Undervoltage Protection and Hiccup Mode

Hiccup mode of operation protects the power supply from being damaged during an over-current fault condition.

If the OCL comparator circuit detects an over-current event the output voltage falls. When the feedback voltage

falls below 68% of the reference voltage, the UVP comparator output goes high and an internal UVP delay

counter begins counting. After counting UVP delay time, the TPS542951 shuts off the power supply for a given

time (7x UVP Enable Delay Time) and then tries to re-start the power supply. If the over-load condition has been

removed, the power supply starts and operates normally; otherwise, the TPS542951 detects another over-current

event and shuts off the power supply again, repeating the previous cycle. Excess heat due to overload lasts for

only a short duration in the hiccup cycle, therefore the junction temperature of the power device is much lower.

UVLO Protection

than the UVLO threshold, the TPS542951 shuts down. As soon as the voltage increases above the UVLO

threshold, the converter starts again.

Thermal Shutdown

TPS542951 monitors its temperature. If the temperature exceeds the threshold value (typically 155°C), the

device shuts down. When the temperature falls below the threshold, the IC starts again.

When VIN1 starts up and VREG5 output voltage is below its nominal value, the thermal shutdown threshold is

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