Datasheet

TPS54325

SLVS932D –MAY 2009–REVISED AUGUST 2012

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Output Discharge Control

The TPS54325 discharges the output when EN is low, or the controller is turned off by the protection functions

(OVP, UVP, UVLO and thermal shutdown). The device discharges outputs using an internal 50-Ω MOSFET

which is connected to VO and PGND. The internal low-side MOSFET is not turned on during the output

discharge operation to avoid the possibility of causing negative voltage at the output, except for OVP protection.

When OVP is activated, both the 50-Ω MOSFET and the internal low side MOSFET are turned on.

Current Protection

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

switch current is monitored by measuring the low-side FET switch voltage between the SW pin and GND. This

voltage is proportional to the switch current. To improve 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 V

OUT

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

decreases linearly. The average value of the switch current is the load current I

OUT

. If the measured voltage is

above the voltage proportional to the current limit, Then the device constantly monitors the low-side FET switch

voltage, which is proportional to the switch current, during the low-side on-time.

The converter maintains the low-side switch on until the measured voltage is below the voltage corresponding to

the current limit at which time the switching cycle is terminated and a new switching cycle begins. In subsequent

switching cycles, the on-time is set to a fixed value and the current is monitored in the same manner.

There are some important considerations for this type of over-current protection. The load current 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. This may cause the output under-voltage protection circuit to be activated. When the over current

condition is removed, the output voltage will return to the regulated value. This protection is non-latching.

Over/Under Voltage Protection

The TPS54325 monitors a resistor divided feedback voltage to detect over and under voltage. When the

feedback voltage becomes higher than 120% of the target voltage, the OVP comparator output goes high and

the circuit latches the high-side MOSFET driver turns off and the low-side MOSFET turns on.

When the feedback voltage becomes lower than 70% of the target voltage, the UVP comparator output goes

high and an internal UVP delay counter begins. After 250 μs, the device latches off both internal top and bottom

MOSFET. This function is enabled approximately 1.7 x soft-start time.

UVLO Protection

The TPS54325 has under voltage lock out protection (UVLO) that monitors the voltage of V

REG5

pin. When the

REG5

voltage is lower than UVLO threshold voltage, the TPS54325 is shut off. This is non-latch protection.

Thermal Shutdown

The TPS54325 monitors the temperature of itself. If the temperature exceeds the threshold value (typically

150°C), the device is shut off. This is non-latch protection.

Product Folder Links: TPS54325