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T Junction Temperature - °C

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TPS54428

SLVSB42A

– NOVEMBER 2011 – REVISED JANUARY 2012

www.ti.com

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 Vin,

Vout, 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 Iout. The TPS54428 constantly

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

If the measured voltage is above the voltage proportional to the current limit, an internal counter is incremented

per each SW cycle and 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. If the over current condition exists for 7 consecutive switching cycles, the internal OCL

threshold is set to a lower level, reducing the available output current. When a switching cycle occurs where the

switch current is not above the lower OCL threshold, the counter is reset and the OCL limit is returned to the

higher value.

There are some important considerations for this type of over-current protection. The load current one half of the

peak-to-peak inductor current is 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 voltage to fall. When the over current condition is removed, the output

voltage returns to the regulated value. This protection is non-latching.

UVLO Protection

Undervoltage lock out protection (UVLO) monitors the voltage of the VREG5 pin. When the VREG5 voltage is

lower than UVLO threshold voltage, the TPS54428 is shut off. This protection is non-latching.

Thermal Shutdown

TPS54428 monitors the temperature of itself. If the temperature exceeds the threshold value (typically 170

°C),

the device is shut off. This is non-latch protection.

TYPICAL CHARACTERISTICS

VIN = 12 V, TA = 25

°C (unless otherwise noted)

Figure 1. VIN CURRENT vs JUNCTION TEMPERATURE

Figure 2. VIN SHUTDOWN CURRENT vs JUNCTION

TEMPERATURE

8

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