TPS54239E Datasheet, PDF (7/21 Pages) Texas Instruments – 4.5V to 23V Input, 2-A Synchronous Step-Down SWIFT Converter With Eco-mode

TPS54239E
www.ti.com
SLVSBT1 – JANUARY 2013
Current Protection
The output overcurrent 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_IN,
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 Iout. The TPS54239E 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 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 VFB voltage becomes lower than 65% of the
target voltage, the UVP comparator detects it. After 6µs detecting the UVP voltage, device will shut down and re-
start after 7 times SS period for Hiccup.
When the over current condition is removed, the output voltage returns to the regulated value. This protection is
non-latching.
Over Voltage Protection
TPS54239E detects over voltage conditions by monitoring the feedback voltage (VFB). This function is enabled
after approximately 1.7 x times the soft start time.
When the feedback voltage becomes higher than 125% of the target voltage, the OVP comparator output goes
high and both the high-side MOSFET driver and the low-side MOSFET driver turn off. This function is non-latch
operation.
UVLO Protection
Undervoltage lock out protection (UVLO) monitors the voltage of the V_REG5 pin. When the V_REG5 voltage is lower
than UVLO threshold voltage, the TPS54239E is shut off. This protection is non-latching.
Thermal Shutdown
TPS54239E monitors the temperature of itself. If the temperature exceeds the threshold value (typically 165°C),
the device is shut off. This is non-latch protection.
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