TPS54226

SLVSA14E

– OCTOBER 2009 – REVISED JULY 2011

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

Over/Under Voltage Protection

The TPS54226 detects over and undervoltage conditions by monitoring the feedback voltage (VFB). This

function is enabled after approximately 1.7 times the soft-start time. 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.

UVLO Protection

than UVLO threshold voltage, the TPS54226 is shut off. This is protection is non-latching.

Thermal Shutdown

Thermal protection is self-activating. If the junction temperature exceeds the threshold value (typically 150

°C),

the TPS54226 shuts off. This protection is non-latching.

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© 2009–2011, Texas Instruments Incorporated

Product Folder Link(s): TPS54226