7.3.9 Low-side MOSFET Zero-Crossing

The device uses a zero-crossing (ZC) circuit to perform the zero inductor current detection during skip-mode

operation. The ZC threshold is set to a small negative value before the low-side MOSFET is turned off, entering

discontinuous conduction mode (DCM) operation. After entering DCM, the ZC threshold hysteresis increases the

threshold to a small positive value after entering DCM. As a result, the device delivers better light-load efficiency.

When the load current increases enough such that the device exits DCM, the ZC circuit must detect 16

consecutive cycles of negative inductor current below the ZC threshold before returning to DCM. Only one

cycle without ZC detection is required to exit DCM.

When the output is enabled, the ZC circuit is also enabled during the first 32 switching cycles while the device

is in soft-start. If the MSEL resistor value is for FCCM, ZC is disabled and the device transitions to FCCM when

soft-start is complete. See Adjustable Soft Start for description on soft-start completion. If there are not at least

32 switching cycles before soft-start is done, such as during start up with a high output pre-bias, the ZC is not

disabled until the first high-side MOSFET on-time after soft-start done is complete.

7.3.10 Current Sense and Positive Overcurrent Protection

For a buck converter, during the on-time of the high-side MOSFET, the switch current increases at a linear

rate determined by the input voltage, output voltage, on-time, and output inductor value. During the on-time of

the low-side MOSFET, the current decreases linearly. The average value of the switch current equals the load

current.

The output overcurrent limit (OCL) in the device is implemented using a cycle-by-cycle valley current detect

control circuit. The inductor current is monitored during the on-time of the low-side MOSFET by measuring the

low-side MOSFET drain-to-source current. If the measured drain-to-source current of the low-side MOSFET is

above the current limit threshold, the low-side MOSFET stays ON until the current level becomes lower than the

current limit threshold. This type of behavior reduces the average output current sourced by the device.

During an overcurrent condition, the current to the load exceeds the current to the output capacitors. Thus, the

output voltage tends to decrease. Eventually, when the output voltage falls below the undervoltage-protection

threshold (80%), the UVP comparator detects it and shuts down the device after a wait time of 70 µs. Depending

on the part number, the device will either hiccup or latch off, as described in Overvoltage and Undervoltage

Protection.

If an OCL condition happens during start-up, the device still has cycle-by-cycle current limit based on low-side

valley current. After soft start is finished, the UV event which is caused by the OCL event shuts down the device

after a wait time of 70 µs. Depending on the part number, the device either hiccups or latches off, as described in

Overvoltage and Undervoltage Protection.

The resistor, RILIM connected from the ILIM pin to AGND sets current limit threshold. TI recommends a

±1% tolerance resistor because a worse tolerance resistor provides less accurate OCL threshold. Equation

4 calculates the RILIM for a given overcurrent limit threshold on the device. Equation 5 calculates the overcurrent

limit threshold for a given RILIM value.

To protect the device from an unexpected connection to the ILIM pin, an internal fixed OCL clamp is

implemented. This internal OCL clamp limits the maximum valley current on the low-side MOSFET when the

ILIM pin has too small of a resistance to AGND, or is accidentally shorted to ground.

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TPS54KB20

SLVSGP3 – MAY 2023

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