the current level becomes OCL level or lower. 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 overcurrent protection. The load current is higher

than the overcurrent threshold by one half of the peak-to-peak inductor ripple current. Also, when the current

is being limited, the output voltage tends to fall as the demanded load current can be higher than the current

available from the converter, which can cause the output voltage to fall. When the FB voltage falls below the

UVP threshold voltage, the UVP comparator detects it and the device shuts down after the UVP delay time

(typically 256 µs) and restarts after the hiccup wait time (typically 14 ms).

When the overcurrent condition is removed, the output voltage returns to the regulated value.

The TPS563257 is a FCCM mode part. In this mode, the device has negative inductor current at light loading.

The device has NOC (negative overcurrent) protection to avoid too large negative current. NOC protection

detects the valley of inductor current. When the valley value of inductor current exceeds the NOC threshold, the

IC turns off the low side then turns on the high side. When the NOC condition is removed, the device returns to

normal switching.

Because the TPS563257 is a FCCM mode port, if the inductance is so small that the device trigger NOC, it

causes the output voltage to be higher than target value. The minimum inductance is identified as Equation 2.

(2)

7.3.7 Undervoltage Lockout (UVLO) Protection

UVLO protection monitors the internal regulator voltage. When the voltage is lower than UVLO threshold voltage,

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

7.3.8 Thermal Shutdown

The device monitors the temperature of itself. If the temperature exceeds the threshold value, the device is shut

off. This is a non-latch protection.

7.4 Device Functional Modes

7.4.1 Eco-mode Operation

The TPS563252 operates in Eco-mode, which maintains high efficiency at light loading. As the output current

decreases from heavy load conditions, the inductor current is also reduced and eventually comes to a point

where the rippled valley touches zero level, which is the boundary between continuous conduction and

discontinuous conduction mode. The rectifying MOSFET is turned off when the zero inductor current is detected.

As the load current further decreases, the converter runs into discontinuous conduction mode. The on time

is kept almost the same as it was in continuous conduction mode so that it takes longer time to discharge

the output capacitor with smaller load current to the level of the reference voltage. This makes the switching

frequency lower, proportional to the load current, and keeps the light load efficiency high.

7.4.2 FCCM Mode Operation

The TPS563257 operates in forced CCM (FCCM) mode, which keeps the converter operating in continuous

current mode during light load conditions and allows the inductor current to become negative. During FCCM

mode, the switching frequency is maintained at an almost constant level over the entire load range, which is

suitable for applications requiring tight control of the switching frequency and output voltage ripple at the cost of

lower efficiency under light load.

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TPS563252, TPS563257

SLUSEQ5 – AUGUST 2022

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