number, the device either hiccups or latches off in response to the undervoltage event caused by the OVP

response. The TPS54KB22 and TPS54KB23 enter hiccup, while the TPS54KB20 and TPS54KB21 latch off.

7.3.13 Output Voltage Discharge

When the device is disabled through EN, the device enables the output voltage discharge mode. This mode

forces both high-side and low-side MOSFETs to latch off, but turns on the internal discharge MOSFET, which

is connected from SW to PGND, to discharge the output voltage. After the FB voltage drops below 50 mV, the

discharge MOSFET is turned off.

While the VCC voltage is sufficient to turn on the discharge switch, the output voltage discharge mode is

activated by any of the following fault events:

1. EN pin goes low to disable the converter.

2. VIN UVLO (falling) is triggered with VCC above its UVLO.

3. VCC UVLO (falling) is triggered with VIN above its UVLO. This event is notably different from the others

because the internal VCC LDO is enabled until FB falls to the same 50-mV threshold for the discharge

MOSFET. As a result, this scenario is only expected if an external circuit is overloading the internal VCC

LDO. Additionally an external circuit overloading the internal VCC LDO can cause the VCC voltage to be

insufficient to turn on the discharge switch, causing the discharge switch to turn off before the output voltage

has discharged FB to the 50-mV threshold.

7.3.14 UVLO Protection

The device monitors the voltage on both the VIN and the VCC pins. If the VCC pin voltage is lower than the

VCCUVLO falling threshold voltage, the device shuts off. If the VCC voltage increases beyond the VCCUVLO rising

threshold voltage, the device turns back on. VCC UVLO is a non-latch protection.

When the VIN pin voltage is lower than the VINUVLO falling threshold voltage but the VCC pin voltage is still

higher than VCCUVLO rising threshold voltage, the device stops switching and discharges the SS pin. After the

VIN voltage increases beyond the VINUVLO rising threshold voltage, the device re-initiates the soft start and

switches again. VIN UVLO is a non-latch protection.

7.3.15 Thermal Shutdown

The device monitors internal junction temperature. If the temperature exceeds the threshold value (typically

165°C), the device stops switching and discharges the SS pin. When the temperature falls approximately 15°C

below the threshold value, the device turns back on with a re-initiated soft start. Thermal shutdown is a non-latch

protection.

7.4 Device Functional Modes

7.4.1 Auto-Skip Eco-mode Light Load Operation

If the MSEL resistor value used selects Skip-mode, the device automatically reduces the switching frequency at

light-load conditions to maintain high efficiency. Multifunction Select (MSEL) Pin describes the selection in detail.

As the output current decreases from heavy load condition, the inductor current also decreases until the valley

of the inductor ripple current touches the zero-crossing threshold (Low-side MOSFET Zero-Crossing). The

zero-crossing threshold sets the boundary between the continuous-conduction and discontinuous-conduction

modes. The synchronous MOSFET turns off when this zero-crossing threshold is detected. As the load current

decreases further, the converter runs into discontinuous-conduction mode (DCM). The on-time is maintained to

a level approximately the same as during continuous-conduction mode operation so that discharging the output

capacitor with a smaller load current to the level of the reference voltage requires more time. The transition point

to light-load operation IOUT(LL) (for example: the boundary between continuous- and discontinuous-conduction

mode) is calculated as shown in Equation 5.

For low output ripple, TI recommends using only ceramic output capacitors for designs that operate in skip-

mode.

IOUTLL =12× VIN−VO×VO

VIN × 1L×fSW

(6)

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TPS54KB20

SLVSGP3 – MAY 2023

Copyright © 2023 Texas Instruments Incorporated

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