drops below the PFM threshold, the device starts to skip switching pulses. In power save mode, the switching

frequency decreases with the load current maintaining high efficiency.

9.4.3 100% Duty-Cycle Operation

The duty cycle of a buck converter operated in PWM mode is given as D = VOUT / VIN. The duty cycle

increases as the input voltage comes close to the output voltage and the off-time gets smaller. When the

minimum off-time of typically 30 ns is reached, the TPS6281x-Q1 skips switching cycles while it approaches

100% mode. In 100% mode, it keeps the high-side switch on continuously. The high-side switch stays turned

on as long as the output voltage is below the target. In 100% mode, the low-side switch is turned off. The

maximum dropout voltage in 100% mode is the product of the on-resistance of the high-side switch plus the

series resistance of the inductor and the load current.

9.4.4 Current Limit and Short Circuit Protection

The TPS6281x-Q1 is protected against overload and short circuit events. If the inductor current exceeds the

current limit ILIMH, the high-side switch is turned off and the low-side switch is turned on to ramp down the

inductor current. The high-side switch turns on again only if the current in the low-side switch has decreased

below the low-side current limit. Due to internal propagation delay, the actual current can exceed the static

current limit. The dynamic current limit is given as:

(

)

L

peak typ

LIMH

PD

V

I

I

t

L

=

+

×

(4)

where

• ILIMH is the static current limit as specified in the electrical characteristics

• L is the effective inductance at the peak current

• VL is the voltage across the inductor (VIN - VOUT)

• tPD is the internal propagation delay of typically 50 ns

The current limit can exceed static values, especially if the input voltage is high and very small inductances are

used. The dynamic high-side switch peak current can be calculated as follows:

(

)

50

IN

OUT

peak typ

LIMH

V

V

I

I

ns

L

-

=

+

×

(5)

9.4.5 Foldback Current Limit and Short Circuit Protection

This is valid for devices where foldback current limit is enabled.

When the device detects current limit for more than 1024 subsequent switching cycles, it reduces the current

limit from its nominal value to typically 1.8 A. Foldback current limit is left when the current limit indication goes

away. For the case that device operation continues in current limit, it can, after 3072 switching cycles, try again

full current limit for again 1024 switching cycles.

9.4.6 Output Discharge

The purpose of the discharge function is to ensure a defined down-ramp of the output voltage when the

device is being disabled but also to keep the output voltage close to 0 V when the device is off. The output

discharge feature is only active after TPS6281x-Q1 has been enabled at least once because the supply voltage

was applied. The discharge function is enabled as soon as the device is disabled, in thermal shutdown, or in

undervoltage lockout. The minimum supply voltage required for the discharge function to remain active typically

is 2 V. Output discharge is not activated during a current limit or foldback current limit event.

9.4.7 Soft Start / Tracking (SS/TR)

The internal soft-start circuitry controls the output voltage slope during start-up. This avoids excessive inrush

current and ensures a controlled output voltage rise time. It also prevents unwanted voltage drops from high

impedance power sources or batteries. When EN is set high to start operation, the device starts switching after

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TPS62810-Q1, TPS62811-Q1, TPS62812-Q1, TPS62813-Q1

SLVSDU1J – AUGUST 2018 – REVISED MARCH 2023

Copyright © 2023 Texas Instruments Incorporated

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