the use of an external RC network to achieve a precise power-up delay. See the Achieving a Clean Start-up by

Using a DC/DC Converter with a Precise Enable-pin Threshold analog design journal for more details.

8.3.2 Output Discharge

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

is disabled and keep the output voltage close to 0V when the device is off. The output discharge feature is

only active once the TPSM8281x has been enabled at least once since 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 is typically 1V.

8.3.3 COMP/FSET

This pin sets two different parameters independently:

• Internal compensation settings for the control loop (three settings available)

• The switching frequency in PWM mode from 1.8MHz to 4MHz

A resistor from COMP/FSET to GND changes the compensation as well as the switching frequency. The change

in compensation adapts the device to different values of output capacitance. The resistor must be placed close

to the pin to keep the parasitic capacitance on the pin to a minimum. The compensation setting is set after

enabling the converter, so a change in the resistor during operation only has an effect on the switching frequency

but not on the compensation.

To save external components, the pin can also be directly tied to VIN or GND to set a pre-defined switching

frequency and compensation. Do not leave the pin floating.

The switching frequency must be selected based on the maximum input voltage and the output voltage to meet

the specifications for the minimum on-time. Using VIN = 5.5V and VOUT = 1.1V as an example, the minimum duty

cycle given with Equation 1 is 0.2, which results in a maximum switching frequency of 2.67MHz according to

Equation 2.

OUT

min

IN, max

V

D

V

(1)

s, max

on, min

min

1

f

t

D

u

(2)

The compensation range has to be chosen based on the minimum effective capacitance used. The capacitance

can be increased from the minimum value as given in Table 8-1 up to the maximum of 470µF in all of the

three compensation ranges. If the capacitance of an output changes during operation, for example, when load

switches are used to connect or disconnect parts of the circuitry, the compensation has to be chosen for the

minimum capacitance on the output. With large output capacitance, the compensation must be done based on

that large capacitance to get the best load transient response. Compensating for large output capacitance but

placing less capacitance on the output can lead to instability.

The switching frequency for the different compensation setting is determined by the following equations.

For compensation (comp) setting 1:

18

(

)

(

)

CF

S

MHz k

R

k

f MHz

× W

W =

(3)

For compensation (comp) setting 2:

TPSM82810, TPSM82813

SLUSDN6B – SEPTEMBER 2019 – REVISED JULY 2024

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