9.3 Feature Description

9.3.1 Precise Enable

The voltage applied at the Enable pin of the TPS6281x-Q1 is compared to a fixed threshold of 1.1 V for a rising

voltage. This allows to drive the pin by a slowly changing voltage and enables the use of an external RC network

to achieve a power-up delay.

The Precise Enable input provides a user-programmable undervoltage lockout by adding a resistor divider to the

input of the Enable pin.

The enable input threshold for a falling edge is typically 100 mV lower than the rising edge threshold. The

TPS6281x-Q1 starts operation when the rising threshold is exceeded. For proper operation, the EN pin must be

terminated and must not be left floating. Pulling the EN pin low forces the device into shutdown, with a shutdown

current of typically 1 μA. In this mode, the internal high-side and low-side MOSFETs are turned off and the entire

internal control circuitry is switched off.

9.3.2 COMP/FSET

This pin allows to set two different parameters independently:

• Internal compensation settings for the control loop

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

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

in compensation allows you to adapt 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 sampled at start-up of 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 / compensation. Do not leave the pin floating.

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

specifications for the minimum on-time and minimum off-time.

For example: VIN = 5 V, VOUT = 1 V --> duty cycle (DC) = 1 V / 5 V = 0.2

• with ton = DC × T --> ton,min = 1 / fs,max × DC

• --> fs,max = 1 / ton,min × DC = 1 / 0.075 µs · 0.2 = 2.67 MHz

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

increased from the minimum value as given in Table 9-1 and Table 9-2, up to the maximum of 470 µF 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:

Space

18

(

)

(

)

CF

S

MHz k

R

k

f MHz

× W

W =

(1)

For compensation (comp) setting 2:

Space

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

SLVSDU1J – AUGUST 2018 – REVISED MARCH 2023

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