8.2.2.7 Soft-Start Capacitor (SS Pin)

The capacitor placed on the SS pin can be used to extend the soft start time past the internal TBD-ms soft

start. This example uses a TBD-ms soft start time and the required external capacitance can be calculated with

Equation 27. In this example a TBD-nF capacitor is used.

TBD

(27)

A minimum capacitor value of TBD nF is required at the SS pin. The SS capacitor must use the AGND pin for its

ground.

8.2.2.8 EN Pin Resistor Divider

A resistor divider on the EN pin can be used to increase the input voltage the converter begins its start-up

sequence. To set the start voltage, first select the bottom resistor (REN_B). The recommended value is between 1

kΩ and 100 kΩ. There is an internal pulldown resistance with a nominal value of 1 MΩ and this must be included

for the most accurate calculations. This is especially important when the bottom resistor is a higher value, near

100 kΩ. This example uses a 100-kΩ resistor and this combined with the internal resistance in parallel results in

an equivalent bottom resistance of 90.9 kΩ. The top resistor value for the target start voltage is calculated with

Equation 28. In this example, the nearest standard value of 453 kΩ is selected for REN_T. When selecting a start

voltage in a wide input range application, be cautious that the EN pin absolute maximum voltage of 6 V is not

exceeded.

TBD

(28)

The start and stop voltages with the selected EN resistor divider can be calculated with Equation 29 and

Equation 30.

TBD

(29)

TBD

(30)

8.2.2.9 VCC Bypass Capacitor

At a minimum, a 1.0-µF, at least 6.3-V rating, X5R ceramic bypass capacitor is needed on VCC pin located as

close to the pin as the layout allows. Use the smallest sized capacitor possible, such as an 0402 package, to

minimize the loop from the VCC pin to the PGND pin.

8.2.2.10 BOOT Capacitor

At a minimum, a 0.1-µF 10-V X5R ceramic bypass capacitor is needed between the BOOT and SW pins located

as close to the pin as the layout allows.

8.2.2.11 Series BOOT Resistor and RC Snubber

An RC snubber on the SW pin can also help reduce voltage overshoot and ringing at the SW pin. For the RC

snubber to be as effective as possible with the symmetrical pinout, place the RC snubber on the opposite side

of the board from the IC with multiple vias in the SW node to minimize routing impedance and with a very low

impedance return to PGND pins.

8.2.2.12 PG Pullup Resistor

The PG pin is open-drain, so a pullup resistor is required when using this pin. The recommended value is

between 1 kΩ and 100 kΩ.

8.3 Power Supply Recommendations

The device is designed to operate from an input voltage supply range between 4 V and 16 V. Both input supplies

(VIN and VCC bias) must be well regulated. Proper bypassing of input supplies (VIN and VCC bias) is also

critical for noise performance, as are PCB layout and grounding scheme. See the recommendations in Layout.

TPS54KB20

SLVSGP3 – MAY 2023

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