between CIN and the power source lead to reduce ringing that can occur between the inductance of the power

source leads and CIN.

9.2.2.5 Output Capacitor Selection

The output capacitor is mainly selected to meet the requirements at load transient or steady state. The loop

is compensated for the output capacitor selected. The output ripple voltage is related to the equivalent series

resistance (ESR) of the capacitor and its capacitance. Assuming a capacitor with zero ESR, the minimum

capacitance needed for a given ripple can be calculated by Equation 9:

OUT

OUT

IN

OUT

SW

OUT

I

(V

V )

C

f

V

V

´

-

=

´ D ´

(9)

where

• COUT is the output capacitor

• IOUT is the output current

• VOUT is the output voltage

• VIN is the input voltage

• ΔV is the output voltage ripple required

• ƒSW is the switching frequency

The additional output ripple component caused by ESR is calculated by Equation 10:

∆VESR=ILpeak× RESR

(10)

where

• ΔVESR is the output voltage ripple caused by ESR

• RESR is the resistor in series with the output capacitor

For the ceramic capacitor, the ESR ripple can be neglected. However, for the tantalum or electrolytic capacitors,

it must be considered if used.

The minimum ceramic output capacitance needed to meet a load transient requirement can be estimated using

Equation 11:

STEP

OUT

BW

TRAN

I

C

2

f

V

D

=

p ´

´ D

(11)

where

• ΔISTEP is the transient load current step

• ΔVTRAN is the allowed voltage dip for the load current step

• ƒBW is the control loop bandwidth (that is, the frequency where the control loop gain crosses zero)

Take care when evaluating the derating of a ceramic capacitor under the DC bias. Ceramic capacitors can

derate by as much as 70% of the capacitance at the respective rated voltage. Therefore, enough margins on the

voltage rating must be considered to ensure adequate capacitance at the required output voltage.

9.2.2.6 Diode Selection

A Schottky diode is the preferred type for D1 due to its low forward voltage drop and small reverse recovery

charge. Low reverse leakage current is important parameter when selecting the Schottky diode. The diode must

be rated to handle the maximum output voltage plus any switching node ringing. Also, it must be able to handle

the average output current.

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TPS61376

SLVSGQ1A – JANUARY 2022 – REVISED SEPTEMBER 2022

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