IND

OUT

L

OUT _ C

SW

OUT

SW

OUT

K

I

i

V

8 f

C

8 f

C

u

'

'

u

u

u

u

OUT_ESR

L

IND

OUT

V

i

ESR

K

I

ESR

'

' u

u

u

IN _ MAX

OUT

OUT

MIN

OUT

IND

IN _ MAX

SW

V

V

V

L

I

K

V

f

u

u

u

OUT

IN _ MAX

OUT

L

IN _ MAX

SW

V

V

V

i

V

L f

u

'

u u

22

LMR23630

SNVSAH2C – DECEMBER 2015 – REVISED JUNE 2017

www.ti.com

Product Folder Links: LMR23630

Submit Documentation Feedback

Copyright © 2015–2017, Texas Instruments Incorporated

9.2.2.4

Inductor Selection

The most critical parameters for the inductor are the inductance, saturation current, and the rated current. The

amount of inductor ripple current relative to the maximum output current of the device. A reasonable value of

inductor current can be high. The inductor current rating should be higher than the current limit of the device.

(12)

(13)

In general, it is preferable to choose lower inductance in switching power supplies, because it usually

corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. However,

inductance that is too low can generate an inductor current ripple that is too high so that overcurrent protection at

the full load could be falsely triggered. It also generates more conduction loss and inductor core loss. Larger

inductor current ripple also implies larger output voltage ripple with same output capacitors. With peak current

mode control, TI does not recommend having an inductor current ripple that is too small. A larger peak-current

ripple improves the comparator signal-to-noise ratio.

nearest standard 8.2 μH ferrite inductor with a capability of 4-A RMS current and 6-A saturation current.

9.2.2.5

Output Capacitor Selection

loop stability, and the voltage over/undershoot during load-current transients.

The output ripple is essentially composed of two parts. One is caused by the inductor current ripple going

through the equivalent series resistance (ESR) of the output capacitors:

(14)

The other is caused by the inductor current ripple charging and discharging the output capacitors:

(15)

The two components in the voltage ripple are not in phase, so the actual peak-to-peak ripple is smaller than the

sum of two peaks.

Output capacitance is usually limited by transient performance specifications if the system requires tight voltage

regulation with presence of large current steps and fast slew rate. When a fast large load increase happens,

output capacitors provide the required charge before the inductor current can slew up to the appropriate level.

The control loop of the regulator usually needs four or more clock cycles to respond to the output voltage droop.

The output capacitance must be large enough to supply the current difference for four clock cycles to maintain

the output voltage within the specified range. Equation 16 shows the minimum output capacitance needed for

specified output undershoot. When a sudden large load decrease happens, the output capacitors absorb energy

stored in the inductor. which results in an output voltage overshoot. Equation 17 calculates the minimum

capacitance required to keep the voltage overshoot within a specified range.