'
V
OUT
=
'
I
P-P
x
1
8 x f
SW
x C
OUT
R
ESR
+
'
I
P-P
=
(V
IN
- V
OUT
) x D
L x f
SW
I
CIN(RMS)
= 3A
0.5 x 0.5 = 1.5A
D =
V
OUT
V
IN
I
CIN(RMS)
= I
OUT
D(1 - D)
Component Selection
5
Component Selection
This section provides a walk-through of the design process of the LM20133 evaluation board. Unless
otherwise indicated all equations assume units of Amps (A) for current, Farads (F) for capacitance,
Henries (H) for inductance, and Volts (V) for voltages.
5.1
Input Capacitor
The required RMS current rating of the input capacitor for a buck regulator can be estimated by
:
(1)
The variable D refers to the duty cycle, and can be approximated by:
(2)
From
, it follows that the maximum I
CIN(RMS)
requirement will occur at a full 3A load current with
the system operating at 50% duty cycle. Under this condition, the maximum I
CIN(RMS)
is given by:
(3)
Ceramic capacitors feature a very large I
RMS
rating in a small footprint, making a ceramic capacitor ideal
for this application. A 47 µF X5R ceramic capacitor from Murata provides the necessary input capacitance
for the evaluation board. For improved bypassing, a small 1 µF high frequency capacitor is placed in
parallel with the 47 µF bulk capacitor to filter high frequency noise pulses on the supply.
5.2
AV
IN
Filter
An RC filter should be added to prevent any switching noise on PV
IN
from interfering with the internal
analog circuitry connected to AV
IN
. These can be seen on the schematic as components R
F
and C
F
. There
is a practical limit to the size of the resistor R
F
as the AV
IN
pin will draw a short 60mA burst of current
during startup, and if R
F
is too large the resulting voltage drop can trigger the UVLO comparator. For the
demo board a 1
Ω
resistor is used for R
F
ensuring that UVLO will not be triggered after the part is enabled.
A recommended 1 µF C
F
capacitor coupled with the 1
Ω
resistor provides roughly 16dB of attenuation at
the 1 MHz switching frequency.
5.3
Inductor
As per the device-specific data sheet recommendations, the inductor value should initially be chosen to
give a peak-to-peak ripple current equal to roughly 30% of the maximum output current. The peak-to-peak
inductor ripple current can be calculated by
:
(4)
Rearranging this equation and solving for the inductance reveals that for this application (V
IN
= 5 V, V
OUT
=
1.2 V, f
SYNC
= 500 kHz, and I
OUT
= 3A) the nominal inductance value is roughly 2.03 µH. Rounding up to
the nearest standard inductor value, a final inductance of 2.5 µH is selected. This results in a peak-to-
peak ripple current of 730 mA and 898 mA when the converter is operating from 5 V and 3.3 V,
respectively. Once an inductance value is calculated, an actual inductor needs to be selected based on a
tradeoff between physical size, efficiency, and current carrying capability. For the LM20133 evaluation
board, a Coilcraft MSS1038-252NL inductor offers a good balance between efficiency (10 m
Ω
DCR), size,
and saturation current rating (5.7A I
SAT
rating).
5.4
Output Capacitor
The value of the output capacitor in a buck regulator influences the voltage ripple that will be present on
the output voltage, as well as the large signal output voltage response to a load transient. Given the peak-
to-peak inductor current ripple (
Δ
I
P-P
) the output voltage ripple can be approximated by
:
(5)
4
AN-1688 LM20133 Evaluation Board
SNVA274B – October 2007 – Revised May 2013
Copyright © 2007–2013, Texas Instruments Incorporated
