R
FB1
= R
FB2
x
-1
V
OUT
0.8V
C
C2
=
C
OUT
x R
ESR
R
C1
x
C
C1
C
OUT
I
OUT
V
OUT
+
18 x D
V
IN
-1
+
1-D
f
SW
x L
R
C1
=
www.ti.com
Component Selection
2.6
C
VCC
The C
VCC
capacitor is necessary to bypass an internal 2.7V subregulator. This capacitor should be sized
equal to or greater than 1 µF, but less than 10 µF. A value of 1 µF is sufficient for most applications..
2.7
C
C1
The capacitor C
C1
is used to set the crossover frequency of the LM20124 control loop. Since this board
was optimized to work well over the full input and output voltage range, the value of C
C1
was selected to
be 3.3 nF. Once the operating conditions for the device are known, the transient response can be
optimized by reducing the value of C
C1
and calculating the value for R
C1
as outlined in the next section.
2.8
R
C1
Once the value of C
C1
is known, resistor R
C1
is used to place a zero in the control loop to cancel the output
filter pole. This resistor can be sized according to the equation:
(7)
For stability purposes the device should be compensated for the maximum output current expected in the
application.
2.9
C
C2
A second compensation capacitor C
C2
can be used in some designs to provide a high frequency pole,
useful for cancelling a possible zero introduced by the ESR of the output capacitor. For the LM20124
evaluation board, the C
C2
footprint is unpopulated, as the low ESR ceramic capacitor used on the output
does not contribute a zero to the control loop before the crossover frequency. If the ceramic capacitor on
the evaluation board is replaced with a different capacitor having significant ESR, the required value of the
capacitor C
C2
can be estimated by the equation:
(8)
2.10 R
FB1
and R
FB2
The resistors labeled R
FB1
and R
FB2
create a voltage divider from V
OUT
to the feedback pin that is used to
set the output of the voltage regulator. Nominally, the output of the LM20124 evaluation board is set to
1.2V, giving resistor values of R
FB1
= 4.99 k
Ω
and R
FB2
= 10 k
Ω
. If a different output voltage is required, the
value of R
FB1
can be adjusted according to the equation:
(9)
R
FB2
does not need to be changed from its value of 10 k
Ω
.
3
SNVA252B – October 2007 – Revised May 2013
AN-1654 LM20124 Evaluation Board
Submit Documentation Feedback
Copyright © 2007–2013, Texas Instruments Incorporated
