C
C2
=
C
OUT
x R
ESR
R
C1
x
C
C1
C
OUT
I
OUT
V
OUT
+
15 x D
V
IN
-1
+
1 - D
f
SW
x L
R
C1
=
t
SS
=
0.8V x C
SS
I
SS
Component Selection
The variable R
ESR
above refers to the ESR of the output capacitor. As can be seen in
, the
ripple voltage on the output can be divided into two parts, one of which is attributed to the AC ripple
current flowing through the ESR of the output capacitor and another due to the AC ripple current actually
charging and discharging the output capacitor. The output capacitor also has an effect on the amount of
droop that is seen on the output voltage in response to a load transient event.
For the evaluation board, a Murata 47 µF ceramic capacitor is selected for the output capacitor to provide
good transient and DC performance in a relatively small package. From the technical specifications of this
capacitor, the ESR is roughly 3 m
Ω
, and the effective in-circuit capacitance is approximately 32 µF
(reduced from 47 µF due to the 1.2 V DC bias). With these values, the peak-to-peak voltage ripple on the
output when operating from a 5 V input can be calculated to be 8 mV.
5.5
C
SS
A soft-start capacitor can be used to control the startup time of the LM20133 voltage regulator. The startup
time of the regulator when using a soft-start capacitor can be estimated by
:
(6)
For the LM20133, I
SS
is nominally 5 µA. For the evaluation board, the soft-start time has been designed to
be roughly 5 ms, resulting in a C
SS
capacitor value of 33 nF.
5.6
C
VCC
The C
VCC
capacitor is necessary to bypass an internal 2.7 V 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..
5.7
C
C1
The capacitor C
C1
is used to set the crossover frequency of the LM20133 control loop. Since this board
was optimized to work well over the full input, output voltage, and frequency range, the value of C
C1
was
selected to be 5.6 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.
5.8
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
:
(7)
For stability purposes the device should be compensated for the maximum output current expected in the
application.
5.9
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 LM20133
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
(8)
5
SNVA274B – October 2007 – Revised May 2013
AN-1688 LM20133 Evaluation Board
Copyright © 2007–2013, Texas Instruments Incorporated
