Datasheet

t
SS
=
0.8V x C
SS
I
SS
C
C2
=
C
OUT
x R
ESR
R
C1
f
Z(FIL)
=
1
2 x S x C
OUT
x R
ESR
LM20123
www.ti.com SNVS524E –OCTOBER 2007–REVISED MARCH 2013
A higher crossover frequency can be obtained, usually at the expense of phase margin, by lowering the value of
C
C1
and recalculating the value of R
C1
. Likewise, increasing C
C1
and recalculating R
C1
will provide additional
phase margin at a lower crossover frequency. As with any attempt to compensate the LM20123 the stability of
the system should be verified for desired transient droop and settling time.
If the output filter zero, f
Z(FIL)
approaches the crossover frequency (f
C
), an additional capacitor (C
C2
) should be
placed at the COMP pin to ground. This capacitor adds a pole to cancel the output filter zero assuring the
crossover frequency will occur before the double pole at f
SW
/2 degrades the phase margin. The output filter zero
is set by the output capacitor value and ESR as shown in the equation below.
(9)
If needed, the value for C
C2
should be calculated using the equation shown below.
where
• R
ESR
is the output capacitor series resistance
• R
C1
is the calculated compensation resistance (10)
AVIN FILTERING COMPONENTS (C
F
and R
F
)
To prevent high frequency noise spikes from disturbing the sensitive analog circuitry connected to the AVIN and
AGND pins, a high frequency RC filter is required between PVIN and AVIN. These components are shown in
Figure 25. as C
F
and R
F
. The required value for R
F
is 1Ω. C
F
must be used. Recommended value of C
F
is 1.0
µF. The filter capacitor, C
F
should be placed as close to the IC as possible with a direct connection from AVIN to
AGND. A good quality X5R or X7R ceramic capacitor should be used for C
F
.
SUB-REGULATOR BYPASS CAPACITOR (C
VCC
)
The capacitor at the VCC pin provides noise filtering and stability for the internal sub-regulator. The
recommended value of C
VCC
should be no smaller than 1 µF and no greater than 10 µF. The capacitor should be
a good quality ceramic X5R or X7R capacitor. In general, a 1 µF ceramic capacitor is recommended for most
applications.
SETTING THE START UP TIME (C
SS
)
The addition of a capacitor connected from the SS pin to ground sets the time at which the output voltage will
reach the final regulated value. Larger values for C
SS
will result in longer start up times. Table 3, shown below
provides a list of soft start capacitors and the corresponding typical start up times.
Table 3. Start Up Times for Different Soft-Start Capacitors
Start Up Time (ms) C
SS
(nF)
1 none
5 33
10 68
15 100
20 120
If different start up times are needed the equation shown below can be used to calculate the start up time.
(11)
As shown above, the start up time is influenced by the value of the Soft-Start capacitor C
SS
(F) and the 5 µA Soft-
Start pin current I
SS
(A). that may be found in the electrical characteristics table.
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM20123