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
LM20125
www.ti.com SNVS525E –OCTOBER 2007–REVISED MARCH 2013
If the output filter zero, f
Z(FIL)
approaches the crossover frequency, 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.
(10)
Where R
ESR
is the output capacitor series resistance and R
C1
is the calculated compensation resistance.
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.
While the Soft-Start capacitor can be sized to meet many start up requirements, there are limitations to its size.
The Soft-Start time can never be faster than 1ms due to the internal default 1 ms start up time. When the device
is enabled there is an approximate time interval of 50 µs when the Soft-Start capacitor will be discharged just
prior to the SS ramp. If the enable pin is rapidly pulsed or the Soft-Start capacitor is large there may not be
enough time for C
SS
to completely discharge resulting in start up times less than predicted. To aid in discharging
the Soft-Start capacitor during long disable periods an external 1 MΩ resistor from SS/TRK to ground can be
used without greatly affecting the start-up time.
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