Datasheet

R
C1
=
x
C
C1
C
OUT
I
OUT
V
OUT
+
-1
2 x D
f
SW
x L
COMP
C
C1
R
C1
C
C2
LM20333
(optional)
LM20333
SNVS558D –MAY 2008–REVISED APRIL 2013 www.ti.com
To achieve a -20dB/decade slope, the error amplifier zero, located at f
Z(EA)
, should be positioned to cancel the
output filter pole (f
P(FIL)
).
Compensation of the LM20333 is achieved by adding an RC network as shown in Figure 28 below.
Figure 28. Compensation Network for LM20333
A good starting value for C
C1
for most applications is 2.2 nF. Once the value of C
C1
is chosen the value of RC
should be approximated using the equation below to cancel the output filter pole (f
P(FIL)
) as shown in Figure 27.
(7)
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 LM20333 the stability of
the system should be verified for desired transient droop and settling time.
For low duty cycle operation, when the on time of the switch node is less than 200ns, an additional capacitor
(C
C2
) should be added from the COMP pin to AGND. The recommended value of this capacitor is 20pF. If low
duty cycle jitter on the switch node is observed, the value of this capacitor can be increased to improve noise
immunity; however, values much larger than 100pF will cause the pole f
P2(EA)
to move to a lower frequency
degrading loop stability.
BOOT CAPACITOR (C
BOOT
)
The LM20333 integrates an N-channel buck switch and associated floating high voltage level shift / gate driver.
This gate driver circuit works in conjunction with an internal diode and an external bootstrap capacitor. A 0.1 µF
ceramic capacitor, connected with short traces between the BOOT pin and SW pin, is recommended. During the
off-time of the buck switch, the SW pin voltage is approximately 0V and the bootstrap capacitor is charged from
VCC through the internal bootstrap diode.
SUB-REGULATOR BYPASS CAPACITOR (C
VCC
)
The capacitor at the VCC pin provides noise filtering for the internal sub-regulator. The recommended value of
C
VCC
should be no smaller than 0.1 µF and no greater than 1 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. The VCC
regulator should not be used for other functions since it isn't protected against short circuit.
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
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