Datasheet

C
C2
=
C
OUT
x R
ESR
R
C1
f
Z(FIL)
=
1
2 x S x C
OUT
x R
ESR
R
C1
=
x
C
C1
C
OUT
I
OUT
V
OUT
+
2.84 x D
V
IN
-1
+
1-D
f
SW
x L
COMP
C
C1
R
C1
C
C2
LM20242
(optional)
LM20242
www.ti.com SNVS534E –OCTOBER 2007–REVISED MARCH 2013
Compensation of the LM20242 is achieved by adding an RC network as shown in Figure 12.
Figure 12. Compensation Network for LM20242
A good starting value for C
C1
for most applications is 4.7 nF. Once the value of C
C1
is chosen the value of RC
should be calculated using Equation 9 to cancel the output filter pole (f
P(FIL)
) as shown in Figure 11.
(9)
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 LM20242 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 Equation 10.
(10)
If needed, the value for C
C2
should be calculated using Equation 11.
(11)
Where R
ESR
is the output capacitor series resistance and R
C1
is the calculated compensation resistance.
BOOT CAPACITOR (C
BOOT
)
The LM20242 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.
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM20242