Datasheet

t
SS
=
0.8V x C
SS
I
SS
LM20343
SNVS559B –MAY 2008–REVISED APRIL 2013 www.ti.com
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 LM20343 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
If different start up times are needed the equation shown below can be used to calculate the start up time.
(9)
As shown above, the start up time is influenced by the value of the soft-start capacitor C
SS
and the 4.5 µA soft-
start pin current I
SS
.
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 1 ms 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 soft-start 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
of soft-start capacitor during long disable periods an external 1MΩ resistor from SS/TRK to ground can be used
without greatly affecting the start up time.
USING PRECISION ENABLE AND POWER GOOD
The precision enable (EN) and power good (PGOOD) pins of the LM20343 can be used to address many
sequencing requirements. The turn-on of the LM20343 can be controlled with the precision enable pin by using
two external resistors as shown in Figure 29 .
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