Datasheet

't = C
I
'V
= 100 PF
3A
3V
= 100 Ps
I
RMS
= I
LOAD
D(1-D)
LM2852
www.ti.com SNVS325D –JANUARY 2005–REVISED APRIL 2013
APPLICATIONS INFORMATION
The LM2852 is a DC-DC synchronous buck regulator belonging to Texas Instrument’s SIMPLE SWITCHER
family. Integration of the PWM controller, power switches and compensation network greatly reduces the
component count required to implement a switching power supply. A typical application requires only four
components: an input capacitor, a soft-start capacitor, an output filter capacitor and an output filter inductor.
INPUT CAPACITOR (C
IN
)
Fast switching of large currents in the buck converter places a heavy demand on the voltage source supplying
PVIN. The input capacitor, C
IN
, supplies extra charge when the switcher needs to draw a burst of current from
the supply. The RMS current rating and the voltage rating of the C
IN
capacitor are therefore important in the
selection of C
IN
. The RMS current specification can be approximated by:
where
• D is the duty cycle, V
OUT
/V
IN
. C
IN
also provides filtering of the supply. (1)
Trace resistance and inductance degrade the benefits of the input capacitor, so C
IN
should be placed very close
to PVIN in the layout. A 22 µF or 47 µF ceramic capacitor is typically sufficient for C
IN
. In parallel with the large
input capacitance a smaller capacitor may be added such as a 1µF ceramic for higher frequency filtering.
SOFT-START CAPACITOR (C
SS
)
The DAC that sets the reference voltage of the error amp sources a current through a resistor to set the
reference voltage. The reference voltage is one half of the output voltage of the switcher due to the 200kΩ
divider connected to the SNS pin. Upon start-up, the output voltage of the switcher tracks the reference voltage
with a two to one ratio as the DAC current charges the capacitance connected to the reference voltage node.
Internal capacitance of 20pF is permanently attached to the reference voltage node which is also connected to
the soft-start pin, SS. Adding a soft-start capacitor externally increases the time it takes for the output voltage to
reach its final level.
The charging time required for the reference voltage can be estimated using the RC time constant of the DAC
resistor and the capacitance connected to the SS pin. Three RC time constant periods are needed for the
reference voltage to reach 95% of its final value. The actual start-up time will vary with differences in the DAC
resistance and higher-order effects.
If little or no soft-start capacitance is connected, then the start-up time may be determined by the time required
for the current limit current to charge the output filter capacitance. The capacitor charging equation I = C ΔV/Δt
can be used to estimate the start-up time in this case. For example, a part with a 3V output, a 100 µF output
capacitance and a 3A current limit threshold would require a time of 100 µs:
(2)
Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to
force the LM2852 to start up in a more controlled fashion based on the charging of the soft-start capacitance. In
this example, suppose a 3 ms start time is desired. Three time constants are required for charging the soft-start
capacitor to 95% of the final reference voltage. So in this case RC=1ms. The DAC resistor, R, is 400 kΩ so C
can be calculated to be 2.5nF. A 2.7nF ceramic capacitor can be chosen to yield approximately a 3ms start-up
time.
SOFT-START CAPACITOR (C
SS
) AND FAULT CONDITIONS
Various fault conditions such as short circuit and UVLO of the LM2852 activate internal circuitry designed to
control the voltage on the soft-start capacitor. For example, during a short circuit current limit event, the output
voltage typically falls to a low voltage. During this time, the soft-start voltage is forced to track the output so that
once the short is removed, the LM2852 can restart gracefully from whatever voltage the output reached during
the short circuit event. The range of soft-start capacitors is therefore restricted to values 1nF to 50nF.
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