Datasheet

I
rms
= I
O
x
D(1 - D)
LM3495
SNVS410F –FEBRUARY 2006–REVISED APRIL 2013 www.ti.com
VLIN5 DECOUPLING CAPACITOR
The VLIN5 pin should always be decoupled with a 2.2 µF, 10V-rated ceramic capacitor placed as close as
possible to the VLIN5 and PGND pins of the LM3495. The decoupling capacitor should have a minimum X5R or
X7R type dielectric to ensure that the capacitance remains stable over the expected voltage and temperature
range.
INPUT CAPACITOR
The input capacitors to a buck regulator are used to smooth the large current pulses drawn by the inductor and
load when the high-side FET is on. Due to this large AC stress, input capacitors are usually selected on the basis
of their AC rms current rating rather than bulk capacitance. Low ESR is beneficial because it reduces the power
dissipation in the capacitors. Although any of the capacitor types mentioned in the OUTPUT CAPACITOR
section can be used, MLCCs are common because of their low ESR and because in general the input to a buck
converter does not require as much bulk capacitance as the output. Input current, I
rms
, can be calculated using
the following equation:
(22)
A good estimate for the maximum AC rms current is one-half of the maximum load current. For this example, the
rms input current can be estimated as 3.5A. Regardless of the type and number of capacitors used, every design
will benefit from the addition of a 0.1 µF to 1 µF ceramic capacitor placed as close as possible to the drain of the
high-side FET and the source of the low-side FET.
In most applications for POL power supplies, the input voltage is the output of another switching converter. This
output often has a lot of bulk capacitance. One 22 µF MLCC provides enough local smoothing and keeps the
input impedance high enough to prevent power supply interaction from the source. For switching power supplies,
the minimum quality dielectric that should be used is X5R. The preferred capacitor voltage rating for a 12V input
voltage is 25V, due to the drop-off in capacitance of MLCCs under a DC bias. Capacitors with a 16V rating can
still be used if size and cost are limiting factors. For this example the current rating of each of the capacitors
should be at least 3Arms. The ESR of large-value ceramic caps is usually below 10 mΩ, which keeps the heating
to a minimum.
CURRENT LIMIT
For this design, the trip point for the current limit circuitry should be below the peak current rating of the output
inductor, which is 18A. To account for the tolerance of the internal current source, the change in the R
DSON
of the
low-side FET, and to prevent excessive heating of the inductor, a target of 15A has been chosen. A 3.8A margin
exists between the expected 11.2A peak current and the current limit threshold to allow for line and load
transients. Following the equation from the Applications Information section the value used for R
LIM
should be
3.32 kΩ 1%.
CONTROL LOOP COMPENSATION
The LM3495 uses emulated peak current-mode PWM control to correct changes in output voltage due to line and
load transients. This unique architecture combines the fast line transient response of peak current mode control
with the ability to regulate at very low duty cycles. As a further advantage, the small signal characteristics of
emulated peak current mode control are almost identical to those of traditional peak current mode control, and
hence compensation can be selected using nearly identical calculations.
The control loop is comprised of two parts. The first is the power stage, which consists of the duty cycle
modulator, output filter, and the load. The second part is the error amplifier, which is a transconductance (g
M
)
amplifier with a typical transconductance of 750 µmho and a typical output impedance of 72 MΩ. Figure 36
shows the regulator and voltage control loop components.
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