Datasheet

LT3742
16
3742fa
applicaTions inForMaTion
voltages, it is also 3× higher at high input voltages when
compared to the low C
RSS
device.
Total gate charge, Q
G
, is closely related to C
RSS
. Low
gate charge corresponds to a small value of C
RSS
. Many
manufacturers have MOSFETs advertised as “low gate
charge” devices (which means they are low C
RSS
devices)
that are specifically designed for low transition loss, and
are ideal for high input voltage applications.
Input Capacitor Selection
For most applications, 10µF to 22µF of input capacitance
per channel will be sufficient. A small 1µF bypass capacitor
between the V
IN
and ground pins of the LT3742, placed
close to the device, is also suggested for optimal noise
immunity. Step-down regulators draw current from the
input supply in pulses with very fast rise and fall times.
The input capacitor is required to reduce the resulting
voltage ripple at the LT3742 and to force this very high
frequency switching current into a tight local loop, minimiz-
ing EMI. The input capacitor must have low impedance at
the switching frequency to do this effectively, and it must
have an adequate ripple current rating. With two control-
lers operating at the same frequency but with different
phases and duty cycles, calculating the input capacitor
RMS current is not simple. However, a conservative value
is the RMS input current for the channel that is delivering
the most power (V
OUT
• I
OUT
):
I
RMS(CIN)
=
I
OUT
V
IN
• V
OUT
• V
IN
– V
OUT
( )
I
RMS(CIN)
is largest (I
OUT
/2) when V
IN
= 2V
OUT
(at DC =
50%). As the second, lower power channel draws input
current, the input capacitor’s RMS current actually de-
creases as the out-of-phase current cancels the current
drawn by the higher power channel, so choosing an input
capacitor with an RMS ripple current rating of I
OUT,MAX
/2
is sufficient.
The combination of small size and low impedance (low
equivalent series resistance, or ESR) of ceramic capacitors
make them the preferred choice. The low ESR results in
very low input voltage ripple and the capacitors can handle
plenty of RMS current. They are also comparatively robust
and can be used at their rated voltage. Use only X5R or
X7R types because they retain their capacitance over wider
voltage and temperature ranges than other ceramics.
An alternative to a high value ceramic capacitor is a lower
value (1µF) along with a larger value (10µF to 22µF) elec-
trolytic or tantalum capacitor. Because the input capacitor
is likely to see high surge currents when the input source
is applied, tantalum capacitors should always be surge
rated. The manufacturer may also recommend operation
below the rated voltage of the capacitor. Be sure to place
the 1µF ceramic as close as possible to the N-channel
power MOSFET.
Output Capacitor Selection
A good starting value for output capacitance is to provide
10µF of C
OUT
for every 1A of output current. For lower
output voltages (under 3.3V) and for applications needing
the best possible transient performance, the ratio should
be 20µF to 30µF of C
OUT
for every 1A of output current.
X5R and X7R ceramics are an excellent choice for the
output capacitance. Aluminum electrolytics can be used,
but typically the ESR is too large to deliver low output
voltage ripple. Tantalum and newer, lower ESR organic
electrolytic capacitors are also possible choices, and the
manufactures will specify the ESR. Because the volume of
the capacitor determines the ESR, both the size and value
will be larger than a ceramic capacitor that would give you
similar output ripple voltage performance.
The output capacitor filters the inductor ripple current to
generate an output with low ripple. It also stores energy
in order to satisfy transient loads and to stabilize the