Datasheet

LT3745
22
3745f
applicaTions inForMaTion
C
IN
, C
VCC
, and C
CAP
Capacitor Selection
A local input bypass capacitor C
IN
is required for buck
converters because the input current is pulsed with fast
rise and fall times. The input capacitor selection criteria are
based on the voltage rating, bulk capacitance, and RMS
current capability. The capacitor voltage rating must be
greater than V
IN(MAX)
. The bulk capacitance determines the
input supply ripple voltage and the RMS current capability
is used to keep from overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple, ∆V
IN
:
C
IN
=
D
MAX
• I
OUT(MAX)
∆V
IN
• f
SW
∆V
IN
is typically chosen at a level acceptable to the user.
100mV is a good starting point. For ceramic capacitors,
only X5R or X7R type should be used because they retain
their capacitance over wider voltage and temperature
ranges than other types such as Y5V or Z5U. Aluminum
electrolytic capacitors are a good choice for high voltage,
bulk capacitance due to their high capacitance per unit area.
The capacitor RMS current is:
I
CIN(RMS)
= I
OUT
•
V
OUT
• ( V
IN
– V
OUT
)
V
IN
2
If applicable, calculate at the worst-case condition,
V
IN
= 2 • V
OUT
. The capacitor RMS current rating specified
by the manufacturer should exceed the calculated I
CIN(RMS)
.
Due to their low ESR, ceramic capacitors are a good choice
for high voltage, high RMS current handling. Note that the
ripple current ratings from aluminum electrolytic capacitor
manufacturers are based on 2000 hours of life. This makes
it advisable to further derate the capacitor or to choose
a capacitor rated at a higher temperature than required.
For a larger high voltage capacitor value, the combination
of aluminum electrolytic capacitors and ceramic capacitors
is an economical approach. Multiple capacitors may also
be paralleled to meet size or height requirements in the
design. Locate the capacitor very close to the MOSFET
switch and the catch diode, and use short, wide PCB traces
to minimize parasitic inductance.
The general discussion above also applies to the capacitor
C
VCC
at the V
CC
pin and the capacitor C
CAP
between the V
IN
and CAP pins. Typically, a 10µF 10V-rated ceramic capaci-
tor for C
VCC
and a 0.47µF 16V-rated ceramic capacitor for
C
CAP
should be sufficient.
C
OUT
Capacitor Selection
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the
LT3745 to produce the DC output containing a con-
trolled voltage ripple. It also stores energy to satisfy load
transients and to stabilize the dual-loop operation. Thus
the selection criteria for C
OUT
are based on the voltage
rating, the equivalent series resistance ESR, and the bulk
capacitance. As always, choose the C
OUT
with a voltage
rating greater than V
OUT(MAX)
.
The LT3745 utilizes the output as the dominant pole
to stabilize the dual loop operation, so the C
OUT
value
determines the unity gain frequency f
UGF
, which is set
around 1/10 of the switching frequency. To stabilize the
FB loop during the startup and precharging phases and the
LED loop during the tracking phase, a low ESR capacitor
(tens of mΩ) should be used and its minimum C
OUT
is
calculated as:
C
OUT
= MAX
0.25
R
S
• f
UGF
,
1.5
V
OUT(MAX)
• R
S
• f
UGF








The adaptive-tracking-plus-precharging technique moves
the V
OUT
with the grayscale PWM dimming frequency to
improve system efficiency, choosing a ceramic capacitor
as the C
OUT
inevitably generates acoustic noise due to the
piezo effect of the ceramic material. In an acoustic noise
sensitive application, low ESR tantalum or aluminum
capacitors are preferred. When choosing a capacitor,
look carefully through the data sheet to find out what the
actual capacitance is under operating conditions (applied
voltage and temperature). A physically larger capacitor, or
one with a higher voltage rating, may be required.