Datasheet
LT3796/LT3796-1
20
3796fa
For more information www.linear.com/3796
require a lower value capacitor than a buck mode converter.
Assuming that a 100mV input voltage ripple is acceptable,
the required capacitor value for a boost converter can be
estimated as follows (T
SW
= 1/f
OSC
):
C
IN
(µF) =I
LED
(A) •
V
LED
V
IN
• T
SW
(µs) •
1µF
A • µs • 2.8
Therefore, a 2.2µF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 48V output
and 500mA load.
With the same V
IN
voltage ripple of less than 100mV, the
input capacitor for a buck converter can be estimated as
follows:
C
IN
(µF)= I
LED
(A)•
V
LED
(V
IN
– V
LED
)
V
IN
2
• T
SW
(µs)•
10µF
A • µs
A 10µF input capacitor is an appropriate selection for a
400kHz buck mode converter with 24V input, 12V output
and 1A load.
In the buck mode configuration, the input capacitor has
large pulsed currents due to the current returned through
the Schottky diode when the switch is off. It is important
to place the capacitor as close as possible to the Schottky
diode and to the GND return of the switch (i.e., the sense
resistor). It is also important to consider the ripple current
rating of the capacitor. For best reliability, this capacitor
should have low ESR and ESL and have an adequate ripple
current rating. The RMS input current for a buck mode
LED driver is:
I
IN(RMS)
= I
LED
• √(1–D)D
D =
V
LED
V
IN
where D is the switch duty cycle.
Table 3. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER WEB
TDK www.tdk.com
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
AVX www.avx.com
Output Capacitor Selection
The selection of the output capacitor depends on the load
and converter configuration, i.e., step-up or step-down
and the operating frequency. For LED applications, the
equivalent resistance of the LED is typically low and the
output filter capacitor should be sized to attenuate the
current ripple. Use of an X7R type ceramic capacitor is
recommended.
To achieve the same LED ripple current, the required filter
capacitor is larger in the boost and buck-boost mode ap-
plications than that in the buck mode applications. Lower
operating frequencies will require proportionately higher
capacitor values.
Power MOSFET Selection
For applications operating at high input or output volt-
ages, the power N-channel MOSFET switch is typically
chosen for drain voltage V
DS
rating and low gate charge
Q
G
. Consideration of switch on-resistance, R
DS(ON)
, is
usually secondary because switching losses dominate
power loss. The INTV
CC
regulator on the LT3796/LT3796-1
has a fixed current limit to protect the IC from excessive
power dissipation at high V
IN
, so the MOSFET should be
chosen so that the product of Q
G
at 7.7V and switching
frequency does not exceed the INTV
CC
current limit. For
driving LEDs be careful to choose a switch with a V
DS
rating that exceeds the threshold set by the FB pin in case
of an open load fault. Several MOSFET vendors are listed
APPLICATIONS INFORMATION