LT3797
22
3797fa
For more information www.linear.com/LT3797
applicaTions inForMaTion
High Side PMOS Disconnect Switch Selection
A PMOS with a minimum V
GS(TH)
of –1V to –2V is rec-
ommended for the high side disconnect switch in most
LT3797
applications to improve the PWM dimming ratio
and protect the LED array from excessive heating during
fault conditions. The PMOS BV
DSS
rating must be higher
than the open-LED regulation voltage set by the FBH pin.
The maximum continuous drain current I
D(MAX)
rating
should be higher than the maximum LED current.
Input Capacitor Selection
The input capacitor C
IN
supplies the AC ripple current to
the power inductor of the converter and must be placed
and sized according to the transient current requirements.
The switching frequency, output current and tolerable input
voltage ripple are key inputs to estimating the required
capacitor value. The X5R or X7R type ceramic capacitors
are usually good choices since they have small variation
with temperature and DC bias. Typically, the boost or SEPIC
converter requires a lower value input capacitor than the
buck mode or buck-boost mode converter, due to the fact
that its inductor is in series with the input, and the input
current waveform is continuous. The input capacitor value
can be estimated based on the inductor ripple ∆I
L
(refer to
Inductor Selection section), the switching frequency, and
the acceptable input voltage ripple ∆V
IN
on C
IN
. C
IN
value
of the boost and SEPIC converter can be calculated by:
C
IN
= 0.125•
L
∆V
• f
C
IN
value of the buck mode and buck-boost mode LED
driver can be calculated by:
C
IN
=I
LED
•
V
LED
• V
IN(MIN)
– V
LED
V
IN(MIN)
2
• ∆V
IN
• f
Output Capacitor Selection
The output filter capacitors should be sized to attenuate
the LED current ripple. Use of X5R or X7R type ceramic
capacitors is recommended. To achieve the same LED
ripple current, the required filter capacitor is smaller in the
buck mode applications than that in the boost, buck-boost
mode and SEPIC applications. This is due to the fact that,
in the buck converter, the inductor is in series with the
output and the ripple current flowing through the output
capacitor is continuous. Lower operating frequencies will
require proportionately higher capacitor values.
The DC Coupling Capacitor Selection for
SEPIC LED Driver
The DC voltage rating of the DC coupling capacitor, C
DC
,
connected between the primary and secondary inductors
should be larger than the maximum input voltage:
V
CDC
> V
IN(MAX)
C
DC
has nearly a rectangular current waveform in CCM.
During the switch off-time, the current through C
DC
is I
VIN
,
while approximately –I
LED
flows during the on-time. The
C
DC
voltage ripple causes distortions on the primary and
secondary inductor current waveforms. The C
DC
should
be sized to limit its voltage ripple. The power loss on the
C
DC
ESR reduces the LED driver efficiency. Therefore, the
sufficient low ESR ceramic capacitors should be selected.
The X5R or X7R ceramic capacitor is recommended for C
DC
.
Integrated INTV
CC
Power Supply
The LT3797 includes an internal switch mode DC/DC con-
verter to generate a regulated 7.5V INTV
CC
power supply
to power the NMOS gate drivers of the three channels
(I
DRIVE
). This INTV
CC
power supply can also be used to
drive external circuits (I
EXT
). This INTV
CC
power supply
