Datasheet

LT3761
17
3761f
Figure 6. OPENLED Logic Block Diagram
+
–
1.2V
FB
PWM
3761 F06
OPENLED
OPEN LED
COMPARATOR
C10
COMPARATOR
1mA
ISN
25mV
ISP
1. OPENLED ASSERTS WHEN V
ISP-ISN
< 25mV AND FB > 1.2V, AND IS LATCHED
2. OPENLED DE-ASSERTS WHEN FB < 1.19V, AND PWM LOGIC 1 = 1V
3. ANY FAULT CONDITION RESETS THE LATCH, SO LT3761 STARTS UP
WITH OPENLED DE-ASSERTED
R
LED
I
LED
+
–
S Q
R
+
–
applicaTions inFormaTion
For monitoring the LED string voltage, if the open LED
clamp voltage is programmed correctly using the FB
resistor divider then the FB pin should not exceed 1.18V
when LEDs are connected. If the OPENLED pulldown is
asserted and the PWM pin transitions low, the pulldown
will continue to be asserted until the next rising edge of
PWM even if FB falls below the OPENLED threshold.
Therefore, a 10μF capacitor is an appropriate selection
for a 400kHz boost regulator with 12V input, 48V output
and 1A load.
With the same V
IN
voltage ripple of 100mV, the input ca-
pacitor for a buck converter can be estimated as follows:
C
IN
(µF) = I
LED
(A) • t
SW
(µs) • 4.7 •
µF
A • µs






A 10μF input capacitor is an appropriate selection for a
400kHz buck mode converter with a 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. In this buck
converter case 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.
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
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 X7R type ceramic capacitors 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.
Input Capacitor Selection
The input capacitor supplies the transient input current for
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 capacitor
value. An X7R type ceramic capacitor is usually the best
choice since it has the least variation with temperature and
DC bias. Typically, boost and SEPIC converters require a
lower value capacitor than a buck mode converter. As-
suming that a 100mV input voltage ripple is acceptable,
the required capacitor value for a boost converter can be
estimated as follows:
C
IN
(µF) = I
LED
(A) •
V
OUT
V
IN
• t
SW
(µs) •
µF
A • µs





