Datasheet

LT3763
14
3763fa
For more information www.linear.com/LT3763
operaTion
SYNC pin which has precise thresholds at 2.175V and
1.5V for rising and falling edges, respectively.
LT3763 also features a PWM driver for LED dimming.
PWM_OUT is high when the PWM pin voltage is higher than
2.175V, and low when PWM is lower than 1.5V. Switching
is terminated when PWM is lower than 1.5V. PWM should
be tied to INTV
CC
when the PWM function is not needed.
The FAULT pin is pulled down to ground when the voltage
at FB becomes less than 0.25V which indicates a short-
circuit condition. It is also pulled down to indicate an
open-circuit condition when the voltage becomes greater
than 1.16V and the inductor current is less than ten percent
of the maximum (C/10), or equivalently, when the volt
-
age between SENSE
+
and SENSE
–
is less than 5mV. To
avoid jitter when recovering from a fault condition, 50mV
hysteresis is employed in the comparators. Additionally,
when the inductor current is lower than C/10, the C/10
comparator disables the low side MOSFET regardless of
the voltage at FB.
The integrated input current and output current monitor
-
ing functions of the LT3763 allow users to acquire system
information such as the input power and output power. The
outputs of the current monitors, IVINMON and ISMON,
range from
0V to 1V when the inputs vary from 0V to
50mV. When using 2.5mΩ sense resistors, for example,
these current monitoring amplifiers sense from 0A to
20A. To filter out the switching portion of the currents
and measure the average current information, the input
pins of the input current monitor, IVINP and IVINN, should
connect to the sense resistor through two 1k resistors
and a capacitor directly between the IVINP and IVINN
pins. The capacitance value can be adjusted according to
the switching frequency and the ripple magnitude. The
output current monitor employs an internal filter to reduce
ripple, and it does not require an external filter, but if one
is added, the corner frequency should be higher than the
switching frequency.
The LT3763 also includes an input current limiting func
-
tion to regulate the input current to a value determined by
the R
SENSE_IN
resistor. When the voltage drop across the
R
SENSE_IN
resistor approaches 50mV, the inductor current
is reduced and regulated so that 50mV is maintained across
the IVINP and IVINN pins.
applicaTions inForMaTion
Programming Inductor Current
The analog voltage at the CTRL1 pin is buffered and pro-
duces a reference voltage, V
CTRL
, across an internal resistor.
The regulated average inductor current is determined by:
I
O
=
V
CTRL
30 •R
S
where R
S
is the external sense resistor and I
O
is the aver-
age inductor current, which is equal to the output current.
Figure 2 shows the maximum output current versus R
S
.
The maximum power dissipation in the resistor will be:
P
RS
=
0.05V
( )
2
R
S
Figure 3 plots the power dissipation in R
S
, and Table 1
lists several resistance values and the corresponding
Figure 2. R
S
Value Selection for Regulated Output Current
maximum inductor current and sense-resistor power dis-
sipation. Susumu, Panasonic and Vishay offer accurate
sense resistors.
R
S
(mΩ)
0
MAXIMUM OUTPUT CURRENT (A)
10
20
30
5
15
25
4 8 12 16
3763 F02
20
20 6 10 14 18