Datasheet

LT3760
14
3760fc
The power rating of RS should be selected to exceed
the I
2
R losses in the resistor. The peak inductor current
should be recalculated for the chosen RS value to ensure
the chosen inductor will not saturate.
Power MOSFET: Overcurrent and Hiccup Mode
For severe external faults which may cause the external
MOSFET to reach currents greater than the peak current
defined by RS and the 52mV sense pin threshold described
above, the LT3760 has an overcurrent comparator which
triggers soft start and turns off the MOSFET driver for
currents exceeding,
I
(OVERCURRENT )
=
100mV
RS
In this fault mode the LT3760 only allows MOSFET turn
on for approximately 100ns every 2ms. This hiccup mode
significantly reduces the power rating required for the
MOSFET.
applicaTions inForMaTion
Soft Start
To limit inductor inrush current and output voltage during
startup or recovery from a fault condition, the LT3760 pro-
vides a soft start function. The LT3760 when entering these
faults will discharge an internal soft start node and prevent
switching at the GATE pin for any of the following faults: V
IN
,
SHDN/UVLO or INTV
CC
voltages too low or MOSFET current
too high (see the timing diagram in Figure 2). When exiting
these faults the LT3760 ramps up an internal soft start
node at approximately 0.5V/ms to control V
C
pin voltage
rise and hence control MOSFET switch current rise. In ad-
dition the soft start period gradually ramps up switching
frequency from approximately 33% to 100% of full scale.
The conditions required to exit all faults and allow a soft
start ramp of the V
C
pin are listed in Figure 2. An added
feature of the LT3760 is that it waits for the first PWM pin
active high (minimum 200ns pulse width) before it allows
Figure 2. LT3760 Fault Detection and Soft Start Timing for V
C
Pin and Internal SS Node
GATE
3760 F02
V
C
0.4V + V
BE
(V
C
SWITCHING
THRESHOLD)
0.1V + V
BE
0.5V/ms
0.5V/ms
SS
(INTERNAL)
0.4V
0.1V
ANY OF THE FOLLOWING FAULTS
TRIGGERS SOFT START LATCH
WITH GATE TURNED OFF
IMMEDIATELY:
V
IN
< 3.7V, SHDN < 1.476V,
INTVCC < 3.4V
I
DSS
(EXTERNAL MOSFET) > 100mV/RS
SOFT-START LATCH RESET REQUIRES
ALL CONDITIONS SATISFIED:
SS (INTERNAL) < 0.2V, V
IN
≥ 4.2V,
SHDN > 1.476V, INTV
CC
> 3.8V,
I
DSS
(EXTERNAL MOSFET) < 100mV/RS,
PWM > 1.4V (FOR AT LEAST 200ns)
SOFT-START
LATCH SET:
V
C MIN
CLAMP