Datasheet

LT3761
18
3761f
applicaTions inFormaTion
Soft-Start Capacitor Selection
For many applications, it is important to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltage overshoot. Connect a capacitor from the DIM/SS
pin to GND to use this feature. The soft-start interval is
set by the softstart capacitor selection according to the
equation:
T
SS
= C
SS
•
1.2V
12µA
= C
SS
•
100µs
nF
provided there is no additional current supplied to the
DIM/SS pin for programming the duty cycle of the PWM
dimming signal generator. A typical value for the soft-start
capacitor is 10nF which gives a 1ms start-up interval. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch.
The soft-start capacitor discharges if one of the follow-
ing events occurs: the EN/UVLO falls below its threshold;
output overcurrent is detected at the ISP/ISN pins; IC
overtemperature; or INTV
CC
undervoltage. During start-
up with EN/UVLO, charging of the soft-start capacitor is
enabled after the first PWM high period. In the start-up
sequence, after switching is enabled by PWM the switch-
ing continues until V
ISP-ISN
> 25mV or DIM/SS > 1V. PWM
pin negative edges during this start-up interval are not
processed until one of these two conditions are met so
that the regulator can reach steady state operation shortly
after PWM dimming commences.
Power MOSFET Selection
The selection criteria for the power MOSFET includes
the drain-source breakdown voltage (V
DS
), the threshold
voltage (V
GS(TH)
), the on-resistance (R
DS(ON)
), the gate
to source and gate to drain charges (Q
GS
and Q
GD
), the
maximum drain current (I
D(MAX)
) and the MOSFET’s thermal
resistances (R
θJC
, R
θJA
).
For applications operating at high input or output voltages,
the power 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
regula-
tor on the LT3761 has a fixed current limit to protect the IC
from excessive power dissipation at high V
IN
, so the FET
should be chosen so that the product of Q
G
at 7.85V 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. The required power MOSFET
V
DS
rating of different topologies can be estimated using
the following equations plus a diode forward voltage, and
any additional ringing across its drain-to-source during
its off-time.
Boost: V
DS
> V
LED
Buck Mode: V
DS
> V
IN(MAX)
SEPIC, Inverting: V
DS
> V
IN(MAX)
+ V
LED
Since the LT3761 gate driver is powered from the 7.85V
INTV
CC
, the 6V rated MOSFET works well for all the LT3761
applications.
It is prudent to measure the MOSFET temperature in
steady state to ensure that absolute maximum ratings
are not exceeded.
Several MOSFET vendors are listed in Table 4. The MOSFETs
used in the application circuits in this data sheet have
been found to work well with the LT3761. Consult factory
applications for other recommended MOSFETs.
Table 4. Recommended Power MOSFET Manufacturers
MANUFACTURER WEB
Vishay Siliconix www.vishay.com
Infineon www.infineon.com
Renesas www.renesas.com