Datasheet

LT3761
16
3761f
applicaTions inFormaTion
Besides the limitation by the minimum off-time, it is
also recommended to choose the maximum duty cycle
below 95%.
D
BOOST
=
V
LED
− V
IN
V
LED
D
BUCK _MODE
=
V
LED
V
IN
D
SEPIC
,D
CUK
=
V
LED
V
LED
+ V
IN
Thermal Considerations
The LT3761 is rated to a maximum input voltage of 60V.
Careful attention must be paid to the internal power dis-
sipation of the IC at higher input voltages to ensure that a
junction temperature of 125°C (150°C for H-grade) is not
exceeded. This junction limit is especially important when
operating at high ambient temperatures. If LT3761 junction
temperature reaches 165°C, the GATE and PWMOUT pins
will be driven to GND and the soft-start (DIM/SS) and PWM
pins will be discharged to GND. Switching will be enabled
after device temperature is reduced 10°C. This function is
intended to protect the device during momentary thermal
overload conditions.
The majority of the power dissipation in the IC comes from
the supply current needed to drive the gate capacitance of
the external power MOSFET. This gate drive current can
be calculated as:
I
GATE
= f
SW
• Q
G
A low Q
G
power MOSFET should always be used when
operating at high input voltages, and the switching fre-
quency should also be chosen carefully to ensure that
the IC does not exceed a safe junction temperature. The
internal junction temperature of the IC can be estimated by:
T
J
= T
A
+ [V
IN
(I
Q
+ f
SW
• Q
G
) • θ
JA
]
where T
A
is the ambient temperature, I
Q
is the quiescent
current of the part (maximum 2mA) and θ
JA
is the package
thermal impedance (43°C/W for the MSE package). For
example, an application has T
A(MAX)
= 85°C, V
IN(MAX)
=
40V, f
SW
= 400kHz, and having a FET with Q
G
= 20nC, the
maximum IC junction temperature will be approximately:
T
J
= 85°C + [40V • (2mA + 400kHz • 20nC) • 43°C/W] =
102°C
The exposed pad on the bottom of the package must be
soldered to a ground plane. This ground should then be
connected to an internal copper ground plane with thermal
vias placed directly under the package to spread out the
heat dissipated by the IC.
Open LED Reporting – Constant Voltage Regulation
Status Pin
The LT3761 provides an open-drain status pin, OPENLED,
that pulls low when the FB pin is within 50mV of its 1.25V
regulated voltage AND output current sensed by V
ISP-ISN
has reduced to 25mV, or 10% of the full-scale value. The
10% output current qualification (C/10) is unique for an LED
driver but fully compatible with open LED indication– the
qualification is always satisfied since for an open load, zero
current flows in the load. The C/10 feature is particularly
useful in the case where OPENLED is used to indicate the
end of a battery charging cycle and terminate charging or
transition to a float charge mode.
Figure 5. Typical Minimum On and Off
GATE Pulse Width vs Temperature
0
100
200
300
50
150
250
3761 F05
TIME (ns)
TEMPERATURE (°C)
–50 0
50
75
–25
25
100
150125
MINIMUM ON-TIME
MINIMUM OFF-TIME
C
GATE
= 3300pF