LT3756-1 LINER [Linear Technology], LT3756-1 Datasheet - Page 16

• Manufacturer Part Number: LT3756-1
• Description: 60VIN LED Controller with Internal PWM Generator
• Manufacturer: LINER [Linear Technology]

LT3761
applicaTions inFormaTion
Besides the limitation by the minimum off-time, it is
also recommended to choose the maximum duty cycle
below 95%.
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.
16
D
D
D
BOOST
BUCK _ MODE
SEPIC
, D
300
250
200
150
100
Figure 5. Typical Minimum On and Off
GATE Pulse Width vs Temperature
=
50
0
CUK
–50
V
C
LED
GATE
V
–25
=
=
LED
= 3300pF
− V
V
V
V
MINIMUM OFF-TIME
LED
LED
MINIMUM ON-TIME
0
IN
IN
V
TEMPERATURE (°C)
LED
+ V
25
IN
50
75
100
125
3761 F05
150
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:
A low Q
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:
where T
current of the part (maximum 2mA) and θ
thermal impedance (43°C/W for the MSE package). For
example, an application has T
40V, f
maximum IC junction temperature will be approximately:
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
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.
I
T
T
102°C
GATE
J
J
SW
= 85°C + [40V • (2mA + 400kHz • 20nC) • 43°C/W] =
= T
A
G
= f
A
= 400kHz, and having a FET with Q
is the ambient temperature, I
power MOSFET should always be used when
+ [V
SW
• Q
IN
(I
G
Q
+ f
SW
• Q
G
A(MAX)
) • θ
JA
]
= 85°C, V
Q
JA
is the quiescent
is the package
G
= 20nC, the
IN(MAX)
ISP-ISN
3761f
=