Datasheet

LT3796/LT3796-1
19
3796fa
For more information www.linear.com/3796
Figure 9. Setting Input Current Limit
For buck applications, filter components, R
IN2(OPT)
and
C
OPT
, are recommended to be placed close to LT3796/
LT3796-1 to suppress the substantial transient signal or
noise at across CSN and CSP pins. For boost and buck-
boost applications, R
IN2(OPT)
and C
OPT
are not required.
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, T
J
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 V
IN
operating
current of the part (2.5mA typical) and θ
JA
is the package
thermal impedance (30°C/W for the TSSOP package). For
example, an application with T
A(MAX)
= 85°C, V
IN(MAX)
=
60V, f
SW
= 400kHz, and having a N-channel MOSFET with
Q
G
= 20nC, the maximum IC junction temperature will be
approximately:
T
J
= 85°C + [60V • (2.5mA + 400kHz • 20nC) • 30°C/W]
≈ 104°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.
It is best if the copper plane is extended on either the top
or bottom layer of the PCB to have the maximum exposure
to air. Internal ground layers do not dissipate thermals as
much as top and bottom layer copper does. See recom-
mended layout as an example.
Input Capacitor Selection
The input capacitor supplies the transient input current for
the power inductor of the converter and must be placed
and sized according to the transient current requirements.
The switching frequency, output current and tolerable input
voltage ripple are key inputs to estimating the capacitor
value. An X7R type ceramic capacitor is usually the best
choice since it has the least variation with temperature
and DC bias. Typically, boost and SEPIC converters
APPLICATIONS INFORMATION
LT3796/LT3796-1
37961 F03
R
IN2(OPT)
R
SNS
R
IN1
CSP
FB2
CSN
TO LOAD
V
IN
I
IN
V
S
CSOUT
V
S
C
OPT
–
+
R
OUT
C
FILT
+V
SNS
–
Thermal Considerations
The LT3796/LT3796-1 are rated to a maximum input volt-
age of 100V. Careful attention must be paid to the internal
power dissipation of the IC at higher input voltages to
ensure that a junction temperature of 150°C is not ex-
ceeded. This junction limit is especially important when
operating at high ambient temperatures. The majority of
the power dissipation in the IC comes from the supply
current needed to drive the gate capacitance of the external
power N-channel MOSFET. This gate drive current can be
calculated as:
I
GATE
= f
SW
• Q
G