LT8310
29
8310f
For more information www.linear.com/LT8310
applicaTions inForMaTion
Thermal Considerations
The LT8310 is rated to a maximum input voltage of 100V.
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. At a junction
temperature of 165°C, the thermal limiter shuts down the
system, which pulls the GATE pin to GND, pulls the SOUT
pin to INTV
CC
, and discharges the soft-start (SS) pin to
GND. Switching can resume after the device temperature
falls by 10°C. This function is intended to protect the device
during momentary thermal overload.
In many applications, 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(s).
For the main switch driven by the GATE pin, and a switch
(if present) on the SOUT pin, the gate-drive current can
be calculated for each as in Equation 7.
A low Q
G
power MOSFET should always be used when op-
erating at high input voltages and the switching frequency
should
also be chosen carefully
to ensure that the IC does
not exceed a safe junction temperature. The internal junc-
tion temperature of the IC can be estimated by:
T
J
= T
A
+ V
IN
• (I
Q
+ I
DRIVE (TOT)
) • θ
JA
[44]
where T
A
is the ambient temperature, I
Q
is the quiescent
current of the part (maximum 4mA), and θ
JA
is the pack-
age’s junction-to-ambient
thermal impedance (38°C/W).
For example, an application having T
A(MAX)
= 85°C,
V
IN(MAX)
= 80V, f
SW
= 200kHz, and having a MOSFET with
Q
G
= 30nC, the maximum IC junction temperature will be
approximately:
T
J
= 85°C + 80V • (4mA + 30nC • 200kHz)
• 38°C/W
≈
115°C [45]
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.
The LT8310’s internal power dissipation can be reduced
by supplying the GATE and SOUT pins (and some internal
circuits) from an external source, such as a regulated
auxiliary transformer winding. The INTV
CC
pin may be
overdriven as long as 10.5V < V
INTVCC(MAX)
< V
IN(MIN)
,
which avoids back-driving the V
IN
pin. The practical up-
per limit of IN
TV
CC
overdrive is 17.4V (typ) where the
regulator’s overvoltage threshold shuts down switching.
PCB Layout / Thermal Guidelines
For proper operation, PCB layout must be given special
attention. Critical programming signals must be able to
coexist with high dv/dt signals. Compact layout can be
achieved but not at the cost of poor thermal management.
The following guidelines should be followed to approach
optimal performance.
1. Ensure that a local bypass capacitor is used (and placed
as close as possible) between V
IN
and GND for the
controller IC(s).
2. The critical programming resistor for timing, R
T
, must
use short traces to both the RT pin and the GND pin
(exposed pad). Keep traces to the RT pin and the DFILT
pin separated.
3. The critical programming resistor for duty cycle, R
SET
,
must use short traces to both the RDVIN pin and the
INTV
CC
pin.
4. The current sense resistor for the forward converter
must use short Kelvin connections to the SENSE pin
and GND pin (exposed pad).
