Datasheet

LT3791
22
3791fa
applicaTions inForMaTion
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T
J
= T
A
+ P • R
TH(JA)
The R
TH(JA)
to be used in the equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(JC)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
diode to be effective, the inductance between it and the
synchronous switch must be as small as possible, mandat-
ing that these components be placed adjacently.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV
CC
pin from the V
IN
supply pin. INTV
CC
powers
the drivers and internal circuitry within the LT3791. The
INTV
CC
pin regulator can supply a peak current of 67mA
and must be bypassed to ground with a minimum of 4.7µF
ceramic capacitor or low ESR electrolytic capacitor. An
additional 0.1µF ceramic capacitor placed directly adjacent
to the INTV
CC
and PGND IC pins is highly recommended.
Good bypassing is necessary to supply the high transient
current required by MOSFET gate
drivers.
Higher input voltage
applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LT3791 to be
exceeded. The system supply current is normally dominated
by the gate charge current. Additional external loading of
the INTV
CC
also needs to be taken into account for the
power dissipation calculations. Power dissipation for the
IC in this case is V
IN
• I
INTVCC
, and overall efficiency is
lowered. The junction temperature can be estimated by
using the equations given
T
J
= T
A
+ (P
D
• θ
JA
)
where θ
JA
(in °C/W) is the package thermal impedance.
For example, a typical application operating in continuous
current operation might draw 24mA from a 24V supply:
T
J
= 70°C + 24mA • 24V • 28°C/W = 86°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V
IN
.
Figure 10. Normalized R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE (Ω)
1.0
1.5
150
3791 F10
0.5
0
0
50
100
2.0
Optional Schottky Diode (D3, D4) Selection
The Schottky diodes D3 and D4 shown in the Typical Ap-
plications section conduct during the dead time between
the conduction of the power MOSFET switches. They
are intended to prevent the body diode of synchronous
switches M2 and M4 from turning on and storing charge
during the dead time. In particular, D4 significantly reduces
reverse-recovery current between switch M4 turn-off and
switch M3 turn-on, which improves converter efficiency
and reduces switch M3 voltage stress. In order for the