Datasheet

LT3791-1
19
37911f
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.
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 LT3791-1 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
.
Top Gate (TG) MOSFET Driver Supply (C1, D1, C2, D2)
The external bootstrap capacitors C1 and C2 connected
to the BST1 and BST2 pins supply the gate drive voltage
for the topside MOSFET switches M1 and M4. When the
top MOSFET switch M1 turns on, the switch node SW1
rises to V
IN
and the BST1 pin rises to approximately V
IN
+
INTV
CC
. When the bottom MOSFET switch M2 turns on, the
switch node SW1 drops low and the bootstrap capacitor
C1 is charged through D1 from INTV
CC
. When the bottom
MOSFET switch M3 turns on, the switch node SW2 drops
low and the bootstrap capacitor C2, is
charged through D2
from
INTV
CC
. The bootstrap capacitors C1 and C2 need to
store about 100 times the gate charge required by the top
MOSFET switch M1 and M4. In most applications a 0.1µF
to 0.47µF, X5R or X7R ceramic capacitor is adequate.
applicaTions inForMaTion
Figure 10. Normalized R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE (Ω)
1.0
1.5
150
37911 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
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 LT3791-1. The