LT3791-1_15 Datasheet, PDF (20/26 Pages) Linear Technology – 60V 4-Switch Synchronous Buck-Boost Controller

LT3791-1
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.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
37911 F10
Figure 10. Normalized R_DS(ON) vs Temperature
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
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.
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