LT3791
23
3791fb
For more information www.linear.com/LT3791
applicaTions inForMaTion
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.
Efficiency Considerations
The power efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all
dissipative
elements in circuits produce losses, four main sources
account for most of the losses in LT3791 circuits:
1. DC I
2
R losses. These arise from the resistances of the
MOSFETs, sensing resistor, inductor and PC board
traces and cause the efficiency to drop at high output
currents.
2. Transition loss. This loss arises from the brief amount
of time switch M1 or switch M3 spends in the saturated
region during switch node transitions. It depends upon
the input voltage, load current, driver strength and
MOSFET capacitance, among other factors. The loss
is significant at input voltages above 20V and can be
estimated from:
Transition Loss ≈ 2.7 • V
IN
2
• I
OUT
• C
RSS
• f
where C
RSS
is the reverse-transfer capacitance.
3. INTV
CC
current. This is the sum of the MOSFET driver
and control currents.
4. C
IN
and C
OUT
loss. The input capacitor has the difficult
job of filtering the large RMS input current to the regu-
lator in
buck operation. The output capacitor has the
difficult
job of filtering the large RMS output current
in boost operation. Both C
IN
and C
OUT
are required to
have low ESR to minimize the AC I
2
R loss and sufficient
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
5.
Other losses. Schottky diode D3 and D4 are respon
-
sible for conduction losses during dead time and light
load conduction periods. Inductor core loss occurs
predominately at light loads. Switch M3 causes reverse
recovery current loss in boost operation.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in the input
current, then there is no change in efficiency.
