LT3755-1 LINER [Linear Technology], LT3755-1 Datasheet - Page 23

• Manufacturer Part Number: LT3755-1
• Description: 60V 4-Switch Synchronous
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V
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
INTV
switch node SW1 drops low and the bootstrap capacitor
C1 is charged through D1 from INTV
MOSFET switch M3 turns on, the switch node SW2 drops
low and the bootstrap capacitor C2, is charged through D2
from INTV
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
MOSFETs, sensing resistor, inductor and PC board
traces and cause the efficiency to drop at high output
currents.
CC
2
. When the bottom MOSFET switch M2 turns on, the
R losses. These arise from the resistances of the
IN
CC
and the BST1 pin rises to approximately V
. The bootstrap capacitors C1 and C2 need to
CC
. When the bottom
IN
.
IN
+
2. Transition loss. This loss arises from the brief amount
3. INTV
4. C
5. Other losses. Schottky diode D3 and D4 are respon-
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.
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:
where C
and control currents.
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
have low ESR to minimize the AC I
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
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.
IN
Transition Loss ≈ 2.7 • V
and C
CC
current. This is the sum of the MOSFET driver
RSS
OUT
is the reverse-transfer capacitance.
loss. The input capacitor has the difficult
IN
IN
and C
2
• I
OUT
2
R loss and sufficient
OUT
• C
are required to
RSS
LT3791
• f
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3791f