lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 17

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Inductor current typically doesn’t reach I
cycles that occur before soft-start becomes active, but can
with high input voltages or small inductors, so the above
relation is useful as a worst-case scenario.
This energy transfer increase in output voltage is typically
small, but for some low voltage applications with relatively
small output capacitors, it can become significant. The
voltage rise can be reduced by increasing output capaci-
tance, which puts additional limitations on C
low voltage supplies. Another approach is to add an
external current limit foldback circuit which reduces the
value of I
An external current limit foldback circuit can be easily
incorporated into an LT3724 DC/DC converter application
by placing a 1N4148 diode and a 47kΩ resistor from the
converter output (V
the peak current to 0.25 • I
limit foldback circuit also has the added advantage of
providing reduced output current in the DC/DC converter
during short-circuit fault conditions, so a foldback circuit
may be useful even if the soft-start function is disabled.
If the soft-start circuit is disabled by shorting the C
to ground, the external current limit foldback circuit must
be modified by adding an additional diode and resistor.
The 2-diode, 2-resistor network shown also provides 0.25
• I
MAX
when V
MAX
Figure 7. Current Limit Foldback Circuit
for Applications that use Soft-Start
during start-up.
OUT
= 0V.
OUT
U
) to the LT3724’s V
V
V
OUT
U
MAX
C
47k
1N4148
when V
3724 F03
W
OUT
C
= 0V. A current
MAX
pin. This limits
OUT
U
in the few
for these
SS
pin
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%.
Express percent efficiency as:
where L1, L2, etc. are individual loss terms as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main contributors usually account for most of
the losses in LT3724 circuits:
1. LT3724 V
2. I
3. MOSFET transition loss
4. Schottky diode conduction loss
1. The V
currents of the LT3724 and the MOSFET drive currents.
The quiescent currents are in the LT3724 Electrical Char-
acteristics table. The MOSFET drive current is a result of
charging the gate capacitance of the power MOSFET each
cycle with a packet of charge, Q
MOSFET data sheet. The average charging current is
calculated as Q
currents can be reduced by backdriving V
voltage than V
% Efficiency = 100% - (L1 + L2 + L3 + ...)
2
Figure 8. Current Limit Foldback Circuit for Applications
that have Soft-Start Disabled (C
R conduction losses
IN
and V
IN
IN
and V
G
CC
such as V
• f
SW
currents are the sum of the quiescent
CC
. The power loss term due to these
39k
current loss
V
OUT
V
OUT
C
27k
1N4148
1N4148
3724 F07
.
SS
Pin Shorted to SGND)
G
. Q
G
is found in the
CC
LT3724
with a lower
17
3724fb