Datasheet

LT3724
17
3724fd
APPLICATIONS INFORMATION
Inductor current typically does not reach I
MAX
in the few
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 volt-
age rise can be reduced by increasing output capacitance,
which puts additional limitations on C
OUT
for these low
voltage supplies. Another approach is to add an external
current limit foldback circuit which reduces the value of
I
MAX
during start-up.
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
OUT
) to the LT3724’s V
C
pin. This limits
the peak current to 0.25 • I
MAX
when V
OUT
= 0V. A cur-
rent 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
SS
pin
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
OUT
= 0V.
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:
% Efficiency = 100% - (L1 + L2 + L3 + ...)
where L1, L2, etc. are individual loss terms as a percent-
age 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
IN
and V
CC
current loss
2. I
2
R conduction losses
3. MOSFET transition loss
4. Schottky diode conduction loss
1. The V
IN
and V
CC
currents are the sum of the quiescent
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
G
. Q
G
is found in
the MOSFET data sheet. The average charging current is
calculated as Q
G
• f
SW
. The power loss term due to these
currents can be reduced by backdriving V
CC
with a lower
voltage than V
IN
such as V
OUT
.
Figure 7. Current Limit Foldback Circuit
for Applications that use Soft-Start
1N4148
47k
V
OUT
V
C
3724 F03
Figure 8. Current Limit Foldback Circuit for Applications
that have Soft-Start Disabled (C
SS
Pin Shorted to SGND)
1N4148
27k
39k
V
OUT
V
C
3724 F07
1N4148