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参数资料
| 型号： | LT3724MPFE#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 225 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件页数： | 9/26页 |
| 文件大小： | 309K |
| 代理商： | LT3724MPFE#TR |
LT3724
17
3724fd
APPLICATIONS INFORMATION
Inductor current typically does not reach IMAX 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
smalloutputcapacitors,itcanbecomesignificant. Thevolt-
age rise can be reduced by increasing output capacitance,
which puts additional limitations on COUT for these low
voltage supplies. Another approach is to add an external
current limit foldback circuit which reduces the value of
IMAX 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 (VOUT)totheLT3724’sVCpin. Thislimits
the peak current to 0.25 IMAX when VOUT = 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 CSS 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
IMAX when VOUT = 0V.
Efficiency Considerations
Theefficiencyofaswitchingregulatorisequaltotheoutput
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 VIN and VCC current loss
2. I2R conduction losses
3. MOSFET transition loss
4. Schottky diode conduction loss
1. The VIN and VCC 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, QG. QG is found in
the MOSFET data sheet. The average charging current is
calculated as QG fSW. The power loss term due to these
currents can be reduced by backdriving VCC with a lower
voltage than VIN such as VOUT.
Figure 7. Current Limit Foldback Circuit
for Applications that use Soft-Start
1N4148
47k
VOUT
VC
3724 F03
Figure 8. Current Limit Foldback Circuit for Applications
that have Soft-Start Disabled (CSS Pin Shorted to SGND)
1N4148
27k
39k
VOUT
VC
3724 F07
1N4148
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相关代理商/技术参数 |
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| LT3726EGN | 制造商:Linear Technology 功能描述: |
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