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參數(shù)資料
| 型號： | LT3724MPFE#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 225 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁數(shù)： | 7/26頁 |
| 文件大小： | 309K |
| 代理商： | LT3724MPFE#TR |
LT3724
15
3724fd
APPLICATIONS INFORMATION
The VFB pin input bias current is typically 25nA, so use
of extremely high value feedback resistors could cause a
converter output that is slightly higher than expected. Bias
current error at the output can be estimated as:
VOUT(BIAS) = 25nA R2
Supply UVLO and Shutdown
The SHDN pin has a precision voltage threshold with
hysteresis which can be used as an undervoltage lockout
threshold (UVLO) for the power supply. Undervoltage
lockout keeps the LT3724 in shutdown until the supply
input voltage is above a certain voltage programmed by
theuser.Thehysteresisvoltagepreventsnoisefromfalsely
tripping UVLO.
Resistors are chosen by first selecting RB. Then:
RA =RB
VSUPPLY(ON)
1.35V
– 1
VSUPPLY(ON) is the input voltage at which the undervoltage
lockout is disabled and the supply turns on.
Example: Select RB = 49.9kΩ, VSUPPLY(ON) =14.5V(based
on a 15V minimum input voltage)
RA = 49.9kΩ
14.5V
1.35V
– 1
= 486.1kΩ (499kΩ resistor is selected)
If low supply current in standby mode is required, select
a higher value of RB.
The supply turn off voltage is 9% below turn on. In the
example the VSUPPLY(OFF) would be 13.2V.
If additional hysteresis is desired for the enable function,
an external positive feedback resistor can be used from
the LT3724 regulator output.
The shutdown function can be disabled by connecting the
SHDN pin to the VIN through a large value pull-up resistor.
Thispincontainsalowimpedanceclampat6V,sotheSHDN
pin will sink current from the pull-up resistor(RPU):
I
SHDN=
VIN – 6V
RPU
Because this arrangement will clamp the SHDN pin to the
6V,itwillviolatethe5Vabsolutemaximumvoltageratingof
the pin. This is permitted, however, as long as the absolute
maximum input current rating of 1mA is not exceeded.
Input SHDN pin currents of <100A are recommended: a
1MΩ or greater pull-up resistor is typically used for this
configuration.
Soft-Start
The soft-start function forces the programmed slew rate
while the converter output rises to 95% of regulation,
which corresponds to 1.185V on the VFB pin. Once 95%
regulation is achieved, the soft-start circuit is disabled.
The soft-start circuit will re-enable when the VFB pin drops
below 70% of regulation, which corresponds to 300mV
of control hysteresis on the VFB pin. This allows for a
controlled recovery from a “brown-out” condition.
Figure 2. Output Voltage Feedback Divider
Figure 3. Undervoltage Lockout Circuit
L1
VFB PIN
R2
R1
VOUT
COUT
3724 F02
SHDN PIN
RA
RB
VSUPPLY
3724 F03
Figure 4.Soft-Start Circuit
RSS
LT3724
VOUT
CSS1
CSS
3724 F04
A
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