参数资料
型号: LT3682IDD#TRPBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: SWITCHING REGULATOR, PDSO12
封装: 3 X 3 MM, LEAD FREE, PLASTIC, DFN-12
文件页数: 11/24页
文件大小: 253K
代理商: LT3682IDD#TRPBF
LT3682
19
3682f
APPLICATIONS INFORMATION
cases the discharged output capacitor will present a load
to the switcher, which will allow it to start. The plots show
the worst-case situation where VIN is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
VIN; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin. At light
loads, the inductor current becomes discontinuous and
the effective duty cycle can be very high. This reduces the
minimum input voltage to approximately 300mV above
VOUT. At higher load currents, the inductor current is
continuous and the duty cycle is limited by the maximum
duty cycle of the LT3682, requiring a higher input voltage
to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3682,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC network to
create a voltage ramp at this pin. Figure 7 shows the startup
and shut-down waveforms with the soft-start circuit. By
choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply 20μA when the RUN/SS
pin reaches 2.5V.
5ms/DIV
VRUN/SS
5V/DIV
VOUT
5V/DIV
IL
1A/DIV
VRUN
5V/DIV
3682 F07
RUN/SS
RUN
GND
15k
0.22μF
Figure 7. To Soft-Start the LT3682, Add a
Resistor and Capacitor to the RUN/SS Pin
LOAD CURRENT (mA)
INPUT
VOL
TAGE
(V)
1
4.5
5
5.5
1000
3.5
4
3
2.5
2
10
100
10
100
6
LOAD CURRENT(mA)
INPUT
VOL
TAGE
(V)
3682 F06
1
6
6.5
7
7.5
1000
4.5
4
5.5
5
3.5
2.5
3
2
8
TO RUN
TO START
(WORST CASE)
VOUT = 3.3V
TA = 25C
L = 10μH
f = 800kHz
TO START
(WORST CASE)
TO RUN
VOUT = 5V
TA = 25C
L = 10μH
f = 800kHz
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 6 shows a plot of minimum load
to start and to run as a function of input voltage. In many
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