LT3682 LINER [Linear Technology], LT3682 Datasheet - Page 19

• Manufacturer Part Number: LT3682
• Description: 1A Micropower Step-Down
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
5.5
4.5
3.5
2.5
6.5
5.5
4.5
3.5
2.5
7.5
4
3
2
5
4
3
2
6
5
8
7
6
1
1
TO RUN
TO RUN
TO START
(WORST CASE)
TO START
(WORST CASE)
LOAD CURRENT (mA)
LOAD CURRENT(mA)
10
10
100
100
V
T
L = 10μH
f = 800kHz
OUT
A
V
T
L = 10μH
f = 800kHz
OUT
A
= 25˚C
= 25˚C
= 3.3V
= 5V
3682 F06
1000
1000
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 V
For lower start-up voltage, the boost diode can be tied to
V
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
V
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.
0.22μF
IN
OUT
RUN
; however, this restricts the input range to one-half of
15k
. At higher load currents, the inductor current is
Figure 7. To Soft-Start the LT3682, Add a
Resistor and Capacitor to the RUN/SS Pin
RUN/SS
GND
V
1A/DIV
5V/DIV
RUN/SS
5V/DIV
5V/DIV
V
V
RUN
OUT
I
L
IN
is ramping very slowly.
5ms/DIV
LT3682
19
3682 F07
3682f