Datasheet

LT3686
17
3686fc
the LT3686 is turned on with its EN/UVLO pin when the
output 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 the 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 9 shows plots of minimum load to start
and to run as a function of input voltage. In many cases
ApplicAtions inForMAtion
the discharged output capacitor will present a load to the
switcher which will allow it to start. 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 400mV above V
OUT
. At
higher load currents, the inductor current is continuous
and the duty cycle is limited by the maximum duty cycle,
requiring a higher input voltage to maintain regulation.
As the LT3686 enters dropout, the boost capacitor voltage
will be limited by V
OUT
, which is fixed by the maximum duty
cycle. If the boost capacitor’s voltage during dropout falls
Figure 9a. Typical Minimum Input Voltage, V
OUT
= 3.3V,
f = 1MHz, L = 15µH, Mode < 0.4V
I
LOAD
(mA)
1
0
V
IN
(V)
1
2
4
5
6
3
7
10 100 1000
3686 F09a
START
RUN
SUSTAIN
I
LOAD
(mA)
1
0
V
IN
(V)
1
2
4
5
6
7
8
3
9
10 100 1000
3686 F09b
START
RUN
SUSTAIN
I
LOAD
(mA)
1
0
V
IN
(V)
1
2
4
5
6
7
3
10 100 1000
3686 F09c
RUN
I
LOAD
(mA)
1
0
V
IN
(V)
1
2
4
5
7
6
8
3
10 100 1000
3686 F09d
RUN
Figure 9b. Typical Minimum Input Voltage, V
OUT
= 5V,
f = 1MHz, L = 22µH, Mode < 0.4V
Figure 9c. Typical Minimum Input Voltage, V
OUT
= 3.3V,
f = 1MHz, L = 15µH, Mode > 0.8V
Figure 9d. Typical Minimum Input Voltage, V
OUT
= 5V,
f = 1MHz, L = 22µH, Mode > 0.8V