LT3493-3 LINER [Linear Technology], LT3493-3 Datasheet - Page 13

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LT3493-3

Manufacturer Part Number
LT3493-3
Description
Manufacturer
LINER [Linear Technology]
Datasheet
APPLICATIO S I FOR ATIO
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. In many cases the discharged
output capacitor will present a load to the switcher which
will allow it to start. For 3.3V applications, the undervolt-
age lockout is high enough (6.8V) that the boost capacitor
always gets charged. For 5V applications with output
loads less than 20mA, the minimum input voltage required
to charge the boost capacitor is 6.8V. Note this higher
input voltage requirements is only in worst-case situation
where V
voltage, the boost diode can be tied to V
Figure 6. To Soft-Start the LT3493-3, Add a Resistor and Capacitor to the SHDN pin. V
IN
is being ramped very slowly. For lower start-up
0.1µF
RUN
15k
RUN
U
SHDN
SHDN
U
GND
GND
W
IN
; however this
U
restricts the input range to one-half of the absolute maxi-
mum rating of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
400mV above V
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3493-3, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT3493-3,
reducing the maximum input current during start-up. The
SHDN pin is driven through an external RC filter to create
a voltage ramp at this pin. Figure 6 shows the start-up
waveforms with and without the soft-start circuit. By
0.5A/DIV
0.5A/DIV
10V/DIV
10V/DIV
2V/DIV
2V/DIV
V
V
V
V
OUT
OUT
SW
SW
I
I
L
L
V
V
L = 10µH
C
V
V
L = 10µH
C
IN
OUT
OUT
IN
OUT
OUT
= 12V
= 12V
= 3.3V
= 10µF
= 3.3V
= 10µF
IN
OUT
= 12V, V
. At higher load currents, the inductor
20µs/DIV
20µs/DIV
OUT
= 3.3V, C
OUT
3493-3 F07a
3493-3 F07b
= 10µF, R
LT3493-3
LOAD
13
= 5Ω
3493-3f

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