LT3480
15
3480fb
APPLICATIONS INFORMATION
the worst-case situation where V
IN
is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
V
IN
; 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 discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V
OUT
. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3480, requiring a higher
input voltage to maintain regulation.
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
GND
LT3480
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
D2
GND
LT3480
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
GND
LT3480
3480 FO5
(5a) For V
OUT
> 2.8V
(5b) For 2.5V < V
OUT
< 2.8V
(5c) For V
OUT
< 2.5V; V
IN(MAX)
= 30V
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3480 F06
LOAD CURRENT (A)
1
INPUT VOL
TAGE (V)
4.0
4.5
5.0
10000
3.5
3.0
2.0
10
100
1000
1
10000
10
100
1000
2.5
6.0
5.5
TO START
(WORST CASE)
TO RUN
LOAD CURRENT (A)
INPUT VOL
TAGE (V)
5.0
6.0
7.0
4.0
2.0
3.0
8.0
TO RUN
V
OUT
= 3.3V
T
A
= 25°C
L = 8.2μH
f = 700kHz
V
OUT
= 5V
T
A
= 25°C
L = 8.2μH
f = 700kHz
TO START
(WORST CASE)
Figure 5. Three Circuits For Generating The Boost Voltage
Soft-Start
The RUN/SS pin can be used to soft-start the LT3480, reduc-
ing the maximum input current during start-up. The RUN/SS
pin is driven through an external RC fi lter to create a voltage
ramp at this pin. Figure 7 shows the start-up 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.
Synchronization
To select Low-Ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
