Datasheet

LT3480
15
3480fe
For more information www.linear.com/LT3480
APPLICATIONS INFORMATION
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 VOLTAGE (V)
4.0
4.5
5.0
10000
3.5
3.0
2.0
10 100 1000
1 1000010 100 1000
2.5
6.0
5.5
TO START
(WORST CASE)
TO RUN
LOAD CURRENT (A)
INPUT VOLTAGE (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
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3480 is turned on with its RUN/SS 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 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 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
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.