Datasheet

LT3972
15
3972fa
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
GND
LT3972
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
D2
GND
LT3972
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7μF
C3
GND
LT3972
3972 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
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3972, requiring a higher
input voltage to maintain regulation.
Soft-Start
The RUN/SS pin can be used to soft-start the LT3972,
reducing 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 shutdown 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.
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
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3972 F06
LOAD CURRENT (mA)
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 (mA)
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)
APPLICATIONS INFORMATION
Figure 5. Three Circuits For Generating The Boost Voltage
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