Datasheet
LT1913
15
1913f
more than 2.3V above the SW pin for best effi ciency. For
outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1µF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1µF boost
capacitor (see Figure 4b). For lower output voltages the
boost diode can be tied to the input (Figure 4c), or to
another supply greater than 2.8V. The circuit in Figure 4a
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
1913 F05
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 = 600kHz
V
OUT
= 5V
T
A
= 25°C
L = 8.2µH
f = 600kHz
TO START
(WORST CASE)
APPLICATIONS INFORMATION
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT1913
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
D2
GND
LT1913
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT1913
1913 FO4
(4a) For V
OUT
> 2.8V
(4b) For 2.5V < V
OUT
< 2.8V
(4c) For V
OUT
< 2.5V
Figure 4. Three Circuits For Generating The Boost Voltage
is more effi cient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT1913 application
is limited by the minimum input voltage (3.6V) and by the
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 LT1913 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