Datasheet

LT3975
18
3975f
APPLICATIONS INFORMATION
Figure 5. The Minimum Input Voltage Depends on Output Voltage and Load Current
BOOST
LT3975
(4a) For 3.2V ≤ V
OUT
≤ 16V
GND
V
IN
V
IN
SW
OUT
V
OUT
BOOST
LT3975
(4d) For V
OUT
< 2.5V, 3.1V ≤ V
S
≤ 16V
GND
V
IN
V
IN
SW
OUT
V
OUT
V
S
BOOST
LT3975
(4e) For V
OUT
> 16V, 3.1V ≤ V
S
≤ 16V
GND
V
IN
V
IN
SW
OUT
V
OUT
3875 F04
V
S
BOOST
LT3975
(4c) For V
OUT
< 2.5V, V
IN
< 27V
GND
V
IN
V
IN
SW
OUT
V
OUT
BOOST
LT3975
(4b) For 2.5V ≤ V
OUT
≤ 3.2V
GND
V
IN
V
IN
SW
OUT
V
OUT
Figure 4. Five Circuits for Generating the Boost Voltage
LOAD CURRENT (A)
0
INPUT VOLTAGE (V)
5.5
6.0
6.5
2
3975 F05a
5.0
4.5
4.0
0.5
1
1.5
2.5
V
OUT
= 5V
f
SW
= 800kHz
TO RUN/TO START
LOAD CURRENT (A)
0
INPUT VOLTAGE (V)
4.0
4.5
5.0
2
3975 F05b
3.5
3.0
2.5
0.5
1
1.5
2.5
V
OUT
= 3.3V
FRONT PAGE APPLICATION
TO RUN/TO START
When the output is above 16V, the OUT pin can not be tied
to the output or the OUT pin abs max will be violated. It
should instead be tied to GND (Figure 4e). This is to pre-
vent the dropout circuitry from interfering with switching
behavior and to prevent the 100mA active pull-down from
drawing power. It is important to note that when the output
is above 16V and the OUT pin is grounded, the dropout
circuitry is not connected, so the minimum dropout will
be about 1.5V, rather than 500mV. If the output is less than
3.2V and an external Schottky is used to charge the boost
capacitor, the OUT pin should still be tied to the output
even though the minimum input voltage of the LT3975 will
be limited by the 4.3V minimum rather than the minimum
dropout voltage.
With the OUT pin connected to the output, a 100mA ac-
tive load will charge the boost capacitor during light load
start-up and an enforced 500mV minimum dropout voltage
will keep the boost capacitor charged across operating
conditions (see Minimum Dropout Voltage section). This
yields excellent start-up and dropout performance. Figure 5
shows the minimum input voltage for 3.3V and 5V outputs.