LT3975
APPLICATIONS INFORMATION
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
drawingpower. Itisimportanttonotethatwhentheoutput
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-upandanenforced500mVminimumdropoutvoltage
will keep the boost capacitor charged across operating
conditions (see Minimum Dropout Voltage section). This
yieldsexcellentstart-upanddropoutperformance.Figure5
showstheminimuminputvoltagefor3.3Vand5Voutputs.
BOOST
LT3975
BOOST
LT3975
BOOST
V
V
SW
V
V
SW
V
V
IN
SW
IN
IN
IN
IN
IN
LT3975
GND
OUT
OUT
OUT
V
V
V
OUT
OUT
OUT
GND
GND
(4a) For 3.2V ≤ V
≤ 16V
(4b) For 2.5V ≤ V
≤ 3.2V
(4c) For V < 2.5V, V < 27V
OUT IN
OUT
OUT
V
S
V
S
BOOST
BOOST
LT3975
V
V
SW
V
IN
V
IN
SW
IN
IN
LT3975
GND
OUT
OUT
V
V
OUT
OUT
GND
3875 F04
(4d) For V
< 2.5V, 3.1V ≤ V ≤ 16V
(4e) For V > 16V, 3.1V ≤ V ≤ 16V
OUT S
OUT
S
Figure 4. Five Circuits for Generating the Boost Voltage
6.5
6.0
5.5
5.0
4.5
4.0
5.0
V
f
= 5V
V
= 3.3V
OUT
SW
OUT
= 800kHz
FRONT PAGE APPLICATION
4.5
4.0
3.5
3.0
2.5
TO RUN/TO START
TO RUN/TO START
0
0.5
1
1.5
2
2.5
0
0.5
1
1.5
2
2.5
LOAD CURRENT (A)
LOAD CURRENT (A)
3975 F05a
3975 F05b
Figure 5. The Minimum Input Voltage Depends on Output Voltage and Load Current
3975f
18
Linear [ Linear ]
