Datasheet
LT3976
19
3976f
For more information www.linear.com/3976
applicaTions inForMaTion
Figure 5. The Minimum Input Voltage Depends on Output Voltage and Load Current
BOOST
LT3976
(4a) For 3.2V ≤ V
OUT
≤ 16V
GND
V
IN
V
IN
SW
OUT
V
OUT
BOOST
LT3976
(4d) For V
OUT
< 2.5V, 3.1V ≤ V
S
≤ 16V
GND
V
IN
V
IN
SW
OUT
V
OUT
V
S
BOOST
LT3976
(4e) For V
OUT
> 16V, 3.1V ≤ V
S
≤ 16V
GND
V
IN
V
IN
SW
OUT
V
OUT
3976 F04
V
S
BOOST
LT3976
(4c) For V
OUT
< 2.5V, V
IN
< 27V
GND
V
IN
V
IN
SW
OUT
V
OUT
BOOST
LT3976
(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
to charge the boost capacitor, the OUT pin should still be
tied to the output even though the minimum input voltage
of the LT3976 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.
Enable and Undervoltage Lockout
The LT3976 is in shutdown when the EN pin is low and
active when the pin is high. The falling threshold of the
EN comparator is 1.02V, with 60mV of hysteresis. The EN
pin can be tied to V
IN
if the shutdown feature is not used.
Undervoltage lockout (UVLO) can be added to the LT3976
as shown in Figure 6. Typically, UVLO is used in situa-
tions where the input supply is current limited, or has a
relatively high source resistance. A switching regulator
draws constant power from
the source, so source cur-
rent
increases as source voltage drops. This looks like a
LOAD CURRENT (A)
0
INPUT VOLTAGE (V)
5.5
6.0
6.5
4
3976 F05a
5.0
4.5
4.0
1
2
3
5
V
OUT
= 5V
f
SW
= 800kHz
TO RUN/TO START
LOAD CURRENT (A)
0
INPUT VOLTAGE (V)
4.0
4.5
5.0
4
3976 F05b
3.5
3.0
2.5
1
2
3
5
V
OUT
= 3.3V
FRONT PAGE APPLICATION
TO RUN/TO START