Datasheet

LT3971A/LT3971A-5
17
3971af
APPLICATIONS INFORMATION
The minimum operating voltage of an LT3971A application
is limited by the minimum input voltage (4.3V) and by
the maximum duty cycle as outlined in the Input Voltage
Range section. For proper start-up, the minimum input
voltage is also limited by the boost circuit. If the input
voltage is ramped slowly, the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on input
and output voltages, and on the arrangement of the boost
circuit. The minimum load generally goes to zero once the
circuit has started. Figure 5 shows a plot 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 discontinuous
and this reduces the minimum input voltage to approxi-
mately 400mV above V
OUT
. At higher load currents, the
inductor current is continuous and the duty cycle is limited
by the maximum duty cycle of the LT3971A, requiring a
higher input voltage to maintain regulation.
Enable Pin
The LT3971A is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the
EN comparator is 1.01V, with 30mV of hysteresis. The EN
pin can be tied to V
IN
if the shutdown feature is not used.
Adding a resistor divider from V
IN
to EN programs the
LT3971A to regulate the output only when V
IN
is above a
desired voltage (see Figure 6). Typically, this threshold,
V
IN(EN)
, is used in situations where the input supply is cur-
rent limited, or has a relatively high source resistance. A
switching regulator draws constant power from the source,
so source current increases as source voltage drops. This
looks like a negative resistance load to the source and can
cause the source to current limit or latch low under low
source voltage conditions. The V
IN(EN)
threshold prevents
the regulator from operating at source voltages where the
problems might occur. This threshold can be adjusted by
setting the values R3 and R4 such that they satisfy the
following equation:
V
IN(EN)
=
R3
R4
+1
where output regulation should not start until V
IN
is above
V
IN(EN)
. Due to the comparator’s hysteresis, switching will
not stop until the input falls slightly below V
IN(EN)
.
3971A F05
LOAD CURRENT (mA)
10
INPUT VOLTAGE (V)
4.0
4.4
4.2
4.6
1000
3.6
3.8
3.4
3.0
100
10 1000100
3.2
5.0
4.8
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
5.8
6.0
6.2
5.6
5.0
5.4
5.2
6.4
TO RUN
V
OUT
= 3.3V
T
A
= 25°C
L = 4.7µH
f = 800kHz
V
OUT
= 5V
T
A
= 25°C
L = 4.7µH
f = 800kHz
TO START
TO RUN
TO START
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
+
–
1V
SHDN
3971A F06
LT3971A
EN
V
IN
R3
R4
Figure 6. Programmed Enable Threshold