Datasheet

LT3991/LT3991-3.3/LT3991-5
16
3991fa
applicaTions inForMaTion
At light loads, the inductor current becomes discontinu-
ous and this reduces the minimum input voltage to ap-
proximately 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 LT3991, requiring
a higher input voltage to maintain regulation.
Enable Pin
The LT3991 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
LT3991 to regulate the output only when V
IN
is above a
desired voltage (see Figure 5). 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, regulation will
not stop until the input falls slightly below V
IN(EN)
.
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred μA. And the part
may be able to switch at cold or for V
IN(EN)
thresholds less
than 7V. Figure 6 shows the magnitude of the increased
input current in a typical application with different pro
-
grammed V
IN(EN)
.
When operating in Burst Mode for light load currents, the
current through the V
IN(EN)
resistor network can easily be
greater than the supply current consumed by the LT3991.
Therefore, the V
IN(EN)
resistors should be large to minimize
their effect on efficiency at low loads.
Figure 5. Programmed Enable Threshold
+
–
1V
SHDN
3991 F05
LT3991
EN
V
IN
R3
R4
3991 F06
INPUT VOLTAGE (V)
12V V
IN(EN)
Input Current
6V V
IN(EN)
Input Current
0 1 2 3 4
INPUT CURRENT (µA)
300
400
12
200
100
0
65 7 8 9 10 11
500
300
400
200
100
0
500
INPUT VOLTAGE (V)
0 1 2 3 4
INPUT CURRENT (µA)
65
V
IN(EN)
= 6V
R3 = 5M
R4 = 1M
V
IN(EN)
= 12V
R3 = 11M
R4 = 1M
Figure 6. Input Current vs Input Voltage
for a Programmed V
IN(EN)
of 6V and 12V