LT3991-5_15 Datasheet, PDF (16/24 Pages) Linear Technology – 55V, 1.2A Step-Down Regulator with 2.8A Quiescent Current

LT3991/LT3991-3.3/LT3991-5
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.
12V V_IN(EN) Input Current
500
400
300
200
100
0
0 1 2 3 4 5 6 7 8 9 10 11 12
INPUT VOLTAGE (V)
V_IN(EN) = 12V
R3 = 11M
R4 = 1M
6V V_IN(EN) Input Current
500
400
300
V_IN
LT3991
R3
1V +
EN
–
R4
SHDN
3991 F05
Figure 5. Programmed Enable Threshold
16
200
100
0
0
1
2
3
4
INPUT VOLTAGE (V)
V_IN(EN) = 6V
R3 = 5M
R4 = 1M
5
6
3991 F06
Figure 6. Input Current vs Input Voltage
for a Programmed V_IN(EN) of 6V and 12V
3991fa