LT3991EMSE-3.3#PBF Linear Technology, LT3991EMSE-3.3#PBF Datasheet - Page 16

• Manufacturer Part Number: LT3991EMSE-3.3#PBF
• Description: IC, DC-DC CONV, 2MHz, MSOP10
• Manufacturer: Linear Technology

Specifications of LT3991EMSE-3.3#PBF

• Primary Input Voltage: 55V
• No. Of Outputs: 1
• Output Voltage: 3.3V
• Output Current: 1.2A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Peak Reflow Compatible (260 C): Yes
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LT3991/LT3991-3.3/LT3991-5
At light loads, the inductor current becomes discontinu-
ous and this reduces the minimum input voltage to ap-
proximately 400mV above V
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
Adding a resistor divider from V
LT3991 to regulate the output only when V
desired voltage (see Figure 5). Typically, this threshold,
V
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
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:
where output regulation should not start until V
V
not stop until the input falls slightly below V
16
IN(EN)
IN(EN)
V
IN(EN)
, is used in situations where the input supply is cur-
. Due to the comparator’s hysteresis, regulation will
=
Figure 5. Programmed Enable Threshold
R3
R4
IN
+ 1
R3
R4
if the shutdown feature is not used.
V
EN
IN
1V
OUT
LT3991
+
–
. At higher load currents,
IN(EN)
IN
to EN programs the
threshold prevents
SHDN
3991 F05
IN
IN(EN)
is above a
IN
is above
.
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
than 7V. Figure 6 shows the magnitude of the increased
input current in a typical application with different pro-
grammed V
When operating in Burst Mode for light load currents, the
current through the V
greater than the supply current consumed by the LT3991.
Therefore, the V
their effect on efficiency at low loads.
500
400
300
200
100
500
400
300
200
100
Figure 6. Input Current vs Input Voltage
for a Programmed V
0
0
IN(EN)
0
0
V
R3 = 11M
R4 = 1M
V
R3 = 5M
R4 = 1M
IN(EN)
IN(EN)
IN(EN)
1
12V V
.
6V V
2
1
= 12V
= 6V
3
resistors should be large to minimize
IN(EN)
IN(EN)
IN(EN)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
4
2
5
resistor network can easily be
Input Current
6
3
Input Current
IN(EN)
7
8
4
of 6V and 12V
9 10 11
IN(EN)
5
3991 F06
thresholds less
12
6
3991fa