LT3991EMSE-5#PBF Linear Technology, LT3991EMSE-5#PBF Datasheet - Page 15

• Manufacturer Part Number: LT3991EMSE-5#PBF
• Description: IC, DC-DC CONV, 2MHz, MSOP10
• Manufacturer: Linear Technology

Specifications of LT3991EMSE-5#PBF

• Primary Input Voltage: 55V
• No. Of Outputs: 1
• Output Voltage: 5V
• 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
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (Figure 3b). For output voltages below 2.5V,
the boost diode can be tied to the input (Figure 3c), or to
another external supply greater than 2.8V. However, the
circuit in Figure 3a is more efficient because the BOOST pin
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
The minimum operating voltage of an LT3991 application
is limited by the minimum input voltage (4.3V) and by
Figure 3. Three Circuits for Generating the Boost Voltage
4.7µF
4.7µF
4.7µF
V
V
V
IN
IN
IN
(3c) For V
(3b) For 2.5V < V
V
V
V
IN
IN
IN
(3a) For V
LT3991
LT3991
LT3991
GND
GND
GND
OUT
BD
BD
BD
BOOST
BOOST
BOOST
< 2.5V; V
SW
SW
SW
OUT
OUT
> 2.8V
IN(MAX)
C3
C3
C3
< 2.8V
D2
= 27V
3991 FO3
V
V
V
OUT
OUT
OUT
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 4 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
For lower start-up voltage, the boost diode can be tied to
V
the absolute maximum rating of the BOOST pin.
LT3991/LT3991-3.3/LT3991-5
IN
; however, this restricts the input range to one-half of
Figure 4. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
10
10
V
T
L = 10µH
f = 400kHz
V
T
L = 10µH
f = 400kHz
OUT
A
A
OUT
TO START
TO RUN
= 25°C
= 25°C
TO START
TO RUN
= 3.3V
= 5V
LOAD CURRENT (mA)
LOAD CURRENT (mA)
100
100
IN
is ramping very slowly.
3991 F04
1000
1000
15
3991fa