LT3991EMSE-5#PBF Linear Technology, LT3991EMSE-5#PBF Datasheet - Page 10

• 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

LT3991/LT3991-3.3/LT3991-5
APPLICATIONS INFORMATION
performance of the LT3991. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
diode should have less than 1µA of typical reverse leak-
age at room temperature. These two considerations are
reiterated in the FB Resistor Network and Catch Diode
Selection sections.
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
While in Burst Mode operation, the burst frequency and the
charge delivered with each pulse will not change with output
capacitance. Therefore, the output voltage ripple will be
inversely proportional to the output capacitance. In a typical
application with a 47μF output capacitor, the output ripple
is about 8mV, and with a 100μF output capacitor the output
ripple is about 4mV. The output voltage ripple can continue
to be decreased by increasing the output capacitance.
At higher output loads (above 86mA for the front page
application) the LT3991 will be running at the frequency
programmed by the R
standard PWM mode. The transition between PWM and low
ripple Burst Mode operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an exter-
nal clock, the LT3991 will pulse skip at light loads. The
10
500mA/DIV
20mV/DIV
5V/DIV
V
V
OUT
SW
I
L
V
V
I
LOAD
IN
OUT
= 48V
= 3.3V
Figure 2. Burst Mode Operation
= 20mA
T
resistor, and will be operating in
5µs/DIV
3991 F02
quiescent current will significantly increase to 1.5mA in
light load situations when synchronized with an external
clock. Holding the SYNC pin high yields no advantages in
terms of output ripple or minimum load to full frequency,
so is not recommended.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
When using large FB resistors, a 10pF phase lead capacitor
should be connected from V
The LT3991-3.3 and LT3991-5 control an internal 10M FB
resistor divider as well as an internal lead capacitor.
Setting the Switching Frequency
The LT3991 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
frequency is in Table 1.
Table 1. Switching Frequency vs R
SWITCHING FREQUENCY (MHz)
R1= R2

 
1.19V
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
V
OUT
− 1

 
T
value for a desired switching
OUT
T
Value
to FB.
R
T
VALUE (kΩ)
71.5
49.9
35.7
28.0
22.1
17.4
14.0
11.0
255
118
3991fa