LT3991EMSE-3.3#PBF Linear Technology, LT3991EMSE-3.3#PBF Datasheet - Page 7

• 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

PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
diode. The BD pin is normally connected to the output.
BOOST (Pin 2): This pin is used to provide a drive volt-
age, higher than the input voltage, to the internal bipolar
NPN power switch.
SW (Pin 3): The SW pin is the output of an internal power
switch. Connect this pin to the inductor, catch diode, and
boost capacitor.
V
internal circuitry and to the internal power switch. This
pin must be locally bypassed.
EN (Pin 5): The part is in shutdown when this pin is low
and active when this pin is high. The hysteretic threshold
voltage is 1.005V going up and 0.975V going down. The EN
threshold is only accurate when V
lower than 4.3V, ground EN to place the part in shutdown.
Tie to V
TYPICAL PERFORMANCE CHARACTERISTICS
100mV/
500mA/
IN
V
OUT
DIV
DIV
(Pin 4): The V
I
L
Transient Load Response,
Load Current Stepped from
0.5A to 1A
V
C
IN
OUT
IN
= 48V, V
= 47µF
if shutdown feature is not used.
OUT
= 3.3V
IN
10µs/DIV
pin supplies current to the LT3991’s
3991 G25
IN
is above 4.3V. If V
500mA/DIV
20mV/DIV
5V/DIV
V
V
OUT
SW
I
L
Switching Waveforms;
Burst Mode Operation
V
I
C
LOAD
IN
OUT
= 48V, V
= 47µF
= 20mA
IN
OUT
is
= 3.3V
5µs/DIV
FB (Pin 6, LT3991 Only): The LT3991 regulates the FB pin
to 1.19V. Connect the feedback resistor divider tap to this
pin. Also, connect a phase lead capacitor between FB and
V
V
3.3 and LT3991-5 regulate the V
respectively. This pin connects to the internal 10MΩ
feedback divider that programs the fixed output voltage.
SS (Pin 7): A capacitor and a series resistor are tied between
SS and ground to slowly ramp up the peak current limit
of the LT3991 on start-up. The soft-start capacitor is only
actively discharged when EN is low. The SS pin is released
when the EN pin goes high. Float this pin to disable soft-
start. The soft-start resistor has a typical value of 100k.
RT (Pin 8): A resistor is tied between RT and ground to
set the switching frequency.
LT3991/LT3991-3.3/LT3991-5
OUT
OUT
. Typically this capacitor is 10pF .
(Pin 6, LT3991-3.3/LT3991-5 Only): The LT3991-
T
A
3971 G26
= 25°C, unless otherwise noted.
500mA/DIV
20mV/DIV
5V/DIV
V
V
OUT
SW
I
L
Switching Waveforms; Full
Frequency Continuous Operation
V
I
C
LOAD
IN
OUT
= 48V, V
= 47µF
= 1A
OUT
OUT
= 3.3V
pin to 3.3V and 5V
1µs/DIV
3991fa
7
3971 G27