Datasheet

LT3991/LT3991-3.3/LT3991-5
7
3991fa
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
IN
(Pin 4): The V
IN
pin supplies current to the LT3991’s
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
IN
is above 4.3V. If V
IN
is
lower than 4.3V, ground EN to place the part in shutdown.
Tie to V
IN
if shutdown feature is not used.
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
OUT
. Typically this capacitor is 10pF.
V
OUT
(Pin 6, LT3991-3.3/LT3991-5 Only): The LT3991-
3.3 and LT3991-5 regulate the V
OUT
pin to 3.3V and 5V
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.
Typical perForMance characTerisTics
Switching Waveforms;
Burst Mode Operation
Switching Waveforms; Full
Frequency Continuous Operation
T
A
= 25°C, unless otherwise noted.
Transient Load Response,
Load Current Stepped from
0.5A to 1A
10µs/DIV
3991 G25
V
OUT
100mV/
DIV
I
L
500mA/
DIV
V
IN
= 48V, V
OUT
= 3.3V
C
OUT
= 47µF
5µs/DIV
3971 G26
V
SW
5V/DIV
V
OUT
20mV/DIV
I
L
500mA/DIV
V
IN
= 48V, V
OUT
= 3.3V
I
LOAD
= 20mA
C
OUT
= 47µF
1µs/DIV
3971 G27
V
SW
5V/DIV
V
OUT
20mV/DIV
I
L
500mA/DIV
V
IN
= 48V, V
OUT
= 3.3V
I
LOAD
= 1A
C
OUT
= 47µF