Datasheet

LT3990
7
3990f
pin FuncTions
FB (Pin 1/Pins 1, 2): The LT3990 regulates the FB pin to
1.21V. Connect the feedback resistor divider tap to this
pin. The two FB pins on the MSE package are connected
internally and provide a redundant path for the feedback
divider. Tie the divider to both pins.
EN/UVLO (Pin 2/Pin 4): The part is in shutdown when this
pin is low and active when this pin is high. The threshold
voltage is 1.21V going up with 32mV of hysteresis. Tie
to V
IN
if shutdown feature is not used. The EN/UVLO
threshold is accurate only when V
IN
is above 4.2V. If V
IN
is lower than 4.2V, ground EN/UVLO to place the part in
shutdown.
V
IN
(Pin 3/Pin 6): The V
IN
pin supplies current to the
LT3990’s internal circuitry and to the internal power switch.
This pin must be locally bypassed.
GND (Pins 4, 5, Exposed Pad Pin 11/Pin 8, Exposed
Pad Pin 17): Ground. The exposed pad must be soldered
to the PCB.
SW (Pin 6/Pin 9): The SW pin is the output of an internal
power switch. Connect this pin to the inductor.
BOOST (Pin 7/Pin 11): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
BD (Pin 8/Pin 13): This pin connects to the anode of the
boost diode. This pin also supplies current to the LT3990’s
internal regulator when BD is above 3.2V.
PG (Pin 9/Pin 14): The PG pin is the open-drain output of
an internal comparator. PG remains low until the FB pin
is within 10% of the final regulation voltage. PG is valid
when V
IN
is above 4.2V and EN/UVLO is high.
RT (Pin 10/Pin 16): A resistor is tied between RT and
ground to set the switching frequency.
NC (Pins 3, 5, 7, 10, 12, 15, MSOP Only): No Connects.
These pins are not connected to internal circuitry and must
be left floating to ensure fault tolerance.
R
SWITCH LATCH
D
BOOST
D
CATCH
BOOST
OSCILLATOR
200kHz TO 2.2MHz
SLOPE COMP
S
Q
–
+
–
+
–
+
Burst Mode
DETECT
ERROR
AMP
1.09V
SHDN
EN/UVLO
1.21V
C1
V
IN
INTERNAL 1.21V REF
–
+
RT
R
T
PG
FB
R2 R1
V
C
GND
V
IN
BD
SW
V
OUT
C2
C3
3990 BD
L1
block Diagram
(DFN, MSOP)