LT3990/LT3990-3.3/LT3990-5
7
3990fa
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Waveforms,
Burst Mode Operation
T
A
= 25°C, unless otherwise noted.
Switching Waveforms, Full
Frequency Continuous Operation
V
OUT
5mV/DIV
V
SW
5V/DIV
I
L
100mA/DIV
2µs/DIV
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 10mA
f = 600kHz
3990 G28
V
OUT
5mV/DIV
V
SW
5V/DIV
I
L
200mA/DIV
1µs/DIV
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 350mA
f = 600kHz
3990 G29
PIN FUNCTIONS
FB (Pin 1/Pins 1, 2 LT3990 Only): 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.
V
OUT
(Pins 1, 2, LT3990-X Only): The LT3990-3.3 and
LT3990-5 regulate the V
OUT
pin to 3.3V and 5V, respec-
tively. This pin connects to the internal feedback divider
that programs the fixed output voltage. The two V
OUT
pins
are connected internally and provide a redundant path to
the output. Tie the output 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 1.19V going up with 35mV 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.
(DFN, MSOP)
