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LT3990

3990p

PIN FUNCTIONS

FB (Pin 1): The LT3990 regulates the FB pin to 1.21V. Con-

nect the feedback resistor divider tap to this pin.

EN (Pin 2): The part is in shutdown when this pin is low

and active when this pin is high. The hysteretic threshold

voltage is 1V going up and 0.97V going down. Tie to VIN

if shutdown feature is not used. The EN threshold is ac-

curate only when VIN is above 4.2V. If VIN is lower than

4.2V, ground EN to place the part in shutdown.

VIN (Pin 3): The VIN 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): Ground.

SW (Pin 6): The SW pin is the output of an internal power

switch. Connect this pin to the inductor.

BOOST (Pin 7): This pin is used to provide a drive volt-

age, higher than the input voltage, to the internal bipolar

NPN power switch.

BD (Pin 8): 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): 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

VIN is above 4.2V and EN is high.

RT (Pin 10): A resistor is tied between RT and ground to

set the switching frequency.

Exposed Pad (Pin 11, DFN Only): Ground. Must be sol-

dered to PCB.

SWITCH LATCH

DBOOST

DCATCH

BOOST

OSCILLATOR

200kHz TO 2.2MHz

SLOPE COMP

–

Burst Mode

DETECT

ERROR

AMP

1.09V

SHDN

VIN

INTERNAL 1.21V REF

–

GND

(4, 5)

VIN

VOUT

3990 BD

BLOCK DIAGRAM

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