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LT3645

36V 500mA Step-Down Regulator and 200mA LDO

Linear Technology 

LT3645

PIN FUNCTIONS (MSOP/QFN)

EN/UVLO (Pin 1/Pin 5): The EN/UVLO pin is used to enable

the buck switching regulator and the low dropout linear

regulator (LDO). An accurate threshold of 1.23V allows the

user to set the undervoltage lockout point with a simple

resistor divider, see Precision Undervoltage Lockout sec-

tion for more information. The EN/UVLO pin can be tied

directly to VIN if the UVLO or shutdown is not used.

FB (Pin 2/Pin 6): The FB pin programs the buck output

voltage. The LT3645 regulates the FB pin to 0.8V. The

feedback resistor divider tap should be connected to this

pin. The output voltage is programmed according to the

following equation:

R1= R2 •

⎛

⎝⎜

VOUT

0.8

– 1⎞⎠⎟

where R1 connects between OUT and FB and R2 connects

between FB and GND. A good value for R2 is 10k.

GND (Pin 3, Exposed Pad Pin 13/Pin 7, Exposed Pad Pin 17):

The GND pin should be tied to a local ground plane below the

LT3645 and the circuit components. Return the feedback

dividers from FB and FB2 to this pin. The exposed pad

must be soldered to the PCB and electrically connected

to ground. Use a large ground plane and thermal vias to

optimize thermal performance.

DA (Pin 4/Pin 8): The DA pin senses the external catch

diode current and prevents the buck regulator from switch-

ing if the sensed current is too high. Connect the anode

of the external Schottky catch diode to this pin.

BOOST (Pin 5/Pin 10): The BOOST pin provides a drive

voltage to the internal bipolar NPN power switch. Tie a

0.1μF capacitor between the BOOST and SW pins.

SW (Pin 6/Pin 9): The SW pin is the output of the internal

buck power switch. Connect the inductor and the cathode

of the external catch Schottky diode to this pin.

VIN (Pin 7/ Pin 11): The VIN pin supplies current to the

LT3645’s internal circuitry, to the internal buck power

switch, and to the LDO. The VIN pin must be locally

bypassed.

VCC2 (Pin 8/Pin 14): The VCC2 pin supplies current to the

linear regulator’s output device. The VCC2 pin is also the

anode of an internal Schottky diode used to generate the

BOOST voltage. The VCC2 pin must be tied to a voltage

source greater than 2.5V to utilize the internal Schottky

boost diode. If the VCC2 pin is tied to a voltage lower than

2.5V, then an external Schottky diode must be connected

between a power supply greater than 2.5V (anode) and

the BOOST pin (cathode). Bypass this pin to ground with

a 0.1μF capacitor close to the part.

OUT2 (Pin 9/Pin 15): The OUT2 pin is the output of the

LDO. Connect a capacitor of at least 0.47μF from this pin

to ground. See Frequency Compensation (LDO) section

for more details.

FB2 (Pin 10/Pin 16): The FB2 pin programs the LDO output

voltage. The LT3645 regulates the FB2 pin to 0.797V. The

feedback resistor divider tap should be connected to this

pin. The output voltage is programmed according to the

following equation:

R3

=

R4

t

⎛

⎝⎜

VOUT2

0.797

– 1⎞⎠⎟

where R3 connects between OUT2 and FB2 and R4

connects between FB2 and GND. A good value for R4 is

10k.

EN2 (Pin 11/Pin 4): The EN2 pin is used to enable the linear

regulator. Pull this pin above 1.6V to enable the LDO. Pull

EN2 below 0.5V to disable the LDO.

NPG (Pin 12/Pin 3): The NPG pin is an open-collector

output used to indicate that both buck and LDO output

voltages are in regulation. The NPG pin pulls low when

FB and FB2 both exceed 720mV.

NC (Pins 1, 2, 12, 13, QFN Only): No Connect Pins. Tie

these to ground.

3645f

7

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