B340A 查看數據表（PDF） - Linear Technology

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B340A

42V, 2.5A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current

Linear Technology 

LT3975

TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, unless otherwise noted.

Load Transient: 0.5A to 2.5A

Load Transient: 20mA to 2A

IL

1A/DIV

VOUT

200mV/DIV

12VIN

3.3VOUT

COUT = 47µF

20µs/DIV

3975 G37

IL

1A/DIV

VOUT

200mV/DIV

12VIN

3.3VOUT

COUT = 47µF

20µs/DIV

3975 G38

PIN FUNCTIONS

FB (Pin 1): The LT3975 regulates the FB pin to 1.197V.

Connect the feedback resistor divider tap to this pin. Also,

connect a phase lead capacitor between FB and the output.

Typically, this capacitor is 10pF.

SS (Pin 2): A capacitor is tied between SS and ground to

slowly ramp up the peak current limit of the LT3975 on

start-up. There is an internal 1.8μA pull-up on this pin.

The soft-start capacitor is actively discharged when the

EN pin goes low, during undervoltage lockout or thermal

shutdown. Float this pin to disable soft-start.

OUT (Pin 3): This pin is an input to the dropout comparator

which maintains a minimum dropout of 500mV between

VIN and OUT. The OUT pin connects to the anode of the

internal boost diode. This pin also supplies the current to

the LT3975’s internal regulator when OUT is above 3.2V.

Connect this pin to the output when the programmed

output voltage is less than 16V.

BOOST (Pin 4): This pin is used to provide a drive volt-

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

NPN power switch.

SW (Pins 5, 6, 7): The SW pin is the output of an internal

power switch. Connect these pins to the inductor, catch

diode, and boost capacitor.

NC (Pins 8, 9): No Connects. These pins are not connected

to internal circuitry.

VIN (Pins 10, 11, 12): The VIN pin supplies current to the

LT3975’s internal circuitry and to the internal power switch.

These pins must be locally bypassed.

EN (Pin 13): The part is in shutdown when this pin is low

and active when this pin is high. The hysteretic threshold

voltage is 1.08V going up and 1.02V going down. The

EN threshold is only accurate when VIN is above 4.3V. If

VIN is lower than 3.9V, internal UVLO will place the part

in shutdown. Tie to VIN if shutdown feature is not used.

RT (Pin 14): A resistor is tied between RT and ground to

set the switching frequency.

PG (Pin 15): The PG pin is the open-drain output of an

internal comparator. PGOOD remains low until the FB pin

is within 8.4% of the final regulation voltage. PGOOD is

valid when VIN is above 2V.

SYNC (Pin 16): This is the external clock synchronization

input. Ground this pin for low ripple Burst Mode operation

at low output loads. Tie to a clock source for synchroni-

zation, which will include pulse skipping at low output

loads. When in pulse-skipping mode, quiescent current

increases to 11µA in a typical application at no load. Do

not float this pin.

GND (Exposed Pad Pin 17): Ground. The exposed pad

must be soldered to the PCB.

3975f

8

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