LT3957AIUHE View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3957AIUHE

Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch

Linear Technology 

LT3957A

PIN FUNCTIONS

NC (Pins 1, 2, 10, 35, 36): No Internal Connection. Leave

these pins open or connect them to the adjacent pins.

SENSE2 (Pin 3): The Current Sense Input for the Control

Loop. Connect this pin to SENSE1 pin directly or through

a low pass filter (connect this pin to SENSE1 pin through

a resistor, and to SGND through a capacitor).

SGND (Pins 4, 23, 24, Exposed Pad Pin 37): Signal

Ground. All small-signal components should connect to

this ground. SGND is connected to GND inside the IC to

ensure Kelvin connection for the internal switch current

sensing. Do not connect SGND and GND externally.

SENSE1 (Pin 6): The Current Sense Output of the Inter-

nal N-channel MOSFET. Connect this pin to SENSE2 pin

directly or through a lowpass filter (connect this pin to

SENSE1 pin through a resistor, then connect SENSE2 to

SGND through a capacitor).

SW (Pins 8, 9, 20, 21, Exposed Pad Pin 38): Drain of

Internal Power N-channel MOSFET.

GND (Pins 12, 13, 14, 15, 16, 17): Ground. These pins

connect to the source terminal of internal power N-channel

MOSFET through an internal sense resistor. GND is con-

nected to SGND inside the IC to ensure Kelvin connection

for the internal switch current sensing. Do not connect

GND and SGND externally.

EN/UVLO (Pin 25): Shutdown and Undervoltage Detect

Pin. An accurate 1.22V (nominal) falling threshold with

externally programmable hysteresis detects when power

is okay to enable switching. Rising hysteresis is generated

by the external resistor divider and an accurate internal

2μA pull-down current. An undervoltage condition resets

soft-start. Tie to 0.4V, or less, to disable the device and

reduce VIN quiescent current below 1μA.

VIN (Pin 27): Input Supply Pin. The VIN pin can be locally

bypassed with a capacitor to GND (not SGND).

INTVCC (Pin 28): Regulated Supply for Internal Loads

and Gate Driver. Supplied from VIN and regulated to

5.2V (typical). INTVCC must be bypassed to SGND with a

minimum of 4.7μF capacitor placed close to pin. INTVCC

can be connected directly to VIN, if VIN is less than 8V.

INTVCC can also be connected to a power supply whose

voltage is higher than 5.5V, and lower than VIN, provided

that supply does not exceed 8V.

VC (Pin 30): Error Amplifier Compensation Pin. Used to

stabilize the voltage loop with an external RC network. Place

compensation components between the VC pin and SGND.

FBX (Pin 31): Positive and Negative Feedback Pin. Re-

ceives the feedback voltage from the external resistor

divider between the output and SGND. Also modulates the

switching frequency during start-up and fault conditions

when FBX is close to SGND.

SS (Pin 32): Soft-Start Pin. This pin modulates compen-

sation pin voltage (VC) clamp. The soft-start interval is

set with an external capacitor between SS pin and SGND.

The pin has a 10μA (typical) pull-up current source to an

internal 2.5V rail. The soft-start pin is reset to SGND by an

undervoltage condition at EN/UVLO, an INTVCC undervolt-

age or overvoltage condition or an internal thermal lockout.

RT (Pin 33): Switching Frequency Adjustment Pin. Set

the frequency using a resistor to SGND. Do not leave this

pin open.

SYNC (Pin 34): Frequency Synchronization Pin. Used to

synchronize the switching frequency to an outside clock.

If this feature is used, an RT resistor should be chosen

to program a switching frequency 20% slower than the

SYNC pulse frequency. Tie the SYNC pin to SGND if this

feature is not used. SYNC is bypassed when FBX is close

to SGND.

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