LT3757 datasheet(9/22 Pages) AD

9 / 22 page

LT3935

Rev. 0

For more information www.analog.com

PIN FUNCTIONS

VIN: Input Voltage Pins. These pins supply power to the

internal,highperformanceanalogcircuitry,andtheysupply

theinductorcurrentwhentheinternalhighsidepowerswitch

is on. Connect capacitors between these pins and GND and

seeSelectingandPlacingtheInputCapacitorsinApplications

Information for advice regarding their placement.

EN/UVLO:EnableandUndervoltageLockoutPin.Avoltage

at this pin greater than 1.16V will enable switching, and

a voltage less than 300mV is guaranteed to shut down

the internal current bias and sub-regulators. A resistor

network between this pin and GND can be used to set the

pin voltage and automatically lockout the part when VIN is

below a certain level. No internal components pull up or

down on this pin, so it requires an external voltage bias

for normal operation. This pin may be tied directly to VIN.

INTVCC: Internally Regulated, Low-Voltage Supply Pin.

This pin provides the power for the converter switch gate

drivers. Do not force any voltage on this pin, but bypass

it with a 2.2µF capacitor to GND.

ISP: Positive Current Sense Pin. This pin is one of the

inputs to the internal current sense error amplifier. It

should be connected to the positive side of the external

sense resistor.

ISN: Negative Current Sense Pin. This pin is one of the

inputs to the internal current sense error amplifier. It

should be connected to the negative side of the external

sense resistor.

ISMON: Output Current Monitoring Pin. This pin provides

a buffered voltage output equal to 10mV for every 1mV

between ISP and ISN.

CTRL: Control Pin. An analog voltage from 250mV to

1.25V at this pin programs the regulated voltage between

ISP and ISN (and therefore, the regulated current supplied

to the load). Alternatively, a digital pulse at this pin with

duty cycle from 12.5% to 62.5% can be used to program

the regulated voltage. Below 200mV or 10% duty cycle,

the CTRL pin voltage disables switching. For more detail,

see Regulated LED Current in Typical Performance Curves

and Programming LED Current with the CTRL Pin in Ap-

plications Information.

VREF: Reference Voltage Pin. This pin provides a buffered

2V reference capable of 2mA drive. It can be used to sup-

ply resistor networks for setting the voltages at the CTRL

and PWM pins. Bypass with a 2.2μF capacitor to GND.

FB: Feedback Pin. When the voltage at this pin is near 1V,

the regulated current is automatically reduced from the

programmed value. A resistor network between this pin

and VOUT can be used to set a limit for the output voltage.

If the voltage at the FB pin reaches 1.05V, an overvoltage

lockout comparator disables switching.

FAULT: Fault Pin. Connect to INTVCC through a resistance

of 100k. When the FB pin voltage is less than 200mV, an

internal switch pulls this pin low to indicate a short-circuit.

When FB is greater than 950mV and the voltage between

ISP and ISN is simultaneously less than 10mV, the switch

pulls this pin low to indicate an open-circuit.

SS: Soft-Start Pin. At startup and recovery from fault

conditions, a 20μA current charges the capacitor and the

FB voltage tracks the rising voltage at this pin until the

load current reaches its programmed level. Typical values

for the capacitor are 10nF to 100nF. A resistor from SS to

INTVCC is used to select one of several fault modes. See

Soft-Start and Fault Modes in Applications Information

for more details.

VC: Compensation Pin. A capacitor connected from this

pin to GND stabilizes the current and voltage regulation.

See Stabilizing the Regulation Loop in the Applications

Information section for more details.

SW: Switch Pins. These two pins are internally connected

to the power devices and drivers. They should always be

tied together. In normal operation, the voltage of these

pins will switch between the input voltage and zero at

the programmed frequency. Do not force any voltage on

these pins.

RT: Timing Resistor Pin. A resistor from this pin to GND

programs the switching frequency between 200kHz and

2MHz. Do not leave this pin open.

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