LT3922 datasheet(11/28 Pages) AD

11 / 28 page

LT8386

Rev. 0

For more information www.analog.com

PIN FUNCTIONS

SW (Pins 1, 27, 28): Switch Pins. These pins are inter-

nally connected to the power devices and high-side gate

driver. They should always be tied together. In normal

operation, the voltage of these pins will switch between

the output voltage and zero at the programmed frequency.

Do not force any voltage on these pins.

BST (Pin 2): Boost Pin. This pin supplies the top power

switch GATE driver. Connect a 33nF capacitor between

this pin and the SW pin as close as possible to the pack-

age. An internal diode from INTVCC to BST will charge the

capacitor when the SW pin switches low.

INTVCC (Pin 3): 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. Place

a 2.2μF bypass capacitor to GND close to the package.

VIN (Pin 4): Input Voltage Pin. This pin supplies power to

the internal, high performance analog circuitry. Connect

a bypass capacitor between this pin and GND.

EN/UVLO (Pin 5): Enable and Undervoltage Lockout Pin.

A voltage at this pin greater than 1.36V (typ.) will enable

switching, and a voltage less than 0.1V is guaranteed to

shutdown the internal current bias and sub regulators.

A resistor network between this pin and ground can be

used to set the pin voltage and automatically lockout the

part when VIN is below a certain level. No internal com-

ponents 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. For more details, see Applications

Information section.

OVLO (Pin 6): Input Overvoltage Lockout Pin. When the

voltage at this pin rises above 1.205V, the system dis-

ables switching and resets the soft-start capacitor. Do not

leave this pin open. Tie this pin to GND when the OVLO

function is not used. For more details, see Applications

Information section.

VREF (Pin 7): Reference Voltage Pin. This pin provides

a buffered 2V reference. It can be used to supply resis-

tor networks for setting the voltages at the CTRL and

PWM pins. Bypass with a 0.1μF capacitor to GND.

CTRL (Pin 8): 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 cur-

rent supplied to the load). Depending on the applications,

the CTRL pin can be affected by the noise in the system. A

10nF bypass capacitor at the CTRL pin can boost the noise

immunity. Alternatively, a digital pulse at this pin with

duty ratio 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 Typical Performance Characteristics and Applications

Information sections.

ISP (Pin 9): 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. Use a Kelvin connection to the sense resis-

tor for accurate current sensing.

ISN (Pin 10): 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. Use a Kelvin connection to the sense resis-

tor for accurate current sensing.

VC (Pin 11): Compensation Pin. A resistor and capaci-

tor connected in series from this pin to GND stabilize

the current and voltage regulation. Typical resistor and

capacitor values are from 0 to 100kΩ and from 10nF to

0.1nF, respectively.

FB (Pin 12): Feedback Pin. When the voltage at this pin is

near 1.2V 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 1.266V, a FB

overvoltage lockout comparator disables switching.

SS (Pin 13): Soft-Start Pin. At start-up and recovery from

fault conditions, a 22μA 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. Using a single resistor

from SS to INTVCC, the LT8386 can be set in two different

fault modes for the shorted LED conditions: hiccup (no

resistor) and latchoff (<470kΩ). Refer to the Applications

Information section for a detailed explanation.

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