LT8610AC datasheet(12/32 Pages) AD

12 / 32 page

LT8636/LT8637

Rev. C

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PIN FUNCTIONS

PG (Pin 1): The PG pin is the open-drain output of an

internal comparator. PG remains low until the FB pin is

within ±8% of the final regulation voltage, and there are

no fault conditions. PG is also pulled low when EN/UV is

below 1V, INTVCC has fallen too low, VIN is too low, or

thermal shutdown. PG is valid when VIN is above 3.4V.

BIAS (Pin 2): The internal regulator will draw current from

BIAS instead of VIN when BIAS is tied to a voltage higher

than 3.1V. For output voltages of 3.3V to 25V this pin

should be tied to VOUT. If this pin is tied to a supply other

than VOUT use a 1µF local bypass capacitor on this pin.

If no supply is available, tie to GND. However, especially

for high input or high frequency applications, BIAS should

be tied to output or an external supply of 3.3V or above.

INTVCC (Pin 3): Internal 3.4V Regulator Bypass Pin. The

internal power drivers and control circuits are powered

from this voltage. INTVCC maximum output current is

20mA. Do not load the INTVCC pin with external circuitry.

INTVCC current will be supplied from BIAS if BIAS > 3.1V,

otherwise current will be drawn from VIN. Voltage on

INTVCC will vary between 2.8V and 3.4V when BIAS is

between 3.0V and 3.6V. Place a low ESR ceramic capaci-

tor of at least 1µF from this pin to ground close to the IC.

GND (Pins 4, 13, Exposed Pad Pin 21): Ground. Place

the negative terminal of the input capacitor as close to

the GND pins as possible. The exposed pads should be

soldered to the PCB for good thermal performance. If

necessary due to manufacturing limitations Pin 21 may

be left disconnected, however thermal performance will

be degraded.

NC (Pins 5, 12): No Connect. This pin is not connected

to internal circuitry and can be tied anywhere on the PCB,

typically ground.

VIN (Pins 6, 11): The VIN pins supply current to the

LT8636/LT8637 internal circuitry and to the internal top-

side power switch. The LT8636/LT8637 requires the use

of multiple VIN bypass capacitors. Two small 1µF capaci-

tors should be placed as close as possible to the LT8636/

LT8637, one capacitor on each side of the device (CIN1,

CIN2). A third capacitor with a larger value, 2.2µF or higher,

should be placed near CIN1 or CIN2. See Applications

Information section for sample layout.

BST (Pin 7): This pin is used to provide a drive voltage,

higher than the input voltage, to the topside power switch.

Place a 0.1µF boost capacitor as close as possible to

the IC.

SW (Pins 8–10): The SW pins are the outputs of the inter-

nal power switches. Tie these pins together and connect

them to the inductor. This node should be kept small on

the PCB for good performance and low EMI.

EN/UV (Pin 14): The LT8636/LT8637 is shut down when

this pin is low and active when this pin is high. The hyster-

etic threshold voltage is 1.00V going up and 0.96V going

down. Tie to VIN if the shutdown feature is not used. An

external resistor divider from VIN can be used to program

a VIN threshold below which the LT8636/LT8637 will shut

down.

SYNC/MODE (Pin 15): For the LT8636/LT8637, this

pin programs four different operating modes: 1) Burst

Mode operation. Tie this pin to ground for Burst Mode

operation at low output loads—this will result in ultralow

quiescent current. 2) Forced Continuous mode (FCM).

This mode offers fast transient response and full

frequency operation over a wide load range. Float this

pin for FCM. When floating, pin leakage currents should

be <1µA. See Block Diagram for internal pull-up and

pull-down resistance. 3) Spread spectrum mode. Tie

this pin high to INTVCC (~3.4V) or an external supply

of 3V to 4V for forced continuous mode with spread-

spectrum modulation. 4) Synchronization mode. Drive

this pin with a clock source to synchronize to an external

frequency. During synchronization the part will operate

in forced continuous mode.

CLKOUT (Pin 16): In forced continuous mode, spread

spectrum, and synchronization modes, the CLKOUT pin

will provide a ~200ns wide pulse at the switch frequency.

The low and high levels of the CLKOUT pin are ground and

INTVCC respectively, and the drive strength of the CLKOUT

pin is several hundred ohms. In Burst Mode operation,

the CLKOUT pin will be low. Float this pin if the CLKOUT

function is not used.

RT (Pin 17): A resistor is tied between RT and ground to

set the switching frequency.

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