LT8646SEVPBF datasheet(12/30 Pages) AD

12 / 30 page

LT8645S/LT8646S

Rev. B

For more information www.analog.com

PIN FUNCTIONS

BIAS (Pin 1): 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 2): Internal 3.4V Regulator Bypass Pin. The

internal power drivers and control circuits are powered

from this voltage. INTVCC maximum output current is

25mA. Do not load the INTVCC pin with external circuitry.

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

otherwisecurrentwillbedrawnfromVIN.VoltageonINTVCC

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

3.0V and 3.6V. This pin should be floated.

NC (Pins 3, 7, 20, 24): No Connect. This pin is not con-

nected to internal circuitry and can be tied anywhere on

the PCB, typically ground.

VIN (Pins 4, 5, 6, 21, 22, 23): The VIN pins supply cur-

rent to the LT8645S/LT8646S internal circuitry and to

the internal topside power switch. These pins must be

tied together and be locally bypassed with a capacitor of

4.7µF or more. Be sure to place the positive terminal of

the input capacitor as close as possible to the VIN pins,

and the negative capacitor terminal as close as possible

to the GND pins. See the Applications Information section

for a sample layout.

GND (Pins 8, 9, 10, 17, 18, 19, Exposed Pad Pins

33–38): Ground. Place the negative terminal of the input

capacitor as close to the GND pins as possible. See the

Applications Information section for a sample layout. The

exposed pads should be soldered to the PCB for good

thermal performance. If necessary due to manufacturing

limitationsPins33to38maybeleftdisconnected,however

thermal performance will be degraded.

BST (Pin 11): This pin is used to provide a drive voltage,

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

This pin should be floated.

SW (Pins 12, 13, 14, 15, 16): The SW pins are the outputs

of the internal power switches. Tie these pins together and

connectthemtotheinductorandboostcapacitor.Thisnode

should be kept small on the PCB for good performance

and low EMI.

EN/UV (Pin 25): The LT8645S/LT8646S is shut down

when this pin is low and active when this pin is high.

The hysteretic threshold voltage is 1.01V going up and

0.965V 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 LT8645S/

LT8646S will shut down.

RT (Pin 26): A resistor is tied between RT and ground to

set the switching frequency.

CLKOUT (Pin 27): In pulse-skipping mode, spread spec-

trum, 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.

SYNC/MODE (Pin 28): This pin programs four different

operating modes: 1) Burst Mode. Tie this pin to ground

for Burst Mode operation at low output loads—this will

result in ultralow quiescent current. 2) Pulse-skipping

mode. This mode offers full frequency operation down to

low output loads before pulse skipping occurs. Float this

pin for pulse-skipping mode. When floating, pin leakage

currents should be <1µA. 3) Spread spectrum mode. Tie

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

3V to 4V for pulse-skipping 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 pulse-

skipping mode.

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