LT8636 datasheet(11/28 Pages) AD

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LT8638S

Rev. 0

For more information www.analog.com

PIN FUNCTIONS

PHMODE (Pin 1): Pin determines the phase relationship

between the LT8638S’s internal clock and CLKOUT. Tie it to

GND for 2-phase operation, float the pin for 3-phase oper-

ation, or tie it to INTVCC for 4-phase operation. See Block

Diagram for internal pull-up and pull-down resistance.

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. Do not load the INTVCC pin with exter-

nal 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

capacitor of at least 1µF from this pin to ground close to

the IC.

BST (Pin 4): This pin is used to provide a drive volt-

age, 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 5–8): 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.

GND (Pins 9–14, 20, Exposed Pad Pins 29–32): 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 Pins 29 to 32

may be left disconnected, however thermal performance

will be degraded.

VIN (Pins 15–18): The VIN pins supply current to the

LT8638S 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.

NC (Pins 19): No Connect. This pin is not connected to

internal circuitry and can be tied anywhere on the PCB,

typically ground.

EN/UV (Pin 21): The LT8638S is shut down when this

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

etic threshold voltage is 0.98V going up and 0.94V 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 LT8638S will shut down.

RT (Pin 22): A resistor is tied between RT and ground to

set the switching frequency.

CLKOUT (Pin 23): Output Clock Signal for PolyPhase

Operation. In forced continuous mode, spread spectrum,

and synchronization modes, the CLKOUT pin provides a

50% duty cycle square wave of the switching frequency.

The phase of CLKOUT with respect to the LT8638S’s

internal clock is determined by the state of the PHMODE

pin. CLKOUT’s peak-to-peak amplitude is INTVCC to GND.

In Burst Mode operation, the CLKOUT pin will be low. Float

this pin if the CLKOUT function is not used.

SYNC/MODE (Pin 24): For the LT8638S, 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 low 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. 3) Spread

spectrum mode. Tie this pin high to INTVCC (or >3V)

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.

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