LTM8002 datasheet(11/26 Pages) LINER

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LTM8065

8065fa

For more information www.linear.com/LTM8065

PIN FUNCTIONS

GND (Bank 1, A1, A4, A6): Tie these GND pins to a local

ground plane below the LTM8065 and the circuit compo-

nents. In most applications, the bulk of the heat flow out of

the LTM8065 is through these pads, so the printed circuit

design has a large impact on the thermal performance of

the part. See the PCB Layout and Thermal Considerations

sections for more details.

VIN (Bank 2): VIN supplies current to the LTM8065’s in-

ternal regulator and to the internal power switches. These

pins must be locally bypassed with an external, low ESR

capacitor; see Table 1 for recommended values.

VOUT (Bank 3): Power Output Pins. Apply the output filter

capacitor and the output load between these pins and

GND pins.

BIAS(PinA2):TheBIASpinconnectstotheinternalpower

bus. Connect to a power source greater than 3.2V. If VOUT

is greater than 3.2V, connect this pin to AUX. If the output

voltage is less, connect this to a voltage source greater

than 3.2V. Decouple this pin with at least 1µF if the voltage

source for BIAS is remote.

PG (Pin A3): The PG pin is the open-collector output of

an internal comparator. PG remains low until the FB pin

voltage is within about 10% of the final regulation volt-

age. The PG signal is valid when VIN is above 3.4V. If VIN

is above 3.4V and RUN is low, PG will drive low. If this

function is not used, leave this pin floating.

RT (Pin A5): The RT pin is used to program the switching

frequency of the LTM8065 by connecting a resistor from

this pin to ground. The Applications Information section of

the data sheet includes a table to determine the resistance

value based on the desired switching frequency. Minimize

capacitance at this pin. Do not drive this pin.

FB (Pin B1): The LTM8065 regulates its FB pin to 0.97V.

Connect the adjust resistor from this pin to ground.

The value of RFB is given by the equation RFB = 241.53/

(VOUT – 0.97), where RFB is in kΩ.

AUX (Pin B2): Low Current Voltage Source for BIAS. In

many designs, the BIAS pin is simply connected to VOUT.

The AUX pin is internally connected to VOUT and is placed

adjacenttotheBIASpintoeaseprintedcircuitboardrouting.

Also, some applications require a feedforward capacitor;

it can be connected from AUX to FB for convenient PCB

routing. Although this pin is internally connected to VOUT,

it is not intended to deliver a high current, so do not draw

current from this pin to the load.

SYNC (Pin B4): External clock synchronization input

and operational mode. 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. Pulse-skipping mode. Float this pin for pulse-skipping

mode. This mode offers full frequency operation down

to low output loads before pulse skipping occurs.

3. Spread spectrum mode. Tie this pin high (between

2.9V and 4.2V) for pulse-skipping mode with spread

spectrum modulation.

4. Synchronizationmode.Drivethispinwithaclocksource

tosynchronizetoanexternalfrequency.Duringsynchro-

nization the part will operate in pulse-skipping mode.

TR/SS (Pin B5): The TR/SS pin is used to provide a soft-

start or tracking function. The internal 2μA pull-up current

in combination with an external capacitor tied to this pin

creates a voltage ramp. If TR/SS is less than 0.97V, the

FB voltage tracks to this value. The soft-start ramp time

is approximated by the equation t = 0.485 • C where C is

in μF. For tracking, tie a resistor divider to this pin from the

tracked output. This pinis pulled toground withan internal

MOSFETduringshutdownandfaultconditions;useaseries

resistor if driving from a low impedance output. This pin

may be left floating if the tracking function is not needed.

RUN (Pin B6): Pull the RUN pin below 0.9V to shut down

the LTM8065. Tie to 1.06V or more for normal operation. If

the shutdown feature is not used, tie this pin to the VIN pin.

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