LTM8021 datasheet(12/28 Pages) LINER

12 / 28 page

LTM8053

8053fc

For more information www.linear.com/LTM8053

PIN FUNCTIONS

GND (Bank 1, A1, A6, B3): Tie these GND pins to a

local ground plane below the LTM8053 and the circuit

components. In most applications, the bulk of the heat

flow out of the LTM8053 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 LTM8053’s

internal regulator and to the internal power switch. 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. Decouple

this pin with at least 1µF if the voltage source for BIAS is

remote. If unused or when generating a negative output,

connect BIAS to LTM8053 GND.

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

of an internal comparator. PG remains low until the FB

pin voltage is between 0.89V and 1.05V typical. 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.

SHARE (Pin A4): Tie this to the SHARE pin of another

LTM8053 to load share. Otherwise leave floating. Do not

drive this pin.

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

frequency of the LTM8053 by connecting a resistor from

this pin to ground. The Applications Information section

of the data sheet includes a table to determine the resis-

tance value based on the desired switching frequency.

Minimize capacitance at this pin. Do not drive this pin.

FB (Pin B1): The LTM8053 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 k.

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 adjacent to the BIAS pin to ease printed circuit

board routing. Also, some applications require a feed-

forward capacitor; it can be connected from AUX to FB

forconvenientPCBrouting.Althoughthispinisinternally

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®. Tie this pin to ground

for Burst Mode operation at low output loads—this will

result in low 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 mode occurs. 3) Spread spectrum mode.

Tie this pin high (between 2.9V and 4.2V) 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 synchro-

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 2A 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. For tracking, tie

a resistor divider to this pin from the tracked output. This

pin is pulled to ground with an internal MOSFET during

shutdown and fault conditions; use a series 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 LTM8053. 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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