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LTM8055

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For more information www.linear.com/LTM8055

PIN FUNCTIONS

GND(Bank1,PinL1):TietheseGNDpinstoalocalground

plane below the LTM8055 and the circuit components.

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

LTM8055 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. Return the RFB1/RFB2 feedback

divider to this net.

VOUT (Bank 2): Power Output Pins. Apply output filter

capacitors between these pins and GND pins.

VIN (Bank 3): Input Power. The VIN pin supplies current to

theLTM8055’sinternalpowerswitchesandtooneterminal

of the optional input current sense resistor. This pin must

be locally bypassed with an external, low ESR capacitor;

see Table 1 for recommended values.

IOUT (Pin D1): Output Current Sense. Tie this pin to the

output current sense resistor. The output average current

sense threshold is 58mV, so the LTM8055 will regulate

the output current to 58mV/RSENSE, where RSENSE is the

value of the output current sense resistor in ohms. The

load is powered through the sense resistor connected at

this pin. Tie this pin to VOUT if no output current sense

resistor is used. Keep this pin within ±0.5V of VOUT under

all conditions.

LL (Pin F1): Light Load Indicator. This open drain pin

indicates that the output current, as sensed through the

resistor connected between VOUT and IOUT, is approxi-

mately equivalent to 6mV or less. Its state is meaningful

only if a current sense resistor is applied between VOUT

and IOUT. This is useful to change the switching behavior

of the LTM8055 in light load conditions.

SVIN (Pins F10, F11): Controller Power Input. Apply a

separate voltage above 5V if the LTM8055 is required to

operate when the main power input (VIN) is below 5V.

Bypass these pins with a high quality, low ESR capacitor.

If a separate supply is not used, connect these pins to VIN.

CLKOUT (Pin G1): Clock Output. Use this pin as a clock

source when synchronizing other devices to the switch-

ing frequency of the LTM8055. When this function is not

used, leave this pin open.

MODE(PinG2):SwitchingModeInput.TheLTM8055oper-

ates in forced continuous mode when MODE is open, and

canoperateindiscontinuousswitchingmodewhenMODE

is low. In discontinuous switching mode, the LTM8055

will block reverse inductor current. This pin is normally

left open or tied to LL. This pin may be tied to GND for the

purpose of blocking reverse current if no output current

sense resistor is used.

RT(PinH1):TimingResistor.TheRTpinisusedtoprogram

the switching frequency of the LTM8055 by connecting a

resistor from this pin to ground. The range of oscillation is

100kHzto800kHz.TheApplicationsInformationsectionof

the data sheet includes a table to determine the resistance

value based on the desired switching frequency. Minimize

capacitance at this pin.

SYNC(PinH2):ExternalSynchronizationInput.TheSYNC

pin has an internal pull-down resistor. See the Synchroni-

zation section in Applications Information for details. Tie

this pin to GND when not used.

FB (Pin J1): Output Voltage Feedback. The LTM8055

regulates the FB pin to 1.2V. Connect the FB pin to a

resistive divider between the output and GND to set the

output voltage. See Table 1 for recommended FB divider

resistor values.

COMP (Pin J2): Compensation Pin. The LTM8055 is

equipped with internal compensation that works well with

most applications. In some cases, the performance of the

LTM8055 can be enhanced by modifying the control loop

compensation by applying a capacitor or RC network to

this pin.

SS (Pin K1): Soft-Start. Connect a capacitor from this pin

to GND to increase the soft-start time. Soft-start reduces

the input power source’s surge current by gradually in-

creasing the controller’s current limit. Larger values of the

soft-start capacitor result in longer soft-start times. If no

soft-start is required, leave this pin open.

CTL (Pin K2): Current Sense Adjustment. Apply a voltage

below 1.2V to reduce the current limit threshold of IOUT.

Drive CTL to less than 50mV to stop switching. The CTL

pin has an internal pull-up resistor to 2V. If not used,

leave open.

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