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LTM8055 Datasheet(PDF) 7 Page - Linear Technology |
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LTM8055 Datasheet(HTML) 7 Page - Linear Technology |
7 / 26 page LTM8055 7 8055fb 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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