Datasheet LTM8071 (Analog Devices) - 10

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PIN FUNCTIONS GND (Bank 1, A1):
Tie these GND pins to a local ground
AUX (Pin E2):
Low Current Voltage Source for BIAS. plane below the LTM8071 and the circuit components. In many designs, the BIAS pin is simply connected to In most applications, the bulk of the heat flow out of the VOUT. The AUX pin is internally connected to VOUT and LTM8071 is through these pads, so the printed circuit is placed adjacent to the BIAS pin to ease printed cir- design has a large impact on the thermal performance of cuit board routing. Also, some applications require a the part. See the PCB Layout and Thermal Considerations feed-forward capacitor; it can be connected from AUX sections for more details. to FB for convenient PCB routing. Although this pin is
V
internally connected to VOUT, it is not intended to deliver
IN (Bank 2):
VIN supplies current to the LTM8071’s internal regulator and to the internal power switch. These a high current, so do not draw current from this pin to pins must be locally bypassed with an external, low ESR the load. capacitor; see Table 1 for recommended values.
SYNC (Pin B1):
External clock synchronization input
V
and operational mode. This pin programs four different
OUT (Bank 3):
Power Output Pins. Apply the output filter capacitor and the output load between these pins operating modes: 1) Burst Mode®. Tie this pin to ground and GND pins. for Burst Mode operation at low output loads—this will result in low quiescent current. 2) Pulse-skipping mode.
BIAS (Pin E1):
The BIAS pin connects to the internal Float this pin for pulse-skipping mode. This mode offers power bus. Connect to a power source greater than 3.2V. full frequency operation down to low output loads before If VOUT is greater than 3.2V, connect this pin to AUX. pulse-skipping mode occurs. 3) Spread spectrum mode. Decouple this pin with at least 1µF if the voltage source Tie this pin high (between 2.9V and 4.2V) for pulse- for BIAS is remote. skipping mode with spread spectrum modulation. 4)
PG (Pin C1):
The PG pin is the open-collector output Synchronization mode. Drive this pin with a clock source of an internal comparator. PG remains low until the FB to synchronize to an external frequency. During synchro- pin voltage is between 0.89V and 1.05V typical. The PG nization the part will operate in pulse-skipping mode. signal is valid when VIN is above 3.6V. If VIN is above
TR/SS (Pin B2):
The TR/SS pin is used to provide a 3.6V and RUN is low, PG will drive low. If this function soft-start or tracking function. The internal 2μA pull-up is not used, leave this pin floating. current in combination with an external capacitor tied
SHARE (Pin C2):
Tie this to the SHARE pin of another to this pin creates a voltage ramp. The output voltage LTM8071 to load share. Otherwise leave floating. Do not tracks to this voltage. For tracking, tie a resistor divider drive this pin. to this pin from the tracked output. This pin is pulled to ground with an internal MOSFET during shutdown and
RT (Pin A2):
The RT pin is used to program the switching fault conditions; use a series resistor if driving from a frequency of the LTM8071 by connecting a resistor from low impedance output. This pin may be left floating if the this pin to ground. The Applications Information section tracking function is not needed. During start-up, if a rela- of the data sheet includes a table to determine the resis- tively low capacitor is used on TR/SS, the output voltage tance value based on the desired switching frequency. may take longer to reach regulation than expected. If Minimize capacitance at this pin. Do not drive this pin. accurate start-up timing is required, please refer to the
FB (Pin D1):
The LTM8071 regulates its FB pin to 0.97V. LTM8071 simulation model in LTspice for help selecting Connect the adjust resistor from this pin to ground. The an appropriate soft-start capacitor. value of RFB is given by the equation RFB = 241.5/(VOUT
RUN (Pin A3):
Pull the RUN pin below 0.9V to shut down – 0.97), where RFB is in kΩ. the LTM8071. Tie to 1.06V or more for normal opera- tion. If the shutdown feature is not used, tie this pin to the VIN pin. Rev 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Package Photos Package Description Typical Application Related Parts