QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948 MOSFET-CONTROLLED POWER SUPPLY TRACKER LTC2926 DESCRIPTION The demonstration circuit board DC948A is intended to This compensation is achieved with automatic remote evaluate the performance of the LTC2926 MOSFET- sense switching. Two integrated switches for slave Controlled Power Supply Tracker. The board contains one supplies and a control signal source for the external LTC2926 Tracker, three power MOSFETs for tracking and switch are implemented in the LTC2926. sequencing three power supply rails, a few resistors for The power supply tracking and sequencing profiles configuring tracking profiles and one small signal known as coincident tracking, ratiometric tracking, offset MOSFET for voltage drop compensation in the master tracking, or supply sequencing are accomplished by se- rail. As assembled, DC948A operates with +5.0V as the lection of the four resistor values per channel (RTA, RTB, master rail, and +3.3V and +2.5V as slave rails. The RFA, and RFB). board features master supply slew rate of S =100V/s, M and slave supply slew rates of S =150V/s. The first S slave supply power up starts 12.5ms later than mas- Design files for this circuit board are available. Call ter supply and second slave starts 25ms later. the LTC factory. The LTC2926 is designed to provide compensation for , LTC, and LT are registered trademarks of Linear Technology Corporation. voltage drop across each power MOSFET and its traces, when an individual power supply is used for each rail. PERFORMANCE SUMMARY SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Voltage Operating Range 2.9 3.3 5.5 V CC V Input Supply Undervoltage Lockout V Rising 2.2 2.4 2.6 V CC(UVLO) CC V ON Pin Threshold Voltage V Rising 1.20 1.23 1.26 V ON(TH) ON V ON Pin Fault Clear Threshold Voltage V 0.465 0.500 0.535 V ON(CLR) ON FALLING V Master Supply Voltage 5.0 V MASTER V Slave Supply 1 Voltage 3.3 V SLAVE1 V Slave Supply 2 Voltage 2.5 V SLAVE2 S Master Gate Voltage Slew Rate 100 V/s MGATE S Slave Supply 1 Slew Rate 150 V/s SLAVE1 S Slave Supply 2 Slew Rate 150 V/s SLAVE2 I Slave Supply Minimal Load Current 20 mA S MIN 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948 MOSFET-CONTROLLED POWER SUPPLY TRACKER OPERATING PRINCIPLES The master rail is controlled like a typical hot swap cir- S =S RFB/RTB. SLAVE MASTER cuit, where the master gate voltage slew rate defines The relation RFB/RTB for initially installed components the output slew rate. With the 10uA gate driver current for both channels equals 1.5. capability, 0.1uF gate capacitor CMGATE placed on the The master signal ramps up and the slave supplies track board, and negligible MOSFET gate capacitance (1.5nF) the master signal, when the ON pin signal rises above the master slew rate is 100V/s. 1.23V. For managing each slave rail the LTC2926 Tracker con- The board allows two options for V source selection. tains individual tracking and gate controller cells. CC Jumper JP1 ON position connects the master supply to The tracking cell servos the TRACK pin to 0.8V by pro- V node, and the OFF position allows the use of an ex- CC viding the required current into the Track resistor to keep ternal voltage source. the voltage on the TRACK pin equal to 0.8V. The current There are also two options for the RAMP pin signal supplied by the TRACK pin is mirrored to the feedback FB source. Jumper JP2 placed in the MASTER position con- pin. The TRACK pin node is connected to the RAMPBUF nects master rail output voltage to the RAMP pin. pin and GND with external resistors RTB and RTA respec- Jumper JP2 EXTERNAL position allows the use of an tively. external source. The gate controller cell servos the FB pin to 0.8V by driv- After master and slave gate drivers reach their maximum ing the gate of the external N-channel power MOSFET. voltages, the LTC2926 closes integrated remote sense This establishes the slave output voltage based on the switches and pulls up the external FET gate with a 10uA TRACK pin current and the feedback divider resistors. current source as an external remote switch control sig- This cell is a bang-bang control system with the inner nal. command (reference) signal equal to 0.8V and feedback provided from the feedback pin node, which is connected For the proper LTC2926 operation, each slave rail out- to two resistors: RFA to GND and RFB to the slave rail put should be loaded to consume a current, which ex- output. When the slave output reaches its nominal out- ceeds 50-100 times the steady state individual feedback put, the feedback divider should provide the FB pin with a divider current. Initially populated DC948A requires a voltage a little bit lower than the inner command. In this 100-150 Ohm resistive load or 150-200uF capacitive case, the gate controller cell develops maximum gate load. voltage to enhance the external MOSFET. Refer to the LTC2926 data sheet for detailed equations Figure 1, borrowed from the Data Sheet, demonstrates and design examples. the structure of the tracking and gate controller cells and simplifies understanding their interaction. The slave output voltage slew rate is defined as: 2