Design Checklist for LTC297X Family of PSM Managers by Mike Peters INTRODUCTION Please follow this checklist to insure a working LTC2977/LTC2974 board design. If you make any exceptions to these rules, please call applications to make sure your design will work properly. POWER CIRCUIT 1. ☐ Addressing 2. ☐ The address select pins (ASELs) are tri-level, check the addressing table on Page 22 of the LTC2978 datasheet (LTC2974 has the same addressing scheme). 3. ☐ Check for collision with other devices on the bus and any global addresses published in their datasheets (i.e. cannot use LTC4306). 4. ☐ Must have a single base address for in system programming. 5. ☐ Connect the output enable pins of LTC297X to the RUN pins of the switchers (call apps engineer immediately if you want otherwise) a. ☐ Insure appropriate pullups on all VOUT_EN b. ☐ The ABS_MAX voltages of the VOUT_EN pins are: i. ☐ VOUT_EN[3:0]: -0.3V to 15V (for both LTC2974 and LTC2978) ii. ☐ VOUT_EN[7:4]: -0.3V to 6V (LTC2978) 2. ☐ Anti-aliasing filters a. ☐ Add anti-aliasing filters to the LTC297X inputs VSENSEP, VSENSEM, ISENSEP and ISENSEM. b. ☐ The recommended filtering for voltage inputs is 100 Ω and 100nF. c. ☐ The recommended filtering for current sensing is 1kΩ and 10nF. d. ☐ Add a first stage matched filter when using DCR current sensing. e. ☐ There is no need to add an external resistive divider to sense 12V, the VIN_SNS has a calibrated divider built-in. 3. ☐ Remote Temperature Sensing (LTC2974 only) a. ☐ Use a diode-connected PNP or NPN. b. ☐ Do not use real diodes such as 1N4148 for temperature sensing! c. ☐ The ground connection should go back to the LTC2974 local ground. d. ☐ Use up to 330nF decoupling capacitance if the layout is noisy. e. ☐ Route the temperature sensing traces away from switch nodes or other noise sources. 4. ☐ Decoupling Capacitors a. ☐ Use 100nF decoupling capacitors for VPWR, VDD33, VDD25 and between REFP and REFM 5. ☐ Connect Logic/Coordination signals a. ☐ All open drain pins (ALERTB, SCL, SDA, SHARE_CLK,FAULTB) should have a single common pull up. b. ☐ Short all ALERTB together, pull up to 3.3V with a 10kΩ resistor. c. ☐ Short all SCL/SDA together, pull up to 3.3V with a 10kΩ resistor. d. ☐ Adjust SDA/SCL pull up or add an I2C bus buffer if stray capacitance is an issue. Check the rising edges of SCL/SDA with an oscilloscope to confirm. e. ☐ Short all SHARE_CLK together, pull up to 3.3V with a 5.49kΩ resistor f. ☐ Short all WP together and pull up to 3.3V with a 10kΩ resistor g. ☐ Do not leave CONTROL pins floating! Pull up to 3.3V with a 10kΩ resistor. 6. FAULT Handling a. ☐ For maximum flexibility and software control, short all FAULTB pins together and pull up to 3.3V with a single 10kΩ resistor. b. ☐ Do not mix power good, fault and control pins to design custom fault-handling or eventbased sequencing schemes. These approaches are extremely difficult to debug and do not allow last minute software fixes. 7. Floating Inputs a. ☐ Connect all unused VSENSEP, VSENSEM and DACM pins to GND. 8. Programming a. ☐ Use the schematic below for each LTC297X if programming with dongle power only is desired. b. ☐ Ensure that VDD33 consumes less than 100mA to avoid overloading the I2C dongle. c. ☐ No body diodes between SDA/SCL from any slave device are allowed. 9. ☐ Trim DAC Resistors a. ☐ Select the trim DAC resistors using the resistor-selection tool in the LTpowerPlay GUI b. ☐ From the main menu "Utilities" -> "Resistor Selection Tool" c. ☐ Enter the required information in the form (feedback voltage, desired trim range etc.) 10. ☐ Use the LTpowerPlay GUI to get Factory Apps support. The email alias automatically copies multiple people depending on availability/vacation etc. REVISION HISTORY Rev Date Description A 11/07/14 Initial Release Page Number