Design Checklist for LTC297X Family of PSM Managers

Design Checklist for LTC297X Family of PSM Managers
Mike Peters
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.
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
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.
Initial Release
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