Application Note
12 of 63
V 1.2
2018-12-12
5 Output PMIC Controller/ IRPS5401
User guide with DB295 and DB296 demo boards
The 5x5 via pattern under the IRPS5401; these help transfer heat from the IC to the copper GND planes
on the PCB
Decouple VCC, MTP, and ADDR_PROT pins directly to AGND (pin 50) and tie AGND to GND with a
single point connection (see arrow above)
SW-A in external mode requires ISENSE and VSENSE lines to be routed back to the IC as differential
pairs
4.1
Setting the Switching Frequency
The phase angle between all switchers is maintained at a constant value. SW-A is at 0°, SW-C is at 90°, SW-B is
at 180° and SW-D is at 270°. A and B are 180° out of phase and C and D are 180° out of phase. In order to
accomplish this, all Switchers must operate at the same switching frequency (fsw). To that end, the fsw for all
switchers is controlled by the FREQUENCY_SWITCH PMBus command for SW-C.
SW-C will ACK any FREQUENCY_SWITCH command from 200 kHz to 2 MHz in increments of 1 kHz. Internally the
command is decoded and the actual FSW is set to the nearest value that can be supported with the 48 MHz
internal clock. For example, 500 kHz can be supported with ninety-six (96) 48 MHz clocks. So if you ask for 500
kHz, you get exactly 500 kHz. But if you wanted 450 kHz, the number of clocks required is 106.6667 (48/0.45). In
this case, the frequency would be set to one hundred and seven (107) 48 MHz clocks or 448.6 kHz. Fractional
values of 0.5 and above are rounded up to the next whole number.
FREQUENCY_SWITCH write commands to SW-A, B, and D will be ACK’d and ignored. FREQUENCY_SWITCH read
commands sent to SW-A, B, and D will read back the value sent in the ignored command.
SW-A FREQUENCY
The user can set the FSW of SW-A to be ½ of SW-C by sending a FREQUENCY_SWITCH command to SW-A
that is less than the FSW of SW-C. It does not have to be the value that is ½ of SW-C. Any FSW smaller
than SW-C FSW will result in SW-A FSW being set to ½ of SW-C. This ‘special’ ½ FSW ability only applies
to SW-A. Setting FSW for SW-A to ½ FSW of SW-C is usually done when SW-A is used in EXT driver mode.
FSW for SW-B and D will always be the same as SW-C
If the FSW of SW-C is less than 400 kHz, the FSW for SW-A will be the same as SW-C regardless of the FSW
setting of SW-A. The FSW of SW-A can’t be less than 200 kHz
SUMMARY
The FSW for all 4 switching regulators is controlled by SW-C. The FSW for SW-A may be set to ½ of SW-C
Because SW-C is the master, FREQUENCY_SWITCH ‘read’ commands to SW-A, B and D may not reflect the actual
FSW values for these outputs.
4.2
AOT Mode
Adaptive on-time (AOT) is a means of saving power at low output currents by reducing the switching frequency
and shutting off the LS FET before the inductor current goes negative. AOT mode is enabled by sending a MFR
specific PMBus command. Sending command D9h with a value 00h will enable AOT mode. AOT is sometimes
called discontinuous current mode (DCM) because the inductor current will go to 0 A.
In the GUI, select command D9h (MFR_FCCM), ‘Adaptive on-time’, then hit ‘write’ to switch between FCCM
(forced continuous conduction mode) and AOT. In AOT mode the switching frequency is a function of the load
so the enforced phase angle between outputs does not apply. As the load increases, the FSW will increase.
When the load is large enough to cause the FSW to be the same as the FSW set in FCCM, the switcher will
automatically switch to FCCM.
