NCP81274
www.onsemi.com
15
Table 8. PHASE SHEDDING CONFIGURATIONS (continued)
Power Zone
PWM Output Status (3 = Enabled, X = Disabled)
PWM Output Configuration
Power Zone
PWM8PWM7PWM6PWM5PWM4PWM3PWM2PWM1
PWM Output Configuration
0
5 Phase
3 3 3 3 3
X X X
3
3
X X X X X X X
4
3
X X X X X X X
0
4 Phase
3 3 3 3
X X X X
2
3
X
3
X X X X X
3
3
X X X X X X X
4
3
X X X X X X X
0
3 Phase
3 3 3
X X X X X
3
3
X X X X X X X
4
3
X X X X X X X
0
2 Phase
3 3
X X X X X X
3
3
X X X X X X X
4
3
X X X X X X X
0
1 Phase
3
X X X X X X X
4
3
X X X X X X X
Power Zone Transition/Phase Shedding
The power zones supported by the NCP81274 are set by
the resistors connected to the LPC
X
pins (PSI = Low) or
PHTH
X
pins (PSI = Mid).
When PSI is set to the Mid-state, the NCP81274 employs
a phase shedding scheme where the power zone is
automatically adjusted for optimal efficiency by
continuously measuring the total output current (voltage at
the IOUT pin) and compare it with the PHTH
X
thresholds.
When the comparison result indicates that a lower power
zone number is required (an increase in the IOUT value), the
controller jumps to the required power zone immediately.
A decrease in IOUT that indicates that the controller needs
to switch into a higher power zone number, the transition
will be executed with a delay of 200 ms set by the phase shed
delay configuration register. The value of the delay can be
adjusted by the user in steps of 10 ms if required. To avoid
excessive ripple on the output voltage, all power zone
changes are gradual and include all intermediate power
zones between the current zone and the target zone set by the
comparison of the output current with the PHTH
X
thresholds, each transition introducing a programmable
200 ms delay. To avoid false changes from one power zone
to another caused by noise or short IOUT transients, the
comparison between IOUT and PHTH
X
threshold uses
hysteresis. The switch to a lower power zone is executed if
IOUT exceeds the PHTH
X
threshold values while
a transition to a higher power zone number is only executed
if IOUT is below PHTH
X
-Hysteresis value. The hysteresis
value is set to 0x10h and can be changed by the user by
writing to the phase shedding configuration register. If
a power zone/PHTH
X
threshold is disabled, the controller
will skip it during the power zone transition process.
When PSI = Low and the user requires to change the
power zone, the transition to the new power zone is identical
to the transition process used when PSI is set to the
Mid-state. The only exception is when the target power zone
is disabled in automatic phase shedding mode. In this case,
the controller will automatically enable the target power
zone and allow the transition. When the controller is set to
automatic phase shedding, the power zone will be
automatically disabled.
Switching Frequency
A programmable precision oscillator is provided. The
clock oscillator serves as the master clock to the ramp
generator circuit. This oscillator is programmed by a resistor
to ground on the FSW pin. The FSW pin provides
approximately 2 V out and the source current is mirrored
into the internal ramp oscillator. The oscillator frequency is
approximately proportional to the current flowing in the
resistor. Table 19 lists the switching frequencies that can be
set using discrete resistor values for each phase
configuration. Also, the switching frequency information is
available in the FSW configuration register and it can be
changed by the user by writing to the FSW configuration
register.
Total Current Sense Amplifier
The controller uses a patented approach to sum the phase
currents into a single temperature compensated total current
signal (Figure 8).
This signal is then used to generate the output voltage
droop, total current limit, and the output current monitoring
functions. The total current signal is floating with respect to
CSREF. The current signal is the difference between
