ncp5372 ON Semiconductor, ncp5372 Datasheet - Page 19

• Manufacturer Part Number: ncp5372
• Description: High Efficiency Two-phase Step-down Power Controller For Imvp-6.5 Mobile Cpu Core Power Supplies
• Manufacturer: ON Semiconductor

Auto Skip Power-saving Mode, Forced-CCM Mode and DCM DIODE EMULATION MODE
When the MODE pin is pulled high, the negative current detector is enabled and the controller is selected to
operate in auto power-saving mode. When the MODE pin is pulled low, the controller has two modes of
operation which depends on the combination of the DPSLPVR and PSI# pins as shown in the Table 2. One is
Forced-CCM and another one is diode emulation mode in DCM. The forced-CCM mode disables the zero-
crossing comparator, allowing the inductor current to reverse at light loads. This causes the low-side gate-drive
waveform to become the complement of the high-side gate-drive waveform. Forced-CCM mode is required
during downward output voltage transitions. While in diode emulation mode, the zero-crossing comparator is
enabled and this allows DCM operation at light-load. In diode emulation mode, the low-side MOSFET is turned
off when the zero-crossing is detected and the switching frequency is reduced with load. The combination of
inputs to the DPRSLPVR and PSI# pins determine the operation mode of the controller. The following table
shows the logic combination of pin DPRSLPVR and PSI#.
Current Sensing
The current sense amplifiers must provide enough bandwidth and slew rate to generate an accurate
representation of the sensed current waveform at the maximum operating frequency. The current sensing has
very low offset voltage so as to provide accurate droop control and current sharing. The sensed current voltage
is gained by 6 and summed with the offset reference voltage from the droop amplifier. The individual current
sense signals are reused in the current sharing function. The current sense signals for each phase is typically
max. 100mV.
Current Sense to Droop Transfer Function and Amplifiers
The droop signal should be an accurate representation of the summed current sense waveforms and the
amplifiers must be stable over a wide range of external loads.
VR_ON
It is the enable for the controller. When the controller is disabled by VR_ON the internal state machines must
be reset into a known starting state to prepare for Soft start. For the fault conditions of over-voltage or over-
current, it is used to restart the power cycle.
Feed-forward
The controller provides both VIN and VOUT feed-forward. For the VIN feed-forward, TON depends on VIN
change. When VIN increases, TON decreases accordingly to fix the time of period. Similarly, for the VOUT
feed-forward, TON also depends on VOUT change. When VOUT increases, TON increases accordingly to fix
the time of period. They give the equation.
TON Selection
With minimum VOUT and maximum VIN, the TON must be NOT LESS than 100nSec. Otherwise, if TON is
smaller than 100nSec, the controller may cause problem.
Current Limit
The current limit is set by voltage developed across an external resistor on the ILIM pin. An accurate bias
current flows out the ILIM pin to create a voltage across the resistor to ground. The voltage signal is heavily low
MODE
Tperiod=VIN/VOUT*TON
1
0
0
0
0
Rev0.3
Confidential and Proprietary – Not for Public Release
DPRSLPVR
X
0
0
1
1
ON Semiconductor CONFIDENTIAL
Table 2. Operation Mode
Mar 19, 2008 - Page 17
PSI#
X
0
1
0
1
Auto Power-saving (Allow DCM)
Allow DCM at Light Load
Allow DCM at Light Load
Allow DCM at Light Load
OPERATION MODE
Forced CCM
(eq. 1)