NCP5385
http://onsemi.com
23
FUNCTIONAL DESCRIPTION
General
The NCP5385 dual edge modulated multiphase PWM
controller is specifically designed with the necessary
features for a high current VR10 or VR11 CPU power
system. The IC consists of the following blocks: Precision
Programmable DAC, Differential Remote Voltage Sense
Amplifier, High Performance Voltage Error Amplifier,
Differential Current Feedback Amplifiers, Precision
Oscillator and Triangle Wave Generators, and PWM
Comparators. Protection features include Undervoltage
Lockout, Soft−Start, Overcurrent Protection, Overvoltage
Protection, and Power Good Monitor.
Remote Output Sensing Amplifier (RSA)
A true differential amplifier allows the NCP5385 to
measure Vcore voltage feedback with respect to the Vcore
ground reference point by connecting the Vcore reference
point to VS+, and the Vcore ground reference point to VS−.
This configuration keeps ground potential differences
between the local controller ground and the Vcore ground
reference point from affecting regulation of Vcore between
Vcore and Vcore ground reference points. The RSA also
subtracts the DAC (minus VID offset) voltage, thereby
producing an unamplified output error voltage at the
DIFFOUT pin. This output also has a 1.3 V bias voltage to
allow both positive and negative error voltages.
Precision Programmable DAC
A precision programmable DAC is provided. This DAC
has 0.5% accuracy over the entire operating temperature
range of the part. The DAC can be programmed to support
either VR10 or VR11 specifications. A program selection pin
is provided to accomplish this. This pin also sets the startup
mode of operation. Connect this pin to 1.25 V to select the
VR11 DAC table, and the VR11 startup mode. Connect this
pin to ground to select the VR10 DAC table and the VR11
startup mode. Connect this pin to VREF to select the VR10
DAC table and the VR10 startup mode.
High Performance Voltage Error Amplifier
The error amplifier is designed to provide high slew rate
and bandwidth. Although not required when operating as
a voltage regulator for VR10 or VR11, a capacitor from
COMP to VFB is required for stable unity gain test
configurations.
Gate Driver Outputs and 2/3/4 Phase Operation
The part can be configured to run in 2−, 3−, or 4−phase
mode. In 2−phase mode, phases 1 and 3 should be used to
drive the external gate drivers as shown in the 2−phase
Applications Schematic. In 3−phase mode, gate output G4
must be grounded as shown in the 3−phase Applications
Schematic. In 4−phase mode all 4 gate outputs are used as
shown in the 4−phase Applications Schematic. The
following truth table summarizes the modes of operation:
Mode
Gate Output Connections
G1 G2 G3 G4
2−Phase Normal OPEN Normal OPEN
3−Phase Normal Normal Normal GND
4−Phase Normal Normal Normal Normal
These are the only allowable connection schemes to
program the modes of operation.
Differential Current Sense Amplifiers
Four differential amplifiers are provided to sense the
output current of each phase. The inputs of each current
sense amplifier must be connected across the current
sensing element of the phase controlled by the
corresponding gate output (G1, G2, G3, or G4). If a phase
is unused, the differential inputs to that phase’s current
sense amplifier must be shorted together and connected
to V
CCP
as shown in the 2− and 3−phase Application
Schematics.
A voltage is generated across the current sense element
(such as an inductor or sense resistor) by the current
flowing in that phase. The output of the current sense
amplifiers are used to control three functions. First, the
output controls the adaptive voltage positioning, where the
output voltage is actively controlled according to the
output current. In this function, all of the current sense
outputs are summed so that the total output current is used
for output voltage positioning. Second, the output signal is
fed to the current limit circuit. This again is the summed
current of all phases in operation. Finally, the individual
phase current is connected to the PWM comparator. In this
way current balance is accomplished.
Oscillator and Triangle Wave Generator
A programmable precision oscillator is provided. The
oscillator ’s frequency is programmed by the resistance
connected from the ROSC pin to ground. The user will
usually form this resistance from two resistors in order to
create a voltage divider that uses the ROSC output voltage
as the reference for creating the current limit setpoint
voltage. The oscillator frequency range is 100 kHz/phase
to 1.0 MHz/phase. The oscillator generates up to 4 triangle
waveforms (symmetrical rising and falling slopes)
between 1.3 V and 2.3 V. The triangle waves have a phase
delay between them such that for 2−, 3−, and 4−phase
operation the PWM outputs are separated by 180, 120, and
90 angular degrees, respectively.
