XRP7725 Datasheet, PDF (14/34 Pages) Exar Corporation – Power Management System

THEORY OF OPERATION
CHIP ARCHITECTURE
REGULATION LOOPS
Vref
DAC
AFE
XRP7725
Intel Node Manager Compatible Programmable
Power Management System
Vin
(VCC)
Fine
Adjust
Vin Feed
Forward
Vdrive
(VCCD)x
VFB
(VOUTx )
Scaler
÷1,2,4
Error
Amp
AFE
ADC
Error
Register
PID
DPWM
Gate
Driver
GHx
GLx
LXx
Window
Comp.
OVS
PFM/
Ultrasonic
Figure 16: XRP7725 Regulation Loops
Current
ADC
PWM -
PFM Sel
Figure 16 shows a simplified functional block
diagram of the regulation loops for one output
channel of the XRP7725. There are four
separate parallel control loops; Pulse Width
Modulation (PWM), Pulse Frequency
Modulation (PFM), Ultrasonic, and Over
Sampling (OVS). Each of these loops is fed by
the Analog Front End (AFE) as shown at the
left of the diagram. The AFE consist of an
input voltage scaler, a programmable Voltage
Reference (Vref) DAC, Error Amplifier, and a
window comparator. Some of the functional
blocks are common and shared by each
channel by means of a multiplexer.
PWM Loop
The PWM loop operates in Voltage Control
Mode (VCM) with optional V_IN feed forward
based on the voltage at the V_CC pin. The
reference voltage (Vref) for the error amp is
generated by a 0.15V to 1.6V DAC that has
12.5mV resolution. In order to provide a 0.6V
to 5.5V output voltage range, an input scaler
is used to reduce feedback voltages for higher
output voltages to bring them within the 0.15V
to 1.6V control range. So for output voltages
up to 1.6V (low range) the scaler has a gain of
1. For output voltages from 1.6V to 3.2V (mid
range) the scaler gain is 1/2 and for voltages
greater than 3.2V (high range) the gain is 1/4.
This results in the low range having an output
voltage resolution of 12.5mV, the mid range
having a resolution of 25mV and the high
range having a resolution of 50mV. The error
amp has a gain of 4 and compares the output
voltage of the scaler to Vref to create an error
voltage on its output. This is converted to a
digital error term by the AFE ADC and is
stored in the error register. The error register
has a fine adjust function that can be used to
improve the output voltage set point
resolution by a factor of 5 resulting in a low
range resolution of 2.5mV, a mid range
resolution of 5mV and a high range resolution
of 10mV. The output of the error register is
then used by the Proportional Integral
Derivative (PID) controller to manage the loop
dynamics.
The XRP7725 PID is a 17-bit five-coefficient
control engine that calculates the required
duty cycle under the various operating
conditions and feeds it to the Digital Pulse
Width Modulator (DPWM). Besides the normal
© 2014 Exar Corporation
14/34
Rev. 1.0.0