XRP7720 Datasheet, PDF (14/28 Pages) Exar Corporation – Quad Output Universal Customizable PMIC with PFM

XRP7720
Quad Output Universal Customizable PMIC with PFM
THEORY OF OPERATION
CHIP ARCHITECTURE
REGULATION LOOPS
Vref
DAC
AFE
Vin
(VCC)
Fine
Adjust
Vin Feed
Forward
Vdrive
(VCCD)x
VFB
(VOUTx)
Scalar
÷1,2,4
Error
Amp
AFE
ADC
Error
Register
PID
DPWM
Gate
Driver
GHx
GLx
LXx
Window
Comp.
PFM/
Ultrasonic
Current
ADC
Figure 11 XRP7720 Regulation Loops
PWM-
PFM Sel
Figure 11 shows a simplified functional block
diagram of the regulation loops for one output
channel of the XRP7720. There are 3 separate
parallel control loops; Pulse Width Modulation
(PWM), Pulse Frequency Modulation (PFM),
and Ultrasonic. 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 scalar, a programmable Voltage
Reference (Vref) DAC, Error Amplifier, and a
window comparator. Some of the function
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 Vin feed forward
based on the voltage at the VCC pin. The
reference voltage (Vref) for the error amp is
created by a 0.15V to 1.6V DAC that has
12.5mV resolution. In order to get a 0.6V to
5.5V output voltage range an input scalar is
used to reduce feedback voltages for higher
output voltages to bring them within the 0.15V
to 1.6V control range. For output voltages up
to 1.6V (low range) the scalar has a gain of 1.
For output voltages from 1.6V to 3.2V (mid
range) the scalar 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, mid range 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 scalar to
Vref to create an error voltage on its output.
This is converted to a digital error term by the
AFE ADC which 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,
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 XRP7720 PID is a 17-bit five coefficient
control engine that calculates the correct duty
cycle under the various operating conditions
and feeds it to the Digital Pulse Width
Modulator (DPWM). Besides the normal
coefficients, the PID also uses the Vin voltage
to provide a feed forward function.
© 2014 Exar Corporation
14/28
Rev. 1.0.0