PM9904BPD
ssaammeess
Power Supply
Power Supply routing and de-coupling
Referring to figure 15, capacitor C10 is charged through D2
during the positive half of the sine wave from the R29, C12
mains voltage dropper. Identical charging circuitry exists for
the other two phases. During the negative sine wave, C11 is
charged through diode D1. The unregulated voltage charged
on C10 and C11 is limited to 47 V by means of zener diode D7.
Resistors R32 and R33 act as current limiting resistors that
feed the unregulated voltage to the positive and negative
voltage regulators U3 and U4. The voltage regulators need a
load capacitance of around 10µF (C8 and C9) to be in a stable
operating region. C15 acts as a supply voltage storage
capacitor.
The 5V supply is de-coupled and routed directly to the power
pins of the SA9904B by means of capacitor C15. Care was
taken not to have current flowing in the node that connects the
voltage reference resistor to VSS as it may introduce power
supply noise on the voltage reference circuit.
Signal Routing
The signal routing is done in such a manner that any signal
coupling in to the measured signal will be a common mode
noise signal and is subsequently rejected. Care should be
taken that the signals to the SA9904B not be influenced by
othersourcessuchas electricfieldsfromtransformersetc.
THE SA8807A LCD DRIVER
Jumpers J4, J5 and J7 allow the power supply to be completely
disconnected form the metering section from the device.
OVERVIEW
PCB DESIGN
The SAMES SA8807A Liquid Crystal Display (LCD) driver is
capable of driving up to 96 LCD segments and is designed for
displays having 3 or 4 track multiplexed back planes. The
SA8807A includes an on-chip oscillator and needs only a
single external capacitor. Communication to the SA8807A is
via the Serial Peripheral Interface (SPI) which is shared with
the SA9904B.
The PM9904AP evaluation module represents a Class 1
meter and is designed to demonstrate the functionality and
performance of the SA9904B metering integrated circuits. The
SA9904B is mainly the analog front end of a meter. The
SA9904B measures the energy, voltage and frequency which
are made available to an external micro-controller, by way of
JP1, or to a PC. When the meter’s PCB is designed, it should
be remembered that the SA9904B inputs are analog and
special care need to be taken with the power supply and signal
routing to the SA9904B.
This LCD driver is ideal for any micro-controller based system
requiring a liquid crystal display of up to 12 seven-segment
digits.
Protection
USING THESA8807A
The SA9904B should be protected from the measuring
environment. This is achieved by using resistor dividers to
scale all the SA9904B’s input signals. MOV's Z1, Z2, Z3
together with resistors R29, R30, R31 protect the power
supply capacitors as well as the voltage sense inputs. The
current setting resistors on the current sense inputs attenuates
anycommonmodeandasymmetricaltransients.
Oscillator
The SA8807A includes an on-chip oscillator that is controlled
by a single external capacitor. Adjusting the capacitor value will
change operating frequency of the SA8807A. The back plane
multiplexing is a function of the SA8807A operating frequency.
It is thus important to select the frequency high enough that the
multiplexing of the display is not noticeable, but still within limits
of theLCDdisplayreactiontime.
Component placement
f =7µFx0.1Hz/ C
All the resistors on the SA9904B’s current sense inputs should
be placed as close as possible to the SA9904B. This
eliminates the possibility of any stray signals coupling into the
input signals.
f =Requiredoscillatorfrequency
f / 8=backplanemultiplexratefora4backplanedisplay
SPI Interface
The SA8807A shares the SPI interface with the SA9904B and
connects directly to the opto-couplers on the PM9904BPD
evaluation board. The CE signal enables the SPI interface for
the display driver and the CS signal enables the SPI interface
for the SA9904B.
Ground Plane
The GND pin of the SA9904B is connected to the neutral
phase, which is halfway between VDD and VSS. Note that
supply bypass capacitors C1 and C2 are positioned as close
as possible to the supply pins of the SA9904B, and is
connected to a solid ground plane. Capacitor C6 is also
positioned as close as possible to the supply pins of the
SA9904Bforpropersupplybypassing.
9/22
http://www.sames.co.za
SAMES [ SAMES ]
