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sames

sames

http://www.sames.co.za

PM9904BPD

Power Supply

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.

Jumpers J4, J5 and J7 allow the power supply to be completely

disconnected form the metering section from the device.

PCB DESIGN

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.

Protection

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

any common mode and asymmetrical transients.

Component placement

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.

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

SA9904B for proper supply bypassing.

Power Supply routing and de-coupling

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

other sources such as electric fields from transformers etc.

THE SA8807A LCD DRIVER

OVERVIEW

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.

This LCD driver is ideal for any micro-controller based system

requiring a liquid crystal display of up to 12 seven-segment

digits.

USING THE SA8807A

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 the LCD display reaction time.

f =7µF x 0.1Hz / C

f = Required oscillator frequency

f / 8 = back plane multiplex rate for a 4 back plane display

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.