Datasheet

MCP3901 Low-Cost Power Monitor Reference Design

DS51915A-page 10  2010 Microchip Technology Inc.

1.2 ANALOG INPUT CIRCUIT

The MCP3901 Low-Cost Power Monitor Reference Design uses an MCP3901 dual

ADC to acquire current and voltage samples. For best performance, the power supply

and ground must be noise free. To ensure low noise, large capacitors are located on

the lines that power the MCP3901 device, i.e., C4 and C5. Additionally, multi-layer

ceramic capacitors are located near the ADC, on the C13 and C14 pins, to ensure that

high-frequency noise is also eliminated.

The V

REF

is potentially another source of noise. Accordingly, it is mandatory to place at

least one 100 nF multi-layer capacitor on the V

REF

pin. For better noise rejection on

REF

, a larger capacitor has been added (C77).

The MCP3901 Low-Cost Power Monitor is provided with a 200 µ shunt as a current

sensor. The user has the option to use the two current transformer footprints U1 and

U10 that are available on the printed circuit board (PCB). Refer to the board schematic

in the appendix, A.1 “Board Schematic – Analog and Power”.

The MCP3901 Power Monitor Reference Design does not contain a crystal – it uses

the clock signal from the output compare pin RC2/CCP1 of the PIC18F25K20 micro-

controller (MCU).

1.3 POWER CIRCUIT

Two voltages are required for the power monitor reference design:

- 3.3V for the MCU

- 5V for the ADC

For this reason, two MCP1703 Low Dropout Voltage Regulators (LDOs) are placed

after the C51 capacitor, with the required voltages at the outputs.

The meter is powered from the capacitive divider, mainly C6 and U53. A parametric

regulator circuit, using Zener diode D5, limits the input voltage of the LDOs to 12V.

Rectifier diode D2 restricts the current flow to a single direction, while ripple is reduced

by C51 and the LDOs.

1.4 PIC18F25K20 MICROCONTROLLER AND LIQUID CRYSTAL DISPLAY (LCD)

A PIC18F25K20 MCU is used in this application for its high speed (16 MIPS) and low

power (nanoWatt XLP technology). It also has an internal EEPROM, where the calibra-

tion constants are saved.

Because the MCU does not include an LCD driver, the LCD used in this reference

design has the driver built in. The connection between the LCD and the MCU carries

four lines of data and three lines of control.