JS202011SCQN datasheet(29/45 Pages) MICROCHIP

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Hardware Description

 2017 Microchip Technology Inc.

DS50002673A-page 29

4.5

HARDWARE FOR I2C/SMBUS COMMUNICATION OVER USB OR EXTERNAL

I2C CONNECTION

Directly above J22 is U2. This is an MCP2221 USB-2 to I2C bridge chip that translates

the USB communications into I2C read/write commands to configure the PAC1934 and

to capture status and results data. To the left of U2 is U3, a level shifter that matches

the I2C signals from the MCP2221 to the VIO voltage that the user is using for the

PAC1934 VDD_IO. On the far right side of the board, there is a switch SW1 and a con-

nector J27. These are both used when connecting an external I2C connection directly

to the board instead of using the USB connection. J27 allows the user to connect the

external VIO, SCA, and SCL signals directly to the PAC1934. SW1 disables the USB

bridge and level shifter and connects the pull-up resistors on SCA and SCL pins to the

external VIO voltage.

4.6

SIGNAL DESCRIPTION IN DEMO MODE

In DEMO mode, all four channels measure the same ISENSE. VSENSE matching is lim-

ited by resistor matching. All four channels measure similar VBUS levels. The VSENSE

signals are equal to the current source value multiplied by 1Ω (25/50 mV).The VBUS

signal for Channel 1 is +5V USB. The VSENSE inputs for the other channels are sepa-

rated from +5V USB by each sense resistor’s voltage drop so they are offset from each

other. Also, the +5V USB is not a low impedance power supply (it is generated from the

USB connection on the PC), so there is a small square wave on each VBUS signal as

the ISENSE current interacts with the output impedance of +5V USB.

All of the plots in the GUI description above are made in the default DEMO mode

configuration.

4.7

PAC1934 CURRENT MEASUREMENTS USING THE PAC1934.

The ADM00805 Evaluation Board can be used to measure the PAC1934 VDD and VIO

currents. For both measurements below, the sense resistor values for Channel 1 (VIO)

and/or Channel 4 (VDD) require changing to 10Ω for proper scaling of the ISENSE and

power measurements.

To measure the current in the VDD_IO pin, move the shunts on J14 and J15 to the VIO

position (see Figure 4-1). This connects Channel 1's inputs across R24, which is con-

nected between VDD_IO and VIO. R24 is a 10Ω sense resistor that can measure the cur-

rent going into the VDD_IO pin on the PAC1934. This is a very low current, and allows

the user to see the benefit of using VSENSE average for making very accurate

VSENSE/ISENSE measurements for low currents.

The VSENSE measurements have a periodic variation, repeating every four measure-

ments. This periodic variation is due to a design feature that caused the average

VSENSE/ISENSE measurement (and the accumulated power) to have nearly zero offset.

Figure 4-2 displays VBUS (VDD_IO) and current into VDD_IO.

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