MAX14921EVKIT Datasheet, PDF (6/25 Pages) Maxim Integrated Products – A high-acuuracy 16-cell battery-management reference design

MAX14921 Evaluation Kit
Evaluates: MAX14921
Analog Cell Voltage Acquisition
The heart of the EV kit is the analog signal path.
The MAX14921 (U1) operates in two major phases. In
the first phase, cell voltages present at the cell stack
connector (P2) are acquired onto the sampling capacitors
C1–C16 through 100Ω series resistors R1 and R40–R54.
These resistors limit the current during fault conditions.
The default GUI sampling time of 4ms is optimized to
work with these resistors and capacitors. All cell voltages
are captured simultaneously.
In the second phase, captured cell voltages are level shift-
ed down and presented to the ADC (U3) for conversion.
A small circuit (R2 and C25) filters the signal between the
MAX14921 and the MAX11163 ADC. To improve accu-
racy, an external voltage reference, MAX6126 (U2), is
used to provide a stable reference to the ADC.
Auxiliary Inputs (T1–T3)
The MAX14921 also includes three general-purpose
analog inputs, ranging from 0 to 5V that can be multi-
plexed to the ADC for conversion. Measuring temperature
through inexpensive thermistors is one common use for
these extra inputs. Inputs T2 and T3 on the EV kit are
preset to accept a 10kΩ thermistor. The MAX9620 op amp
(U4) buffers the reference voltage to 10kΩ pullup resistors
(R59 and R60), while filtering is done with R56, R57, C27,
and C28.
Input T1 is uncommitted. The pullup resistor (R61) for this
input is not installed, and different filter components (R58
and C29) are used, optimized for faster input signals.
Cell Balancing
The EV kit includes transistors Q1–Q16 and 33Ω power
resistors for cell-balancing applications. These transistors
can be enabled or disabled, using the SPI interface, in
any combination at any time for passive cell-balancing
purposes.
Power and Isolation
The EV kit accepts power from either USB con-
nector, P102 or P105. To use the supplied GUI,
connect to P102. P105 is used only when reprogram-
ming microprocessor U101. Because a different protocol
is used for reprogramming, the GUI does not recognize
the EV kit if the USB cable is plugged in to P105. Two
MAX14523A power switches (U113 and U114) auto-
matically power the EV kit from either USB connector.
A MAX8880 linear regulator (U109) provides a clean 3.3V
supply for the microprocessor (U101) and related circuitry.
The microprocessor uses the Q101 and Q102 circuit to
switch in or out a 1.5kΩ pullup resistor on the USB DP
line, to implement USB suspend mode.
To avoid any damage to the PC in the case of accidental
miswiring, the cell voltage acquisition circuitry is elec-
trically isolated from the microprocessor circuitry and
USB interface. Two MAX14850 digital isolators (U107,
U108) provide this isolation for the SPI and other digital
signals and a MAX256 transformer driver (U111), along
with transformer T101 provide an isolated supply to a
MAX8880 linear regulator (U110). The MAX8880 provides
a clean 5V supply for all circuits in the analog capture
path, with the exception of the internal supply of the
MAX14921 and the analog supply of the MAX11163 ADC,
which power from the MAX14921’s internal 5V regulator.
Detailed Description of Software
The Windows-based graphical-user interface (GUI)
application is simple to use and demonstrates all the
important features of the device. The application auto-
matically detects the presence of an EV kit. If the applica-
tion is run before any EV kit is attached, the fields appear
mostly blank, as seen in Figure 3. Once an EV kit is
detected, fields are automatically filled in.
Status Section
Figure 5 shows the “Status” window, located at the
bottom-right corner of the application window. It indicates
the status of the EV kit connection to the PC.
The text in the window indicates whether the application
sees an EV kit or not. It indicates either “EVK Attached,”
or “EVK Detached.”
The third line details the real-time instructions to the EV kit
from the application. Table 1 includes a detailed explana-
tion for each label.
Control Section
Much of the control and status information for the EV kit is
available in the top-right section of the application. Figure
6 and Table 2 detail the available controls and indicators.
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