Table of Contents
Advertisement
Related Documents From Texas Instruments
Communication with the bq77908A from other hosts must be straightforward. See to the data sheet for
timing and interface details. On the EVM, zero delay mode can be set to allow communication using a
0.1-inch shunt (not provided) at J6 pins 1 to 2, or a signal at ZD on J3. To program, the onboard
programming voltage switch can be used by controlling the PGM signal on J3. Signals must be 3.3-V
CMOS levels.
6
Related Documents From Texas Instruments
To obtain a copy of any of the following TI document, call the Texas Instruments Literature Response
Center at (800) 477-8924 or the Product Information Center (PIC) at (972) 644-5580. When ordering,
identify this document by its title and literature number. Updated documents can also be obtained through
the TI Web site at
Document
bq77908A Multi-Cell Lithium-Ion/Polymer Precision Protector
Data Sheet
7
bq77908AEVM Circuit Description and Configuration
This section describes the circuit on the bq77908A circuit module and how to configure it for changes the
user may want to make to the board for specific evaluations. In many cases, an EVM schematic is used
as a starting point for a circuit design. This can result in unnecessary extra components on the product
design that take up space and add cost. Refer to the schematic in
descriptions. Note that the schematic has two pages: the first shows the main functions, and the second
page shows primarily cell count configuration and protection components.
7.1
Battery Voltage Clamp
The TVS diode D9 provides a clamp for transients on the pack. A common transient is due to system
inductances and sudden load shutoff. The breakdown voltage of D9 must be set near or below the 5-V per
cell absolute maximum of the device. As a transient pushes the voltage up the I-V curve, a substantial
device here reduces the demands on other clamping components in the system, which may be needed to
protect the device. Alternatives to D9 may be to control the system inductance and current switching rate,
use capacitance to absorb transients, or other transient protectors such as MOVs where appropriate.
7.2
Device Power
The device is powered through the BAT pin. C12 is a bypass capacitor near the IC; the footprint is large in
order to make it easy to mount larger capacitors, in the event that this is desired in evaluation. R5 and J1
provide a way to measure current to the device during evaluation and are not needed in an application. D2
provides power from the battery to the IC and prevents the IC power from being pulled down when the
battery voltage dips suddenly due to a heavy load on the battery. C2 provides an operating reservoir for
device current during battery voltage droop. D1 limits the voltage to the BAT pin preventing overvoltage
from rectification of transients by D2.
Power is provided through R4. The 200-Ω resistor provides for a voltage drop when an overvoltage
charger pulls down BAT through D8. For lower cell counts, this resistor may be a smaller value. R3 is a
0-Ω connection to the high-current battery positive connection BATT+ which is shared with the PACK+.
This jumper is provided in case the user wants to power the part from the current monitor line instead.
7.3
Cell Monitor Inputs and Configuration
The cell monitor inputs are provided through an RC filter network. The series resistors R
cells to the device to allow voltage measurement and limit the balancing current. Balancing current is
approximately the cell voltage divided by twice the resistor. The EVM is circuit-designed to use the internal
balancing path at near the maximum current. The 47-Ω R
the 50-Ω minimum recommended in the data sheet and prevents exceeding the absolute maximum
balancing current up to 4.3 V per cell. The input capacitance is typically connected between cell inputs,
except the capacitances at VC8 and VC9 are connected to ground to avoid pushing the inputs below
ground during transients. The capacitance must not be increased beyond the 1 µF shown because the
18
bq77908AEVM Evaluation Module
www.ti.com
Copyright © 2012, Texas Instruments Incorporated
Literature Number
SLUSAV5
Section 8
while reading the following
value on the EVM is nominally within 10% of
VCX
Submit Documentation Feedback
www.ti.com
connect the
VCX
SLUU854 – February 2012
Advertisement
Table of Contents
