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Related Documents From Texas Instruments
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 www.ti.com.
Document
bq77910A Multi-Cell Li-Ion/Polymer Precision Protector Data
Sheet
7
bq77910A EVM Circuit Description and Configuration
This section describes the circuit on the bq77910A 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 extra components on the product design which
are not needed but take up space and in some cases add unnecessary expense. Refer to the schematic
in the physical construction section while reading the following descriptions. Note that the schematic has 2
pages, the first shows the main functions, 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 should be set near or below the 5V
per cell abs max 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 might 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 to
make it easy to mount larger capacitors in case 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 over voltage from rectification of
transients by D2.
Power is provided through R4. The 200-Ω resistor provides for a voltage drop when an over voltage
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 a 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 is designed to use the
internal balancing path at near the maximum current. The 47 Ω R
10% of the 50-Ω minimum recommended in the datasheet and will prevent exceeding the absolute
maximum balancing current up to 4.3V per cell. The input capacitance is typically connected between cell
inputs, except the capacitances at VC10 and VC11 are connected to ground to avoid pushing the inputs
below ground during transients. The capacitance should not be increased beyond the 1 μF shown since
the capacitors are discharged into the device during balancing. The filter effect will both reduce cell
transients to the device inputs, and slow voltage recovery after balancing. The resulting time constant is
complicated due to the multiple components but relatively short due to the small resistors used for high
balancing current. Due to the cell input voltage measurement after balancing, the time constant (R
should not be made too large.
18
bq77910AEVM
Copyright © 2012, Texas Instruments Incorporated
Literature Number
SLUSAV6
VCX
value on the EVM is nominally within
VCX
SLUU855 – February 2012
Submit Documentation Feedback
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connect the
)
VCX
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