bq769x0 Circuit Module Use
6.6
Connecting to a Host
After initial operation of the AFE with the bq76940/bq76930/bq76920 software, it may be desirable to
operate the board connected to a microcontroller board. J14 can be used to connect to the microcontroller
board. No voltages should be applied to the gauge terminals. Alternately, the microcontroller could be
connected to the signal test points or J8 and the ALERT test point. The interface voltage for the installed
AFE IC is 2.5 V.
6.7
Gauge Circuits
The EVM contains a gauge circuit consisting of U2 and an SMBus interface connector, J14. This bq78350
IC and circuit can be used to control the AFE if configured and connected at the J12 header. By default
the AFE on the EVM is a 2.5V output device, only connect the gauge circuitry to a 2.5-V output AFE IC.
Shunts may be placed on the /KEYIN and /PRES headers to simulate control of these signals. An
alternate SMBus address may be selected using the Addr select header. Refer to the bq78350 TRM for
details and configuration selections for these device features.
The BV header typically has a shunt on the H position for the full number of supported cells. When the cell
count or maximum cell voltage is reduced, refer to the schematic and the bq78350 documents for
alternate positions which may better use the 1 V input range of the BAT pin. Making a selection which
would exceed the 1V limit is not recommended.
6.8
Unused Components
The EVM contains a number of component patterns which may be useful for evaluation. Test points are
not typically populated. The patterns may be used as probe points or wires or test points can be soldered
to provide probing if desired.
The Q1 and Q3 patterns are for parallel power FETs.
A heatsink pattern is provided at each power FET position (HS1 - HS4) for optional heatsink attachment.
Other heatsink options may be available in the evaluation environment.
D4 is a flyback diode to prevent PACK- from rising significantly above PACK+. The D5 pattern provides a
place to mount a higher current diode or other transient suppression component.
D16 to D21 and D32 to D37 are patterns for Schottky diodes. When the battery is short circuited, the cell
voltages will drop and the inputs are pulled below the group power reference pin and current flows from
the inputs. If this causes problems, diodes at these locations would conduct to prevent high current from
the input pins. These were not needed in EVM testing.
The ALERT line will switch high and low in normal operation as status bits are asserted and cleared. A
large load is not desired, since it consumes power. If it is useful to slow the transition, the pattern C20 is
available. C20 should not be large to avoid current and slowing the edge to where the bq769x0 would see
the ALERT high as an input and set the OVRD_ALERT condition.
Normally the power filter components (Rf and Cf in the datasheet) R57-59 and C34-C36 keep the supply
voltage for the AFE cell groups in a safe operating range. For situations with large transients, the D38-40
patterns provide positions for clamp diodes, if needed. The pattern is large and it may be easy to fit other
component sizes, if needed. Be aware that if the system transients are large enough that a clamp is
needed at the supply pins, the cell inputs should also be inspected for excessive voltages and an
improved filter or clamp be added there, if needed.
The REGSRC pin also has the D42 pattern as a provision for a clamp diode. REGSRC is referenced to a
filtered node by the gate of Q14 and should not normally have transients which need clamping.
R60-62 and R79 are current measurement shunt resistors. Current into the power pins of U1 can be
determined by measuring the voltage across these resistors at the related test points and dividing by the
resistance (100). These resistors are populated on the EVM but could be shorted or combined with other
series resistance when using the EVM as a reference for a circuit design.
Q21 is an alternate transistor pattern for Q20 when a lower power transistor is suitable such as when
using fewer LEDs or lower voltage.
R91 and R92 are options to pull down unused output pins on U2
30
bq76930 and bq76940 Evaluation Module User's Guide
SLVU925B – April 2014 – Revised July 2014
Copyright © 2014, Texas Instruments Incorporated