Texas Instruments | bq77905 Shutdown for Current Reduction | Application notes | Texas Instruments bq77905 Shutdown for Current Reduction Application notes

Texas Instruments bq77905 Shutdown for Current Reduction Application notes
Application Report
SLUA826 – July 2017
bq77905 Shutdown for Current Reduction
Willy Massoth ................................................................................. BMS: Battery Monitoring & Protection
ABSTRACT
The bq77905 3-5S Low-Power Protector for lithium-ion batteries features a low 6-µA typical supply current
to extend battery life. Power tools or other applications where the battery is in storage or unused for long
periods may want to prevent battery discharge during idle times. The schematic examples and test results
in this document help the battery electronics designer when implementing a circuit topology to reduce
battery current.
1
2
3
4
Contents
Introduction ................................................................................................................... 2
Single Device ................................................................................................................ 3
Stacked Devices ............................................................................................................ 7
References .................................................................................................................. 12
1
Common Circuit Implementation........................................................................................... 2
2
Ineffective VDD Switch ...................................................................................................... 3
3
Shutdown Circuit ............................................................................................................. 4
4
Single Device Shutdown Example
5
Single Device Turn Off
List of Figures
6
7
8
9
10
11
........................................................................................ 5
..................................................................................................... 6
Ineffective Switch With Stacked Devices Due To Leakage Path ..................................................... 7
Stacked Circuit Shutdown With CTRC and CTRD Leakage ........................................................... 8
Stacked Device Shutdown Circuit ......................................................................................... 9
Stacked Switch Example Test Circuit ................................................................................... 10
Overtemperature Fault on Top Device .................................................................................. 11
Stacked Devices Turn Off Using Test Circuit .......................................................................... 12
Trademarks
All trademarks are the property of their respective owners.
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1
Introduction
1
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Introduction
The bq77905 3-5S low-power protector is an easy-to-use component for lithium-ion battery circuits. With
the common simple schematic shown in Figure 1, the part is continuously powered. The current load on
the battery is the low IC operating current, the load current of the RGS resistors, and the open wire test
currents. The low operating current of the bq77905 can give a long battery life, but some systems which
are infrequently used may want to reduce the current.
PACK+
RVDD
CVDD
RIN
RIN
RIN
RIN
RIN
CVDD
CIN
CIN
VDD
DVSS
AVDD
CTRD
VC5
CTRC
VC4
CCFG
VC3
VTB
VC2
VC1
CIN
CIN
bq77905
RTS_PU
RTS
TS
LD
AVSS
CHG
SRP
CHGU
SRN
DSG
RCHG
RDSG
RLD
CIN
RGS
RSNS
RGS
PACKCopyright © 2016, Texas Instruments Incorporated
Figure 1. Common Circuit Implementation
2
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Single Device
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2
Single Device
A typical attempt to reduce current is to disconnect power to the IC VDD pin while leaving the remainder
of the IC connected similar to Figure 2. This approach is not effective with the bq77905 since there are
internal leakage paths between VC5 and VDD. Disconnecting VDD with VC5 connected biases the part in
a way which increases total current and provides an incorrect voltage for the top cell due to the voltage
drop on the VC5 resistor.
IVdd
VDD
bq77905
VC5
IVC5
VC4
IVC4
VC3
IVC3
VC2
DVSS
IVC2
AVSS
VC1
IVC1
Copyright © 2017, Texas Instruments Incorporated
Figure 2. Ineffective VDD Switch
Table 1. Input Current Comparison
Input Current, µA
Switch VDD Only
Input
ON
OFF
VDD
6.03
VC5
0.109
VC4
Switch Cell 5
ON
OFF
0.000
6.03
0.000
37.6
0.109
0.000
0.114
0.114
0.115
0.000
VC3
0.112
0.112
0.113
0.000
VC2
0.101
0.101
0.102
0.000
VC1
0.108
0.108
0.109
0.000
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Single Device
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Moving the switch from the VDD to the common net from cell 5 breaks both the VDD and VC5 inputs to
the part. This eliminates the current into the part. When the part shuts down, the open wire test current to
the lower input pins is turned off. Figure 3 shows the switch in the cell 5 connection. Table 1 shows a
comparison of switching only VDD and switching the common cell 5 connection.
IVdd
VDD
bq77905
Icell5
VC5
IVC5
VC4
IVC4
VC3
IVC3
VC2
IVC2
VC1
DVSS
AVSS
IVC1
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Figure 3. Shutdown Circuit
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Single Device
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A switch can be implemented and controlled as desired in the system. One method is to control a Pchannel FET with either an N-channel FET or NPN. Figure 4 shows an example circuit with an ENABLE
signal that could be connected to PACK+ to enable the pack. Table 2 shows the value used for the
example test. With a low duty cycle the ON current is not a significant concern, but values could be
optimized and transient protection added for a system design.
PACK+
Q1
BSS84
R1
Q2
BC846B
R2
R4
bq77905
R3
VDD
ENABLE
VC5
VC4
VC3
VC2
VC1
DVSS
AVSS
Copyright © 2017, Texas Instruments Incorporated
Figure 4. Single Device Shutdown Example
Table 2. Single Device Shutdown Example Component
Values
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Reference Designator
Value (kΩ)
R1
510
R2
510
R3
5.1
R4
30
bq77905 Shutdown for Current Reduction
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5
Single Device
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Figure 5 shows example waveforms when ENABLE is disconnected from PACK+. The 10-MΩ scope
probes load the circuit and will decrease the turn off time. The gate voltage falls as VDD drops until the
gate is turned off by undervoltage or the shutdown threshold. With the drop in gate voltage, the part
should not be turned off when the pack is loaded.
Figure 5. Single Device Turn Off
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Stacked Devices
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3
Stacked Devices
When devices are stacked for higher cell count packs the top cell for each part needs to be switched, but
there is another consideration. The stacking interface includes a clamp to the VDD of the next lower part.
This clamp can power the lower part from the FET outputs of the upper part since, even when off, the
DSG and CHGU pins cannot fall substantially below the VSS voltage of the upper device. Figure 6 shows
a switching concept and the internal clamps that prevent the power down of the lower part.
bq77905
CTRD
VDD
CTRC
VC5
VC4
CHGU
VC3
DSG
VC2
DVSS
VC1
AVSS
10M
10M
Icell5
bq77905
VDD
VC5
CTRD
CTRC
VC4
IVC4
VC3
CHG
DSG
IVC3
VC2
DVSS
IVC2
VC1
AVSS
IVC1
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Figure 6. Ineffective Switch With Stacked Devices Due To Leakage Path
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Stacked Devices
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Figure 7 shows an example of switching only the upper cells to the devices. The scope probes load the
circuit but the variation of VDD1 can be observed as the lower part oscillates between the VSHUT and VPOR
and levels.
Figure 7. Stacked Circuit Shutdown With CTRC and CTRD Leakage
To avoid the leakage from the upper FET outputs to the lower device power through the substrate diodes,
the upper FET control signals must be allowed to fall below the VSS level. This function is built into the
CHG pin, but must be externally implemented with the DSG pin. To prevent the CTRD and CTRC pins of
the lower device from falling to VSS, and re-enabling the FETs in the case of a fault with the upper device
during operation, diodes are used from the VDD pin of the lower device.
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Stacked Devices
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Figure 8 shows a circuit proposal to allow shutdown of stacked devices avoiding the leakage into CTRC
and CTRD. A specification concern may exist for some user's since the maximum VCTR(DIS) for the stacking
input is 0.7 V while the maximum V(FETOFF) is 0.5 V. This apparently provides little margin, however CHG is
not held at the maximum and is allowed to fall below VSS. The designer should satisfy themselves the
circuit will work in their application before implementing this design.
bq77905
IVdd2
CTRD
VDD
Icell10
CTRC
VC5
IVC10
VC4
IVC9
CHG
VC3
DSG
VC2
DVSS
VC1
AVSS
10M
10M
Icell5-2
bq77905
IVdd1
VDD
Icell5-1
VC5
CTRD
CTRC
IVC5
VC4
IVC4
VC3
CHG
DSG
VC2
VC1
DVSS
AVSS
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Figure 8. Stacked Device Shutdown Circuit
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Stacked Devices
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Figure 9 shows a test circuit which implements switches for both upper and lower devices and the
proposal to prevent pullup through the CTRC and CTRD pins. The pack is enabled when ENABLE is
connected to PACK+. R5 and R6 provide a load on the entire battery while R1 and R2 load only the lower
cells. A compensating load could be added from Q3 drain to the upper VSS, but was not implemented in
this circuit. Other switch configurations could be implemented, use appropriate transient protection for a
system design.
Q3
BSS84
PACK+
R5
R6
bq77905
CTRD
VDD
CTRC
VC5
VC4
CHG
VC3
DSG
VC2
Q4
BSS84
DVSS
VC1
AVSS
10M
Q1
BSS84
10M
Q2
BC846B
R2
R4
R3
R1
ENABLE
bq77905
VDD
VC5
CTRD
CTRC
VC4
VC3
CHG
DSG
VC2
VC1
DVSS
AVSS
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Figure 9. Stacked Switch Example Test Circuit
10
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Stacked Devices
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The circuit was tested with the component values shown in Table 3. With the circuit enabled, the circuit
operates as expected. Figure 10 shows the operation of the FET controls in response to an
overtemperature on the top device. When the upper device CHG and DSG are off, CTRC and CTRD of
the lower device are held near the VDD level and the lower DSG and CHG turn off.
Table 3. Stacked Switch Circuit Component Values
Reference Designator
Value (Ω)
R1
510 k
R2
510 k
R3
5.1 k
R4
62 k
R5
510 k
R6
1.5 M
(diodes)
1N4148 type
Figure 10. Overtemperature Fault on Top Device
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References
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Figure 11 shows the response when ENABLE is disconnected from PACK+. Like the single device, the
discharge gate voltage drops as VDD1 falls. The upper device VDD falls to the cell 5 level. VDD1, the
lower device VDD falls toward VSS. Without the leakage into CTRC and CTRD, VDD1 continues to fall
slowly after the device enters shutdown. The load on the bottom 5 cells which cause voltage variation of
VDD1 shown in Figure 7 is not present with this circuit.
Figure 11. Stacked Devices Turn Off Using Test Circuit
4
References
For additional information, refer to the following documents available at www.ti.com.
• Texas Instruments, bq77905, bq77904 3-5S Ultra Low-Power Voltage, Current, Temperature, and
Open Wire Stackable Lithium-ion Battery Protector Data Sheet
• Texas Instruments, bq77905 3-5S Low-Power Protector Evaluation Module User's Guide
• Texas Instruments, (bq77905 20S Cell Stacking Configuration Application Report)
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