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Texas Instruments Bq77905 Using Multiple FETs Application notes
Application Report
SLUA773 – July 2016
bq77905 Using Multiple FETs
Willy Massoth ........................................................................................ BMS: Monitoring and Protection
ABSTRACT
The basic implementation of the bq77905 3-5S Low Power Protector for lithium ion batteries uses single
charge and discharge FETs for current control. This document shows circuit implementation examples of
driving multiple charge and discharge FETs when using the bq77905. The schematic examples and test
results will help the battery electronics designer when designing a pack with multiple FETs for charge and
discharge.
1
2
3
4
Contents
Introduction ...................................................................................................................
Multiple FETs ................................................................................................................
Increasing the Charge Turn Off Speed ...................................................................................
References ...................................................................................................................
1
Common Implementation ................................................................................................... 2
2
Schematic With Multiple FETs ............................................................................................. 3
3
Short Circuit With Multiple FETs ........................................................................................... 4
4
Undervoltage With Multiple FETs.......................................................................................... 5
5
Overtemperature Charge With Multiple FETs............................................................................ 6
6
Schematic With Faster Charge Disable
1
2
7
8
List of Figures
7
..................................................................................
Overtemperature Charge With Faster Charge Disable .................................................................
7
8
List of Tables
1
Introduction
The bq77905 3-5S low power protector is an easy to use component for lithium ion battery circuits. The
common schematic uses series FETs where the current for both charge and discharge flows through
single charge and discharge FETs in series. See Figure 1. RDSG and RCHG are sized for appropriate
switching speeds of the transistors, and in some cases additional circuitry may be used in the charge path
to accommodate system voltages. Refer to the datasheet for additional description.
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Multiple FETs
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PACK+
RVDD
CVDD
RIN
RIN
CVDD
CIN
RIN
RIN
DVSS
CTRD
VC5
CTRC
VC4
CCFG
VC3
CIN
RIN
VDD
AVDD
VC2
VC1
CIN
CIN
bq77905
RTS_PU
VTB
RTS
TS
LD
AVSS
CHG
SRP
CHGU
SRN
DSG
RCHG
RDSG
RLD
CIN
RGS
RSNS
RGS
PACKCopyright © 2016, Texas Instruments Incorporated
Figure 1. Common Implementation
2
Multiple FETs
In some cases, it may be desirable to use multiple FETs in parallel for both the charge and discharge
FETs. This may be either for increased current handling or to spread heat over a wider area. Consult
industry references or your FET manufacturer for recommendations on paralleling FETs. It is generally
recommended to keep the circuit symmetrical and the FETs at the same temperature for equal current
sharing. With parallel FETs, the driver sees a larger load. The bq77905 has drivers with low internal
resistance and can accommodate the larger total gate load, refer to the datasheet feature description and
electrical characteristics for details. When driving the parallel FETs, it is generally desirable to keep the
signal to the gates the same while maintaining some isolation between the gates to avoid oscillation.
Ferrite beads or a small portion of the total resistance is often used for gate isolation. Figure 2 shows a
circuit implementation with 4 parallel MOSFETs. In the DSG drive circuit the RDSG is split between a
common resistance R8 and smaller individual gate resistors R14, R19, R21, and R23. The CHG circuit is
more complex since it limits the gate voltage to the FETs during transients and blocks the current into the
CHG pin during a discharge protection with load. The RCHG for turn on is provided by the common R9 and
the individual R15, R20, R22 and R24. For CHG turn off the drive resistance is dominated by R13 in
parallel with R17 until PACK- falls below GND, then the remaining turn off is through R17.
2
bq77905 Using Multiple FETs
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Multiple FETs
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R1
VDD
1.0k
C1
1µF
U1
R2
R3
1
2
3
4
5
6
C5
C4
C3
C2
C1
3
VC4
4
1.0k
R5
5
VC3
VC2
1.0k
R7
C3
0.1µF
C4
0.1µF
C5
0.1µF
C6
0.1µF
VTB
TS
VC4
LD
VC3
VC2
VC1
DSG
9
SRP
AVSS
10
C2
0.1µF
VC5
7
VC1
1.0k
CTRD
CTRC
CCFG
CHG
CHGU
6
1.0k
R6
AVDD
VC5
1.0k
J1
VDD
2
C7
1µF
SRN
DVSS
19
18
17
16
15
R4
14
10.0k
13
12
t°
GND
1
8
GND
20
BQ77905PWR
GND
GND
GND
GND
GND
GND
R8
2.2k
R9
5.1k
R11
100
GND
R12
100
C10
0.1µF
R13
1.0M
R18
D1
100V
GND
R14
R15
220
220
R16
10M
BATT-
R10
470k
C8
0.1µF
GND
C9
0.1µF
RT1
10.0k ohm
11
R17
3.0M
D2
16V
PACK-
BATSR-
0.003
J2
Q1
CSD18502KCS
NT1
Q2
CSD18502KCS
R19
R20
220
220
C12
C13
C11
0.1µF
0.1µF
0.1µF
C14
0.1µF
GND
C5
PACK+
J3
R21
Q3
CSD18502KCS
220
Q4
CSD18502KCS
R22
220
Q5
CSD18502KCS
R23
220
Q6
CSD18502KCS
R24
220
Q7
CSD18502KCS
Q8
CSD18502KCS
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Figure 2. Schematic With Multiple FETs
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Multiple FETs
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When PACK+ is shorted to PACK-, a high current flows from the battery limited by the capability of the
cells and the resistances of the circuit, interconnect and short. When the current exceeds the short circuit
threshold for the protection delay, both DSG and CHG are turned off. Figure 3 shows the turn off of the
discharge FET from a short circuit protection event with the Figure 2 circuit. When DSG is turned off, the
RDSG resistances allow the discharge FET gates to fall slowly so the current drops from over 100 A to 0
slow enough to avoid a large transient from the cell inductance and current transition. Although CHG turns
off, the high resistance of the turnoff path and the FET's gate capacitance keep the charge gates high
during the short time shown in the figure.
NOTE: Waveform captures in this document show charge current as positive.
Figure 3. Short Circuit With Multiple FETs
4
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Multiple FETs
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During an undervoltage protection turn off as shown in Figure 4, the discharge FET gates again fall slowly
but the transient on PACK- is lower due to the smaller current and higher resistance of the load. The
bq77905 keeps CHG on during the undervoltage event, but it is not apparent since the charge gates rise
again in the short term due to the gate capacitance.
Figure 4. Undervoltage With Multiple FETs
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Multiple FETs
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When the charge output is turned off from a protection event such as overtemperature during charge as
shown in Figure 5, the high resistance of the turn-off path provides a slow turn off of the FET. The typical
RGS_CHG is large to avoid the CHG output voltage from preventing load removal when a bq77905 with the
load removal for UV recovery option is used and to reduce constant current into the resistor when CHG is
on. The turn-off time can be faster with a smaller RGS_CHG resistance when the load removal for UV
recovery option is not used. For systems where the charge current is small, the delay time to the drop in
current and the slow transition time may be acceptable, but for higher charge currents a different
technique may be desired.
Figure 5. Overtemperature Charge With Multiple FETs
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Increasing the Charge Turn Off Speed
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3
Increasing the Charge Turn Off Speed
When using multiple FETs, the RGS_CHG resistance which turns off the charge FETs may allow a long turnoff time and result in FET heating or failure. It may be desirable to add a circuit to speed up the turn off of
the charge FETs. With high impedance drivers and fast switching, adding a PNP transistor to pull down
the FET gate as shown in figure 31 of SNVA008 is a common technique. Since RGS_CHG is large in the
bq77905 circuit, voltage control of a P-channel FET is more appropriate as shown in Figure 6. In this
circuit the RGS_CHG is split between R17 and R25. D3 turns on the FET gates when CHG is high, Q9 pulls
down the gates when CHG is turned off and R17 pulls Q9's gate down. It is important that the VGSTH of Q9
is lower magnitude than VGSTH of the charge FETs so that Q9 is still conducting as the charge FETs turn
off. The charge FET gate voltage pull down continues by Q9 until it turns off, then by R25.
R1
VDD
1.0k
C1
1µF
U1
R2
R3
1
2
3
4
5
6
C5
C4
C3
C2
C1
VC4
5
VC3
1.0k
R6
VC2
1.0k
R7
VC5
4
1.0k
R5
AVDD
3
VC5
1.0k
J1
VDD
2
C3
0.1µF
C4
0.1µF
C5
0.1µF
C6
0.1µF
VTB
TS
16
15
LD
14
13
12
VC4
VC3
6
VC2
7
VC1
DSG
11
9
SRP
AVSS
8
SRN
DVSS
10
C2
0.1µF
19
18
17
CHG
CHGU
VC1
1.0k
CTRD
CTRC
CCFG
C7
1µF
R4
10.0k
t°
GND
1
GND
20
BQ77905PWR
GND
GND
GND
GND
GND
GND
R11
100
GND
R12
100
R18
R9
5.1k
R10
470k
D1
100V
R13
1.0M
C10
0.1µF
GND
R14
R15
220
220
R16
10M
BATT-
R8
2.2k
C8
0.1µF
GND
C9
0.1µF
RT1
10.0k ohm
D3
3V
R25
10M
R17
5.1M
Q9
D2
16V
PACK-
BATSR-
0.003
J2
Q1
CSD18502KCS
NT1
Q2
CSD18502KCS
R19
R20
220
220
C12
C13
C11
0.1µF
0.1µF
0.1µF
C14
0.1µF
GND
C5
PACK+
J3
R21
Q3
CSD18502KCS
220
Q4
CSD18502KCS
R22
220
Q5
CSD18502KCS
R23
220
Q6
CSD18502KCS
R24
220
Q7
CSD18502KCS
Q8
CSD18502KCS
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Figure 6. Schematic With Faster Charge Disable
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References
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Figure 7 shows the improved charge turn-off time of the circuit in Figure 6. The circuit was tested with a
FDN340P FET for Q9 although other FETs such as CSD25483F4 may be appropriate depending on the
power FET selected. Other circuits to speed up the switching may be desired depending on the
application.
Figure 7. Overtemperature Charge With Faster Charge Disable
4
References
For additional information, refer to the documents below available at www.ti.com.
• bq77904 / bq77905: 3-5S Low Power Protector data sheet (SLUSCM3)
• bq77905 EVM User's Guide (SLVUAN2)
• AN-558 Introduction to Power MOSFETs and Their Applications (SNVA008)
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