SA57608 Datasheet, PDF (7/10 Pages) NXP Semiconductors – One-cell Lithium-ion battery protection with over/undercharge and overcurrent protection

Philips Semiconductors
One-cell Lithium-ion battery protection with
over/undercharge and overcurrent protection
Product data
SA57608
Undervoltage condition
When the cell voltage falls below the overdischarge threshold,
(V_UV1), as measured between V_CC (pin 5) and GND (pin 6), the gate
of the discharge MOSFET (DF, pin 1) is brought LOW (OFF) after an
internal time delay. The SA57608 then assumes a sleep condition
where its I_CC assumes a very low state (I_CC(SLP)) The gate is then
brought HIGH (ON) when a charge current is detected, or when the
VM pin (pin 2) is brought to 0.7 V higher than the negative terminal
of the cell (GND, pin 6) or when the cell voltage is higher than the
hysteresis voltage (V_UV2).
Discharge overcurrent condition
If a discharge overcurrent condition is experienced as seen when a
short-circuit is experienced across the battery terminals, the
SA57608 views a high voltage across the MOSFET’s R_DS(on). If this
voltage exceeds the threshold voltage (V_SC), the discharge gate is
brought to a LOW condition (OFF) after an internally set of time
delays are exceeded. If the overcurrent is LOW, then the t_SC1 is
enacted. If the the overcurrent is higher, as experienced in a hard
short-circuit, the time delay is less than 400 ns. This prevents the
MOSFETs from failing from an FBSOA failure.
The gate of the discharge MOSFET is turned on again only when
the voltage of the VM pin is allowed to fall within the 0.7 volts of the
negative terminal of the cell (GND, Pin 6). If the short-circuit
persists, the gate of the discharge MOSFET is immediately brought
LOW (OFF) again until the short-circuit condition is again removed.
APPLICATION INFORMATION
The typical single-cell lithium-ion or polymer protection circuit based
upon the SA57608 is seen in Figure 6.
V+
Li-ION
CELL
100 Ω
0.1 µF
0.01 µF
5
V_CC
4
C_DLY
SA57608
2
VM
6
GND _DF
CF
1
3
0.1 µF
1 kΩ
V–
DISCHARGE
FET
CHARGE
FET
SL01570
Figure 6. Typical protection circuit
The SA57608 drives the series N-Channel MOSFETs to states
determined by the cell’s voltage and the battery pack load current.
During normal periods of operation, both the discharge and charge
MOSFETs are in the ON state, thus allowing bidirectional current
flow.
If the battery pack is being charged, and the cell’s voltage exceeds
the overvoltage threshold, then the charge MOSFET is turned OFF
(FET towards the pack’s external terminal). The cell’s voltage must
fall lower than the overvoltage hysteresis voltage (V_OV(Hyst)) before
the charge MOSFET is again turned ON.
If the battery pack is being discharged and the undervoltage
threshold (V_UV(Th)) is exceeded, then the discharge MOSFET is
turned OFF. It will not run back ON until a charger is applied to the
pack’s external terminals and the cell’s voltage rises above the
undervoltage hysteresis voltage (V_UV(Hyst)).
When the battery pack is being discharged, the load current causes
the voltage across the discharge MOSFET to increase past the
overcurrent threshold voltage (V_OC(TH)), then the discharge
MOSFET is turned OFF after a fixed 7–18 ms delay. If short-circuit
is placed across the pack’s terminals, then the discharge MOSFET
is turned OFF after a 100–300 µs time delay to avoid damaging the
MOSFETs.
The R-C filter on the V_CC pin
One needs to place an R-C filters on the V_CC pin. It is to primarily
shield the IC from electrostatic occurrences and spikes on the
terminals of the battery pack. A secondary need is during the
occurrence of a short-circuit across the battery pack terminals. Here,
the Li-ion cell voltage could collapse and cause the IC to enter an
unpowered state. The R-Cs then provide power during the first
instant of the short circuit and allow the IC to turn OFF the discharge
MOSFET. The IC can then enter an unpowered state. Lastly, the
R-C filter filters any noise voltage caused by noisy load current.
The values shown in Figure 6 are good for these purposes.
Selecting the Optimum MOSFETs:
For a single-cell battery pack, a logic-level MOSFET should be
used. These MOSFETs have turn-on thresholds of 0.9 V and are
considered full-ON at 4.5 V VGS. Some problem may be
encountered in not having enough gate voltage to fully turn-ON the
series MOSFETs over the battery pack’s entire operating voltage. If
one deliberately selects an N-Channel MOSFET with a much
greater current rating, a lower R_DS(on) over the entire range can be
attained.
The MOSFETs should have a voltage rating greater than 20 V and
should have a high avalanche rating to survive any spikes
generated across the battery pack terminals.
The current rating of the MOSFETs should be greater than four
times the maximum “C-rating” of the cells. The current rating,
though, is more defined by the total series resistance of the battery
pack. The total resistance of the battery pack is given by Equation 2.
R_bat(tot) = R_DS(on) + R_cell
(Equation 2)
The total pack resistance is typically determined by the system
requirements. The total pack resistance directly determines how
much voltage droop will occur during pulses in load current.
Another consideration is the forward-biased safe operating area of
the MOSFET. During a short-circuit, the discharge current can easily
reach 10–15 times the “C-rating” of the cells. The MOSFET must
survive this current prior to the discharge MOSFET can be turned
OFF. So having an FBSOA envelope that exceeds 20 amperes for
5 ms would be safe.
2003 Oct 29
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