ICS1708
QuickSaver® Charge Control IC for Nickel-Cadmium
and Nickel-Metal Hydride Batteries
General Description
The
ICS1708
is a low cost 8 pin CMOS control IC for the intelligent charging of either nickel-cadmium (NiCd) or nickel-metal hydride
(NiMH) batteries. The
ICS1708
uses a pulsed-current charging technique together with multiple voltage slope termination methods. The
ICS1708
employs a four stage charge sequence that provides a complete recharge without overcharging. The controller has three (3) user
selectable charge rates, user accessible clock, and two (2) charge status output pins. On power-up, the
ICS1708
monitors for the presence of
a battery and begins charging. The
ICS1708
is for applications where battery temperature protection is not required of the charge controller.
Applications
Embedded and charger stands for: Portable consumer products, Power tools, Audio/video products, Communications products, RC toys,
and Wireless products
Features
•
•
•
•
•
•
•
•
•
Charge termination methods include: Voltage slope (+
∆
V
/dt and +/- peak detect), and Fast charge time out to maintenance mode
Four stage charge sequence:
SoftStart
conditioning, Fast charge, Topping charge, and Maintenance charge
Three (3) user selectable charge rates: 15 minutes (4C), 60 minutes (1C), 150 minutes (C/2.5)
Continuous polling mode for battery detection
Adjustable open circuit (no battery) voltage reference
Benefits
-compared to other methods
Peak battery performance and extended cycle life
Improved battery efficiency and reliability
Lower internal resistance build-up
Lower capacity fade
Block Diagram
OPEN CIRCUIT
REFERENCE
VOLTAGE
0.5V
BATTERY
VOLTAGE
SENSE
MICROCODE CONTROL
FAST
CHARGE
PROCESSOR
OUTPUT
CONTROL
CHARGE
COMPLETE
CURRENT
SOURCE
ADC
RAM
RC (CLOCK)
ROM
OSCILLATOR
ICS1708
Pin Configuration
CHG
MMN
CMN
VSS
1
2
3
4
ICS1708
8
7
6
5
VDD
VIN
OPREF
RC
8-Pin DIP or SOIC
Pin Definitions
Pin
1
2
3
4
5
6
7
8
Note:
Name
CHG
CMN
MMN
VSS
RC
OPREF
VIN
VDD
Type
OUT
OUT
OUT
IN
IN
IN
Definition
Active high (PFET), active low (NFET) 25mA max. TTL compatible signal used to turn on and off an external current source to
provide pulsed current to charge the battery.
Charge mode indicator. NFET drain rated at 40mA max. turns on an external indicator to show the controller is applying either a
SoftStart
charge or fast charge.
Maintenance mode indicator. NFET drain rated at 40mA max. turns on an external indicator showing the battery is ready to use as the
topping charge or maintenance charge is applied.
Ground.
An external resistor and capacitor sets the frequency of the internal clock.
Open circuit (no battery) voltage reference. A pull down or external resistor divider from 5V sets a voltage reference on this pin used
to detect the presence of a battery.
Battery voltage normalized to one cell with an external resistor divider.
Device supply =+5.0 VDC +/- 5% regulated. The ICS1708 requires 11mA max. average that includes brief 50mA peak currents.
When used, LEDs, pull-up resistors, and drivers require additional current from the +5VDC supply. An electrolytic and ceramic
capacitor between (or very close to) both VDD and VSS is recommended for bypassing.
Pin 6 has an internal pull-up.
Input and output pins have internal ESD protection diodes to VDD and VSS for 2KV protection per MIL STD 883 method 3015.7.
Controller Operation
Charging Stages
The charging sequence consists of four stages. The application of current is shown graphically in Figure 1. The
SoftStart
stage gradually
increases current levels up to the user selected fast charge rate during the first two minutes. The
SoftStart
stage is followed by the fast
charge stage, which continues until termination. After fast charge termination, a reduced duty cycle topping charge is applied, followed by
a further reduced cycle maintenance charge. After fast charge termination, the battery is charged and ready to use.
97%
18%
A v e ra g e
C u r re n t
( no t t o s c a le )
Fast Charge
Topping Charge
2.5 to 20%
Maintenance Charge
0.6 to 5%
S ta g e 1
0
2 m in
S t a ge 2
t e rm in a tion
S t a ge 3
t erm in atio n +
S t ag e 4
Tim e (n o t to s c a le)
Figure 1: Graphical representation of average current levels during the four charging stages
2
ICS1708
SoftStart
Conditioning Charge
New, over-discharged, and batteries out of long term storage may exhibit an initial high impedance conditions in response to charging, as
shown in Figure 2. Unless dealt with, this high impedance condition can cause a voltage peak and -
∆
V condition at the beginning of the
charge cycle that can appear to be the response of an already full battery. To manage this condition, the
ICS1708
applies a
SoftStart
conditioning charge.
Figure 2: High impedance voltage spike at the beginning of charge
The
SoftStart
charge eases batteries into the fast charge stage by gradually increasing the duty cycle of the charge. The gradual increase
alleviates the voltage peak and the -
∆
V condition. The duty cycle of the applied current is increased to the fast charge rate, as shown in
Figure 3, by extending the current pulse on every cycle until the pulse is about 1.1 seconds in duration. The initial current pulse is
approximately 200ms. The CMN indicator is a continuous active low during the
SoftStart
conditioning stage.
Initial Pulse
Width
Initial Pulse
Width
increment
time
Initial Pulse
Width
2 x increment
time
cycle time
cycle time
cycle time
Figure 3: Cycle-to-cycle increase of the
SoftStart
current pulse widths
3
ICS1708
Fast Charge
In the second stage, the
ICS1708
applies the charging current in a repetitive sequence consisting of positive current charging pulses followed
by a rest time and voltage acquisition time. The cycle, shown with charge, rest, and data acquisition periods in Figure 4, repeats every 1.1
seconds until the battery is fully charged.
rest time
fast charge pu lse w id th
cycle time
voltage
a cqu isit ion tim e
Figure 4: Charge cycle showing fast charge current pulses
The amplitude of the current pulse is determined by system parameters such as the current capability of the charging system, the desired
recharge time, battery capacity and the ability of the battery to accept the charge current in the charging environment. The
ICS1708
can be
set for three (3) user-selectable fast charge rates: 15 minutes (4C), 60 minutes (1C), or 150 minutes (C/2.5). The CMN indicator is a
continuous active low during this stage. The charge rate is determined by the voltage level on the CMN indicator when the
ICS1708
is first
powered-up.
A voltage acquisition window immediately follows a brief rest time after the charge pulse. No charge is applied during the rest time or
during the acquisition window to allow the battery chemistry to settle. Since no current is flowing, the measured cell voltage is not obscured
by any internal or external IR drops or noise. The
ICS1708
takes samples of battery voltage during the acquisition window. The voltage
samples are averaged for comparisons to previous and subsequent averages. This method provides a most accurate representation of the true
state of charge of the battery.
Topping Charge
The third stage is a topping charge that applies current at a rate low enough to prevent cell heating but high enough to equalize cells in a
multiple cell packs. The topping stage is not required before putting the battery into immediate service after fast charge completes.
The topping charge is applied for a minimum of two (2) hours. The current consists of the same pulse technique used during the fast charge
stage; however, the delay time is extended as shown in Figure 5. Extending the delay time between charge pulses allows the same charging
current used as in the fast charge stage so that no changes to the current source are necessary. For example, the same charge pulse that
occurs every second at a 1C (1 hour) fast charge rate will occur every eleven (11) seconds for a topping charge rate of C/11. The MMN
indicator is a continuous active low during the topping and maintenance charge stages.
cycle
t im e
de la y tim e
cycle
tim e
Figure 5: Representative timing diagram for topping and maintenance charge
Maintenance Charge
The maintenance charge is intended to offset the natural self-discharge of NiCd or NiMH batteries by keeping their cells primed at peak
charge. After the topping charge ends, the
ICS1708
begins this charge stage by once again extending the delay time between charge pulses.
The maintenance charge will last for as long as the divided down battery voltage is greater than 0.5V at VIN pin 7. The MMN indicator is a
continuous active low during this stage.
4
ICS1708
Charge Termination Methods
Charge termination methods include voltage slope and fast charge time out.
Voltage Slope Termination
The most distinctive point on the voltage curve of a battery in response to charging by a constant amplitude current is the voltage peak that
occurs as the battery approaches full charge. By mathematically calculating the first derivative of the voltage, a second curve can be
generated showing the change in voltage with respect to time as shown in Figure 6. The slope will reach a maximum just before peak
voltage. Using milestone voltage slope data as charging progresses, the
ICS1708
determines when the point of full charge will occur and
accurately terminates the applied current when the battery reaches that point. The actual termination point is based on the actual charging
characteristics of the particular battery. Typically, fast charge ends around peak voltage.
Figure 6: Voltage and slope curves showing inflection and zero slope points
New, over-discharged, old, and already full batteries often produce a voltage profile that varies from that shown in Figure 6. The
ICS1708
fast charge termination in these cases is based on a slight decrease in the voltage. Some new batteries may need several charge/use cycles
before their response fully stabilizes to that shown in Figure 6.
Charge Timer Termination
The controller uses a timer to limit fast charge duration. These times are internally set according to the charge rate selected and can be
adjusted slightly by changing the clock frequency. Fast charge timer termination is a safety backup feature to limit the duration of the fast
charge stage. The fast charge timer is always active and cannot be disabled. See Table 3
Charge Rate List.
5
京公网安备 11010802033920号
EEWORLD
