INTEGRATED CIRCUITS
SAA1501T
Battery charge level indicator
Objective specification |
December 1994 |
File under Integrated Circuits, IC11 |
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Philips Semiconductors
Philips Semiconductors |
Objective specification |
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Battery charge level indicator |
SAA1501T |
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FEATURES
∙High level of integration to allow assembly in intelligent battery packs
∙Accurate charge and discharge account
∙Large dynamic range of charge and discharge currents
∙Independent settings of charge and discharge efficiency
∙2 V minimum supply voltage (2 cell operation)
∙Temperature protection of batteries during charging
∙Temperature controlled self-discharge
∙Accurate charge current regulation
∙Two charge amount display modes, LCD and LED.
QUICK REFERENCE DATA
GENERAL DESCRIPTION
The SAA1501T is intended to be used as a battery monitor and charge current control circuit in rechargeable battery systems.
The SAA1501T is processed in BiCMOS technology where the benefits of mixed bipolar and CMOS technology is fully utilized to achieve high accuracy measurements and digital signal processing in the same device. The general function of the integrated circuit is a Coulomb counter. During battery charging, the charge current and charge time are registered in a Coulomb counter. During discharge, the discharge current and time are recorded. The momentary charge amount of the batteries can be displayed either on an LCD screen or on an LED bargraph. Using the SAA1501T, intelligent batteries or intelligent battery powered systems can be easily designed with only a few external components.
SYMBOL |
PARAMETER |
CONDITIONS |
MIN. |
TYP. |
MAX. |
UNIT |
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VCC |
supply voltage |
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2.0 |
3.0 |
4.3 |
V |
ICC |
supply current |
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VCC = 3 V; |
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1.2 |
1.7 |
mA |
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Ic = Id = 60 μA |
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ICCstb |
supply current in standby mode |
VCC = 3 V; |
− |
− |
100 |
μA |
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VCSI = VDSI = 0 V |
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fosc |
fixed oscillator frequency |
Cosc = 820 pF; |
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4.2 |
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kHz |
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Rref = 51.5 kΩ |
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Vi(s) |
input sense voltage (pins 9 and 10) |
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0 |
− |
VCC − 1.6 |
V |
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Tamb |
operating ambient temperature |
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0 |
− |
+70 |
°C |
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ORDERING INFORMATION |
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TYPE NUMBER |
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PACKAGE |
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NAME |
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DESCRIPTION |
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VERSION |
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SAA1501T |
SO24 |
plastic small outline package; 24 leads; body width 7.5 mm |
SOT137-1 |
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December 1994 |
2 |
Philips Semiconductors |
Objective specification |
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Battery charge level indicator |
SAA1501T |
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BLOCK DIAGRAM |
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Block diagram. |
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Fig.1 |
December 1994 |
3 |
Philips Semiconductors |
Objective specification |
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Battery charge level indicator |
SAA1501T |
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PINNING |
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SYMBOL |
PIN |
DESCRIPTION |
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VCC |
1 |
supply voltage |
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EN |
2 |
enable output |
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Ccy |
3 |
duty cycle capacitor output |
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CCC |
4 |
charge counter capacitor output |
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Ich |
5 |
maximum average charge current |
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setting input |
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Rref |
6 |
current reference resistor input |
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RDCC |
7 |
discharge current conversion resistor |
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input |
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RCCC |
8 |
charge current conversion resistor |
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input |
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CSI |
9 |
charge sense input |
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DSI |
10 |
discharge sense input |
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RTEMP1 |
11 |
temperature sensing resistor 1 input |
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RTEMP2 |
12 |
temperature sensing resistor 2 input |
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Cosc |
13 |
oscillator capacitor input |
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BUZ |
14 |
buzzer output |
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FULL |
15 |
battery full indication output |
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L100 |
16 |
100% segment indication output |
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L80 |
17 |
80% segment indication output |
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L60 |
18 |
60% segment indication output |
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L40 |
19 |
40% segment indication output |
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L20 |
20 |
20% segment indication output |
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BP |
21 |
LCD back plane drive |
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BLI |
22 |
battery low indicator LED output |
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POL |
23 |
power-on LED output |
Pin configuration. |
GND |
24 |
Fig.2 |
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power ground |
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December 1994 |
4 |
Philips Semiconductors |
Objective specification |
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Battery charge level indicator |
SAA1501T |
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FUNCTIONAL DESCRIPTION
The most important function of the SAA1501T is the charge account in rechargeable battery systems. Both NiCd and NiMH batteries in all sizes can be used. The system can operate alone as a charge monitor with a charge amount display function or, can operate in conjunction with a charger. If the SAA1501T operates together with a charger, it delivers a control signal at output EN, for charge current regulation or for battery voltage regulation.
Fast charging systems and charge current regulation
The SAA1501T is especially designed to be used in fast charging systems. In fast charging systems, the charge time is lowered by raising the charge current. Signal EN controls the charger current. The counters register the state of charge of the batteries and at the 80% level the charge current is reduced via a smaller duty cycle regulation of signal EN. The second (slow) level fully charges the batteries which is not possible with the first (fast) level. After the slow charge mode the counter switches over to an even smaller duty cycle of EN and thus enters the third (trickle) charge mode, to overcome the self-discharge of the batteries.
Current sensing and charge account
The charge current is sensed by means of a very low resistance (e.g. 70 mΩ) sense resistor Rsc (see Fig.8) to save power at high charge rates. Via the V/I charge converter and external resistor RCCC (see Fig.8), the sensed voltage is converted into a charge current Ic (the same is applicable for the discharge current). In the I/F converter the charge current is converted into a frequency for up-counting the counter. For the discharge current (Id) the converted frequency is used for down-counting. The up and down counting is registered in counters CNT1 and CNT2, depending on the actual charge and discharge current levels of the batteries. This is called dynamic charge account.
Charge display
The charge amount represented by the Coulomb counter can be displayed via an LCD screen or via an LED bargraph. If the charge amount is reduced to 0%, the battery low indicator (BLI) LED is turned on at the end of a battery discharge session. A flashing BLI, in combination with a repeating buzzer alarm, informs the user about the low charge state. A new charge session should then be started.
Protections
In the temperature control block, the absolute temperature is used as a protection to end the fast charge cycle. Fast charging at high temperature is not permitted because of degradation of the battery cells. If the batteries are disconnected, an open-battery condition is recognized and the SAA1501T enters the standby mode.
Mode detection
The mode detector detects whether there are any charge or discharge currents, whether the system is powered, whether loads are connected or whether the system is in the standby mode. If power is connected, the power-on LED (POL) is on. In the standby mode, the Coulomb counter will count down in accordance with the self-discharge speed of the batteries, which is temperature controlled. The following subsections describe the various blocks of the block diagram in more detail.
Supply and reference
During the period when VCC rises from 0 V to the internal reset level, all counters are reset. The internal reset is released before VCC reaches 1.7 V. The operating supply voltage ranges from 2 V to the open battery level of
4.3 V (min). The characteristics are guaranteed at
VCC = 3 V. In order to protect the SAA1501T against high supply voltages during open battery in a flyback converter, a voltage clamp circuit is made active at 6.35 V (typ). The clamping current must not exceed 80 mA. A band gap reference block is included to generate accurate voltages i.e. for the oscillator. Moreover, together with Rref, accurate currents are generated which are used in the I/F and V/I converters and the oscillator block. In the standby mode only the oscillator and the digital parts are active to limit the discharge current of the batteries to a current level of less than 100 mA. The circuits that are needed temporarily are switched on and off during standby (see “Timing characteristics” tsom).
Voltage-to-current charge and discharge
In the V/I converter, the input charge current is translated into acceptable levels for the circuit. The conversion formula is:
( Ich arge × Rsc)
Ic = --------------------------------------- ; where RCCC > Rsc (see Fig.7)
RCCC
December 1994 |
5 |
Philips Semiconductors |
Objective specification |
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Battery charge level indicator |
SAA1501T |
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With RCCC, the charge efficiency can be manipulated depending on the charge level. The restriction of the SAA1501T is a maximum average charge current of 60 mA and a minimum momentary charge current of 0.6 mA. The same formula is applicable for the discharge current. The discharge efficiency can now also be changed by RDCC depending on the discharge current levels, but independent of the charge current. As both sense levels are referenced to ground, the sensing elements could be combined into one. The outputs are used combined as 1¤6 ´ (Ic - Id) in the I/F converter and combined as (Ic - Id) in the pulse width modulator block and made separately available in the mode detector. The conversion is made lower by a factor of 6 in the I/F converter block, thereby enabling the use of poor leakage capacitors on pin 4. All V/I converter pins are sensitive to capacitive loading
(Cout ´ Rconv < 1 ms), the conversion resistors should be mounted as close as possible to the output pins.
I/F converter
This block produces up-counts while charging and down-counts while discharging. The I/F converter translates the charge/discharge currents into a frequency. This frequency is determined by
(I c ( d) ´ Rsense ´ 6 )
f = ----------------------------------------------------------------------------------
(C CC ´ DVosc ´ RCCC ( RDCC) )
During the time period ‘t’, the charge current, expressed as a ‘Charge Parcel’, will be counted in the Coulomb counters (CNT1 and CNT2). During discharge the ‘Charge Parcel’ is the product of the discharge current and the ‘t’ from the I/F converter generated frequency. The momentary contents of the Coulomb counter is a multiple of the ‘Charge Parcels’.
Coulomb counters CNT1 and CNT2
The SAA1501T has been designed for average maximum charge and discharge current levels of 5 C and minimum charge and discharge current levels of 0.05 C. This means that counter CNT1 will be full, or empty, after a minimum time period of 12 minutes at maximum charge and discharge currents at the recommended oscillator frequency. Higher charge and discharge rates than 5 C are possible, but only by changing the oscillator frequency. It should be noted that the self-discharge time and the display functions are influenced by a higher oscillator frequency. The SAA1501T enables top-up charging in order to account for the decrease of charge efficiency at high charge rates. The SAA1501T switches to the slow charge mode at full recognition when CNT1 is at its maximum. As soon as the batteries are completely full (when CNT2 is at its maximum), the SAA1501T switches
to the trickle charge mode to overcome the self-discharge of the batteries. The top-up charge volume of
CNT2 = 0.2 ´ CNT1 = 0.2 C (where Q is rated as Ampere hours of the battery). The slow and trickle charge current levels are dependent on the k-factor. Signal EN controls the external charger e.g. TEA1400 (see Fig.8). When an LED bargraph display is used, the LED currents are also considered as a battery discharge current, and therefore influence the duty cycle of the charge current regulation signal EN. The SAA1501T also enables temperature protection. In the event that the battery temperature exceeds a certain maximum temperature level
(Tbattery > Tmax), which can be set by an external NTC resistor, the SAA1501T switches to the slow charge mode.
In the standby mode (self-discharge mode), which is recognized by the SAA1501T in the mode detector when both the charge and discharge currents are zero
(Ic = Id = 0), the self-discharge of the batteries is registered by counting down in 200 days (based on fosc = 4 kHz) if
Tbattery < Tself or in 100 days (based on fosc = 4 kHz) if Tbattery > Tself. Tself is also set by means of an external NTC resistor.
Band gap generation
From the band gap voltage block, two reference voltages
are derived Vref and Vmax. Voltage Vref at pin Rref sets the reference currents, Iref1 (I/F converter); Iref2 (mode
detector) and Iref3 (oscillator). Voltage Vmax sets the current Imax which is used in the pulse width modulation block to accurately control the charge current.
Charge current regulation
While charging, the SAA1501T produces a charge current regulation signal EN in the pulse width modulation block which is used for controlling an external charger. This digital signal EN is derived from the signal produced at pin Ccy. The duty cycle is determined by
dk ´ Imax
=-------------------
Ic –Id
in which the value of k depends on the state of the counters CNT1 and CNT2:
CNT1 is not full; k = 1 (fast charging).
CNT1 is full; CNT2 is not full; k = 0.1 (slow charging).
CNT1 and CNT2 are full; k = 0.025 (trickle charging).
December 1994 |
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