SAA1501T Datasheet, PDF (6/20 Pages) NXP Semiconductors – Battery charge level indicator

Philips Semiconductors
Battery charge level indicator
Objective specification
SAA1501T
With R_CCC, the charge efficiency can be manipulated
depending on the charge level. The restriction of the
SAA1501T is a maximum average charge current of 60 µA
and a minimum momentary charge current of 0.6 µA. The
same formula is applicable for the discharge current. The
discharge efficiency can now also be changed by R_DCC
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
(C_out × R_conv < 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
f = -₍---C------_C---_C---⁽--_×^I---c--_∆--⁽--^d_V--⁾--_o--^×-_s---_c-R--_×--^s---^e_R---ⁿ--_C-^s--^eC----_C^×----₍-6-_R---_D-⁾---_C----_C---₎----₎--
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
(T_battery > T_max), 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
(I_c = I_d = 0), the self-discharge of the batteries is registered
by counting down in 200 days (based on f_osc = 4 kHz) if
T_battery < T_self or in 100 days (based on f_osc = 4 kHz) if
T_battery > T_self. T_self is also set by means of an external NTC
resistor.
Band gap generation
From the band gap voltage block, two reference voltages
are derived V_ref and V_max. Voltage V_ref at pin R_ref sets the
reference currents, I_ref1 (I/F converter); I_ref2 (mode
detector) and I_ref3 (oscillator). Voltage V_max sets the
current I_max 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
C_cy. The duty cycle is determined by
δ = -k--I--^×_c----_–-I--^m-I--_d-^a--^x-
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
6