Cypress CY8C29x66 Gina V Vbat Max Nmax, Vref, Nnew nold N4.2 V new, N4.2 V old, Rref Rterm

The voltage measurement also is performed by the INA on the corresponding resistor. The resistive dividers (R7, R6), (R13, R12), and (R18, R19) transform cell voltage into signals suitable for the PSoC device. It is very important to use the high precision resistors in the resistive divider to obtain a high value common mode signal rejection. The recommended R6, R7, R12, R13, R18, and R19 tolerances are 0.1 percent. The following equation depicts the voltage measurement scheme:

VADC			Gina V Vbat
n nmax		nmax		Equation 18
	V		V
	ref		ref

The value n is the ADC code without influence of the INA and the ADC offset voltage ( n nmeas noffset ) . The value nmax is the maximum ADC code and is equal to 2048 for 12-bit incremental ADC in bipolar mode. The value

Vbat is the battery voltage, Gina is INA gain (1),				Vref	is
the bandgap reference	voltage (1.3V), and		V	is	the
resistive divider coefficient (0.25):
	1		Equation 19
V
	1	R7
		R6

To provide higher voltage measurement accuracy in decision-making charging voltages, the following calibration technique is used. All voltage thresholds are stored as calibrated ADC codes. During operation, the ADC code of the battery voltage is compared with these calibrated values. For this purpose, an external precision 4.2V voltage source and calibration procedure after assembly are used. All voltage thresholds are tuned from this precision voltage:

nnew nold	n4.2V _ new	Equation 20
	n4.2V _ old

The value nnew is the new voltage threshold ADC code. The value nold is the old voltage threshold ADC code that

is calculated by using Equation 16 on page 9. The value n4.2V _ new is the input ADC code during the calibration

procedure. The value n4.2V _ old is the old voltage

threshold ADC code for 4.2V, which is calculated by using Equation 16 on page 9. In this way, the calibration is performed for all decision-making charging voltages simultaneously. All devices must be calibrated during the manufacturing process by using external reference.

AN2309

For temperature measurement, a reference voltage resistive divider is employed based on a thermistor and a precision resistor (R6). Thermistor resistance is calculated according to the voltage drop on the precision resistor and the value of the reference voltage. To provide the necessary temperature measurement accuracy, the RefHI reference voltage is first set, and then AGND. After this, the second value of the resistor voltage drop is subtracted from the first. Bias voltages RefHi (2.6V) level in the first step and AGND (1.3V) in the next step are formed by using the continuous time user module TestMux. This technique allows compensation for both the ADC/INA offset error and the variation in the voltage drop on the current-sense resistor during the charging/discharging process. The following equations represent the temperature measurement scheme:

2	1			Rref	Equation 21
Vt	Vt	VAGND
			Rref	Rterm

	2	1				Rterm		Equation 22
nt		nt	nAGND
						Rref	Rterm
The value		V 1 is	the		voltage level on			the temperature
		t
reference resistor during application of the								VAGND (1.3V)
reference	voltage.			V	2 is the		voltage	level	on	the
				t
temperature		reference			resistor		during	application		of
VREFHI	(2.6V) reference voltage. Rterm is the thermistor
resistance.		Rref	is the temperature reference resistance
R24 (10K).		n 1 and		n	2 are the ADC codes of				V 1	and
		t		t					t

Vt 2 , respectively. The value nAGND is the ADC code of

the AGND input level and is equal to 2048 for 12-bit incremental ADC in unipolar mode.

The battery charge/discharge algorithm only needs to check for temperatures that fall in allowed ranges: during charging (typical values are 0 to 45 degrees Celsius) and discharging (typical values are -20 to 60 degrees Celsius). During the charge phase a hysteresis is added for the lower and upper bounds in/out temperature. This prevents multiple triggering when the temperature is close to the preset range. If the temperature is outside the discharge range, the LOAD connector is turned off and the PSoC device goes into sleep mode. Therefore, a hysteresis for the discharge range is not needed. The temperature profile is shown in Figure 7 on page 11.

November 25, 2007 Document No. 001-17394 Rev. *B - 10 -

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