LP3910SQ-AN Datasheet, PDF (41/60 Pages) National Semiconductor (TI) – Power Management IC for Hard Drive Based Portable Media Players

ANALOG TO DIGITAL CONVERTER
LP3910SQ-AN is equipped with an 8-bit dual-slope integrat-
ing analog to digital converter. Dual-slope converters provide
effective filtering of >500 kHz and <125 kHz noise compo-
nents on the input voltage, and does not require a sample and
hold stage. The A/D converter core digitizes the input voltage
ranging from V_REF to 2V_REF, where V_REF is the voltage mea-
sured on the V_REFH pin. After an initial 2 ms warm-up for the
first activation of the ADC enable bit, the dual-slope converter
integrates the input signal during the first phase for approxi-
mately 2 ms, followed by a second phase that integrates
V_REF for 0 ms to 2 ms depending on the level of the input
signal. As a result the total conversion time varies from 2 ms
to 4 ms.
Simplified ADC Block Diagram
The A/D converter multiplexes 4 different sources:
1. The battery voltage
2. The battery charge current
3. External source ADC1
4. External source ADC2
The voltage ranges for the first two sources are scaled to
match the input voltage interval of the A/D converter: [V_REFH,
2V_REFH]. This is accomplished by using two internal scalars.
Battery Voltage Measurement
The battery voltage scalar transforms the battery voltage
ranging from 2.6V–3.5V to the reference voltage interval:
[V_REFH, 2*V_REFH]. A wider voltage range (2.6V–4.4V) can be
selected through I²C by setting the voltage range bit D7 in
register 0xA to 0’b1.
Battery Charge Current Measurement
The battery charge current is indirectly measured by measur-
ing the voltage across the ISENSE resistor, R_SENSE. A fixed
portion of the battery charge current is mirrored over the
R_SENSE and hence:
V_ISENSE = K * I_CHARGE* R_SENSE
where K is a ratio between the I_SENSE current and the charge
current.
The battery charge current scalar transforms the voltage
across the external ISENSE resistor to the [V_REFH, 2*V_REFH]
input voltage interval of the A/D converter.
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External General Purpose Sources
Two additional A/D converter sources are available on the
ADC1 and ADC2 pins of the LP3910SQ-AN. These two ex-
ternal A/D converter sources are not internally scaled and
have an input voltage range of [V_REFH, 2*V_REFH]. The system
designer can use these two sources for general purpose ap-
plications such as resistive keyboard matrix scanning, tem-
perature measurements, battery load current, battery ID
resistor measurement, etc.
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