CS35L32 datasheet(20/51 Pages) CIRRUS

20 / 51 page

DS963F5

CS35L32

4.8 Signal Monitoring

4.8.2

Monitoring Voltage across the Load—VMON

As shown in Fig. 4-5, monitoring on VMON is accomplished via the VSENSE± pins. Table 3-7 gives operating and

performance specifications for this ADC path. The following equation determines the VMON voltage (in Volts):

DOUT is the 16-bit digital output monitoring word in signed decimal format (–32,768 to +32,767) and VA is the voltage on

the VA pin. Relative to VSENSE+, negative DOUT values equate to a negative load voltage and positive DOUT values

equate to a positive load voltage. When VA is 1.8 V, the full-scale signal is 6.25 V.

If VMON is a 12-bit word, its equivalent 16-bit representation for the computational purposes of this section positions the

12 bits in the 12 MSBs and the 4 LSBs are cleared in the computation.

4.8.3

Monitoring Current through the Load—IMON

As shown in Fig. 4-5, monitoring of output current is accomplished via the ISENSE± pins, which are provided to measure

a voltage drop across a sense resistor in the output path, as described in Section 3. A precision resistor (

1%) is chosen

for high accuracy when calculating the current from the voltage measured across the resistor. Likewise, to avoid thermal

drift, the resistor is chosen to have a low thermal coefficient of 100 ppm/°C. Table 3-7 gives operating and performance

specifications for this ADC path.

The following equation determines the IMON current (in Amps) when using a 0.1-

 sense resistor:

DOUT is the 16-bit digital output monitoring word in signed decimal format (–32,768 to +32,767) and VA is the voltage on

the VA pin. Relative to ISENSE+, negative DOUT values equate to a negative current and positive DOUT values equate to

a positive current. The default IMON_SCALE, as described in Section 4.8.3.1, is used for the example equation. If the

IMON_SCALE value is increased by 1 bit, the 215 power in the IMON equation increases to 215+1. If the IMON_SCALE

value is decreased by 1 bit, the 215 power in the IMON equation decreases to 215–1.

If IMON is a 12-bit word, its equivalent 16-bit representation for the computational purposes of this section positions the

12 bits in the 12 MSBs, and the 4 LSBs are cleared in the computation.

4.8.3.1

IMON Signal Scaling (IMON_SCALE)

Because the voltage is measured across a resistor of very small value and because output current can vary significantly

depending on the program material, a gain-scaling block (shown in Fig. 4-5) is included to improve the reported sample

resolution for low-level signals. This control, configured through IMON_SCALE (see p. 38), allows the system processor

to determine the range of bits to be received from the available 26-bit word on the IMON ADC’s data bus. The default

IMON_SCALE configuration (22 down to 7) configures the ADC data MSB (bit 22) to be the 16-bit IMON data packet MSB.

ADC bits 23–25 allow the signal to be divided down.

If IMON is a 12-bit word, its equivalent 16-bit representation for the computational purposes of this section positions the

12 bits in the 12 MSBs. The 4 LSBs are cleared in the computation.

4.8.3.2

IMON Sense Resistor

A 0.1-

sense resistor is used to generate a differential voltage that is captured by the IMON circuitry to monitor the load

current. If PWM output filtering components, such as ferrite beads, are placed in series with the output load, the sense

resistor must be placed between the SPKOUT+ pin and the external series filter component, minimizing any performance

effects produced by the output filter. If the sense resistor is placed after the series-filtering component, the signal being

measured across the sense resistor will have been altered from its expected form.

VMON

DOUT

–

-------------------







6.25 VA



1.8

--------------------------







IMON

DOUT

–

-------------------







0.82 VA



0.1



--------------------------







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