Cirrus Logic CS35L32 - Manual (Page 20)

20 DS963F4
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):
D
OUT
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 D
OUT
values equate to a negative load voltage and positive D
OUT
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:
D
OUT
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 D
OUT
values equate to a negative current and positive D
OUT
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 2 15 power in the IMON equation increases to 2 15+1 . If the IMON_SCALE
value is decreased by 1 bit, the 2 15 power in the IMON equation decreases to 2 15–1 .
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
2
15
1 –
-------------------
 
 
  6.25 VA 
1.8
--------------------------
 
 
 =
IMON
DOUT
2
15
1 –
-------------------
 
 
 
0.82 VA 
0.1 
--------------------------
 
 
 =