pc87591l-n05 National Semiconductor Corporation, pc87591l-n05 Datasheet - Page 183

• Manufacturer Part Number: pc87591l-n05
• Description: Lpc Mobile Embedded Controller
• Manufacturer: National Semiconductor Corporation

Revision 1.2
4.0 Embedded Controller Modules
4.12.6 Usage Hints
Power Consumption
When a channel is enabled and no load is connected, the DAC current consumption depends on the value set in DACDAT
register. Minimal current is consumed when the data is 00
DAC Output Protection
To maintain the high performance of the analog circuits, the DAn (n=0 to 3) pins are not back-drive protected. Therefore, the
voltage applied to these pins must be within the AGND to AV
External circuits should not drive currents into these pins when the PC87591L-N05 power is off because this may cause the
internal Power-Up reset circuit to fail.
Output Voltage Accuracy
Besides the intrinsic accuracy of the D/A channels, the output voltage accuracy directly depends on the accuracy of the
AV
AV
DACDATA7-0 in DACDATn registers (see Section 4.12.3 on page 179).
The external load on DA0-3 pins may also affect the final output voltage of the DAC. Since the output resistance of these
pins is typically 3 K
drivers.
For the worst case calculation, if the output resistance is 4 K
than 2 M
To work with loads of 3 K (1 mA at 3V) with an error lower than 1/2 LSB, the output resistance of the external driver should
be lower than:
Output Settling Time
The DAC output settling time depends on the external load characteristics and the required accuracy. Figure 65 shows the
equivalent circuit used for evaluating DAC behavior. Each DAC output has a typical output impedance of 3 K
if the total load is a 50 pF capacitor only, the output settles to 1/2 LSB within 1 s. The total load capacitance is comprises
the analog output capacitance (C
Filtering Noise on Output Signals
Output signals may have unwanted noise caused by nearby digital circuits. When using slow changing signals in a noisy
environment, a low-pass filter (LPF) may be added externally. This may also be required in applications where the DAC out-
puts control sensitive circuits like audio amplifiers. This can be implemented as a simple RC circuit. The cutoff frequency of
this LPF should be above the required signal frequency.
CC
CC
power supply, which serves as reference voltage. In order to improve the accuracy of the output voltage, the actual
value, measured by the ADC module (see Section 4.11 on page 166), should be used when computing the value of
see Figure 65). In this case, the error caused by the load is lower than 1/2 LSB.
use high-impedance loads; if high accuracy or high output currents are required, use external analog
R
AO
OEXT
) and the external load capacitance (C
V
OUT
Equivalent Circuit
Figure 65. DAC Output Equivalent Circuit
< 3 K
DAC Output
(Continued)
RO
/ (2
C
AO
*
16
256) = 5.8
183
; maximum current is consumed when the data is 55
CC
(maximum limit), the external load (R
range; otherwise the device may be damaged.
Load Circuit
Application
L
).
C
L
R
L
L
) must not be lower
For example,
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16
.