
ADuC7019/20/21/22/24/25/26/27/28
Rev. B | Page 84 of 92
HARDWARE DESIGN CONSIDERATIONS
POWER SUPPLIES
The ADuC7019/20/21/22/24/25/26/27/28 operational power
supply voltage range is 2.7 V to 3.6 V. Separate analog and
digital power supply pins (AVDD and IOVDD, respectively) allow
AVDD to be kept relatively free of noisy digital signals often
present on the system IOVDD line. In this mode, the part can
also operate with split supplies, that is, it can use different
voltage levels for each supply. For example, the system can be
designed to operate with an IOVDD voltage level of 3.3 V while
the AVDD level can be at 3 V, or vice versa. A typical split supply
04
95
5-
04
4
ADuC7026
0.1F
ANALOG
SUPPLY
10F
73
74
AVDD
75
DACVDD
8
GNDREF
70
DACGND
71
AGND
67
REFGND
26
IOVDD
54
25
IOGND
53
0.1F
+
–
DIGITAL
SUPPLY
10F
+
–
Figure 74. External Dual Supply Connections
As an alternative to providing two separate power supplies, the
user can reduce noise on AVDD by placing a small series resistor
and/or ferrite bead between AVDD and IOVDD, and then decoupling
AVDD separately to ground. An example of this configuration is
shown in
Figure 75. With this configuration, other analog circuitry
(such as op amps, voltage reference, and others) can be powered
from the AVDD supply line as well.
0
495
5-
0
45
ADuC7026
0.1F
BEAD
1.6
73
74
AVDD
75
DACVDD
8
GNDREF
70
DACGND
71
AGND
67
REFGND
26
IOVDD
54
25
IOGND
53
0.1F
DIGITAL SUPPLY
10F
+
–
Figure 75. External Single Supply Connections
reservoir capacitor sits on IOVDD, and a separate 10 μF capacitor
sits on AVDD. In addition, local small-value (0.1 μF) capacitors are
located at each AVDD and IOVDD pin of the chip. As per standard
design practice, be sure to include all of these capacitors and ensure
the smaller capacitors are close to each AVDD pin with trace
lengths as short as possible. Connect the ground terminal of
each of these capacitors directly to the underlying ground plane.
Finally, note that the analog and digital ground pins on the
ADuC7019/20/21/22/24/25/26/27/28 must be referenced to
the same system ground reference point at all times.
IOVDD Supply Sensitivity
The IOVDD supply is sensitive to high frequency noise because it
is the supply source for the internal oscillator and PLL circuits.
When the internal PLL loses lock, the clock source is removed
by a gating circuit from the CPU, and the ARM7TDMI core
stops executing code until the PLL regains lock. This feature is
to ensure that no flash interface timings or ARM7TDMI
timings are violated.
Typically, frequency noise greater than 50 kHz and 50 mV p-p
on top of the supply causes the core to stop working.
section do not sufficiently dampen all noise soures below
50 mV on IOVDD, a filter such as the one shown in Figure 76 is recommended.
ADuC7026
26
IOVDD
54
25
IOGND
53
0.1F
DIGITAL
SUPPLY
10F
+
–
1H
04
95
5-
0
87
Figure 76. Recommended IOVDD Supply Filter
Linear Voltage Regulator
Each ADuC7019/20/21/22/24/25/26/27/28 requires a single
3.3 V supply, but the core logic requires a 2.6 V supply. An on-
chip linear regulator generates the 2.6 V from IOVDD for the
core logic. The LVDD pin is the 2.6 V supply for the core logic.
An external compensation capacitor of 0.47 μF must be
connected between LVDD and DGND (as close as possible to
these pins) to act as a tank of charge as shown in
Figure 77.
04
955
-04
6
ADuC7026
0.47
μF
27
LVDD
28
DGND
Figure 77. Voltage Regulator Connections
The LVDD pin should not be used for any other chip. It is also
recommended to use excellent power supply decoupling on
IOVDD to help improve line regulation performance of the on-
chip voltage regulator.