mpc5645s Freescale Semiconductor, Inc, mpc5645s Datasheet - Page 94

• Manufacturer Part Number: mpc5645s
• Description: Mpc5645s Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

Electrical characteristics
The two transients above are not influenced by the voltage source that, due to the presence of the R
provide the extra charge to compensate the voltage drop on C
the filter is very high with respect to the sampling time (T
94
•
Equation 6
faster, but the A/D converter circuitry has been designed to be robust also in the very worst case: the sampling time T
is always much longer than the internal time constant:
The charge of C
according to
A second charge transfer involves also C
R
would be faster), the time constant is:
In this case, the time constant depends on the external circuit: in particular imposing that the transient is completed
well before the end of sampling time T
Of course, R
impedance) and R
(at the end of the charge transfer transient) will be much higher than V
balance assuming now C
L
: again considering the worst case in which C
can again be simplified considering only C
L
Equation
shall be sized also according to the current limitation constraints, in combination with R
P1
V A2
F
and C
(filter resistance). Being C
8:

P2
S

C S C P1 C P2 C F
already charged at V
is redistributed also on C
V A1
+
MPC5645S Microcontroller Data Sheet, Rev. 6
10  2



+
C S
 1
 1
 2 R L
S
=
, a constraints on R
F
=
+

10 R L
(that is typically bigger than the on-chip capacitance) through the resistance

C P1


+
R SW
R SW

F
A1

+
definitively bigger than C
P2

S


):
). The filter is typically designed to act as anti-aliasing.
C P2
C S
+
+
=
and C

S
C S
R AD
R AD
S
V A C F
, determining a new value of the voltage V
+
with respect to the ideal source V
S

+
C P1
as an additional worst condition. In reality, the transient is

=
S
C P1


were in parallel to C
L
V A


C S T S
+
sizing is obtained:
-------------------- -
C P
C P C S
+
+
C P2

V A1
«
+

C P2

C P1
C S



A1


+
C P1 C P2
.
C P2
T S
P1
Equation 11
, C
+

P2
P1
(since the time constant in reality
and C
+
C S
must be respected (charge
A
S
, then the final voltage V

; the time constant R
F
C
Freescale Semiconductor
F
filter, is not able to
A1
on the capacitance
S
(source
Eqn. 10
Eqn. 11
Eqn. 6
Eqn. 7
Eqn. 8
Eqn. 9
F
C
F
A2
of
S