MOTOROLA
2
AN1257/D
Figure 1. On-Chip Voltage Regulator Block Diagram
The primary voltage regulator circuit is a classic three terminal design using an integrated series-pass
transistor. The output of the pass transistor is fed into an error amplifier in the primary regulator where it is
compared to a reference voltage produced by a band-gap reference circuit. The output of the error
amplifier, in turn, controls the current through the pass transistor. As the load on the regulator output
increases or decreases, the output of the error amplifier changes proportionally, increasing or decreasing
the current through the pass transistor. This enables the regulator to maintain a stable 5-volt output under
changing load conditions.
The 5-volt output of the primary regulator supplies power directly to the digital logic portion of the MCU.
The primary regulator output also is connected to the V
DD
pins to allow for easy connection to external
decoupling capacitors and the 10
μ
F supply stabilization capacitor. Connection of the power supply output
to the V
DD
pins also allows 5 volts to be passed externally to the analog circuitry of the MCU and to other
components in an application. Should a short circuit occur between the V
DD
and V
SSD
pins, an over-current
protection circuit has been included to prevent damage to the primary voltage regulator. Refer to the
MC68HC05V7 and MC68HC705V8 general release specifications, Motorola document numbers
HC705V7GRS/D and HC705V8GRS/D, for the maximum supply current available from the voltage
regulator on each MCU.
The secondary voltage regulator has two functions. The first is to supply V
EE
to the small amount of power
supply control logic necessary for enabling and disabling the primary voltage regulator. The secondary
regulator’s other function is to act as a “pre-regulator” for the input voltage supplied to the primary
regulator. This helps reduce the variation in the input voltage seen by the primary regulator, improving the
line rejection specification of the primary regulator output with respect to changes in the input voltage.
NONOTE
The primary voltage regulator can be enabled only if the mask option enabling the internal
voltage regulation system is selected. However, the secondary regulator and the power
supply control logic will always be enabled whenever the supply voltage to the V
BATT
pin is
above the minimum operating voltage specified for correct regulator operation.
IGNS
PDC
...
V
IGN
BUS
MDLCPU
VDDC
REGULATOR
REGULATOR
CLAMP
CPU
V
BATT
V
DD
V
SSD
MISC
REGISTER
LVR
MCU RESET
V
EE
SERIES-PASS
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INTEGRATED VOLTAGE REGULATOR
10
μ
F (EXTERNAL)
+