MOTOROLA
SEMICONDUCTOR APPLICATION NOTE
Order this document
by AN1257/D
Motorola, Inc., 1995
AN1257/D
AN1257
Using the M68HC05 Family
On-Chip Voltage Regulator
By Chuck Powers
MCTG Strategic Engineering
INTRODUCTION
Two 1993 additions to the M68HC05 Family of microcontrollers (MCUs) are the MC68HC705V8 and the
MC68HC05V7. These two MCUs, manufactured with a state-of-the-art mixture of ultra-high voltage CMOS
(UHVCMOS) and bipolar analog processing, each include an integrated 12-volt (nominal) to 5-volt voltage
regulation system. This voltage regulation circuit, which is enabled via mask option, can provide the 5 volts
necessary for powering the MCU in an application.
This on-chip voltage regulator is specified for normal operation over an input voltage range of 8 V to 16 V,
optimizing it for use in the automotive environment. In automotive applications, either of these two devices
can be connected directly to the vehicle's battery via the V
BATT
pin, thus eliminating the external voltage
regulator normally required to provide the 5-V supply for the MCU. In addition to supplying the 5 volts for
internal use, this integrated regulator also can supply additional current off chip, powering other application
components.
Although designed to operate over a normal input voltage range of 8 volts to 16 volts, this voltage regulator
is specified for correct operation with an input voltage of up to 26.5 volts for five minutes. This ensures that
the device will operate during double-battery situations, such as during a jump start from a wrecker. In
addition, the regulator's design and processing enable it to survive many of the severe electrical transients
common in an automotive electrical system, such as the load dump voltage spikes which can occur in a
vehicle’s 12-volt supply when the battery becomes disconnected while the engine is running.
This application note provides the user with an overview of the voltage regulator architecture and an
outline of the external components and software necessary for correct operation of this regulator as it has
been implemented on the above mentioned devices. Although this voltage regulator currently is available
on two M68HC05 MCUs only, Motorola is likely to make this regulator available on more devices. In
addition, the maximum operating specifications described in this application note apply to the
MC68HC705V8 and the MC68HC05V7 and may be modified and expanded in future designs.
POWER SUPPLY ARCHITECTURE
The M68HC05 Family’s integrated voltage regulation circuit consists of three main parts: the primary
regulator, the secondary regulator, and the power supply control logic. (See Figure 1.) A low-voltage reset
(LVR) circuit also has been included on the MC68HC705V8 and MC68HC05V7 to prevent unpredictable
operation of the MCU in the event the 5-volt supply level drops below the specified operating level.