Datasheet

 2011 Microchip Technology Inc. Preliminary DS41576B-page 161
PIC12F752/HV752
18.0 SHUNT REGULATOR
(PIC12HV752 ONLY)
The PIC12HV752 devices include a permanent internal
5 volt (nominal) shunt regulator in parallel with the V
DD
pin. This eliminates the need for an external voltage
regulator in systems sourced by an unregulated supply.
All external devices connected directly to the VDD pin
will share the regulated supply voltage and contribute
to the total VDD supply current (ILOAD).
18.1 Regulator Operation
A shunt regulator generates a specific supply voltage
by creating a voltage drop across a pass resistor R
SER.
The voltage at the V
DD pin of the microcontroller is
monitored and compared to an internal voltage refer-
ence. The current through the resistor is then adjusted,
based on the result of the comparison, to produce a
voltage drop equal to the difference between the supply
voltage V
UNREG and the VDD of the microcontroller.
See Figure 18-1 for voltage regulator schematic.
FIGURE 18-1: SHUNT REGULATOR
An external current limiting resistor, RSER, located
between the unregulated supply, V
UNREG, and the VDD
pin, drops the difference in voltage between VUNREG
and VDD. RSER must be between RMAX and RMIN as
defined by Equation 18-1.
EQUATION 18-1: RSER LIMITING RESISTOR
18.2 Regulator Considerations
The supply voltage VUNREG and load current are not
constant. Therefore, the current range of the regulator
is limited. Selecting a value for RSER must take these
three factors into consideration.
Since the regulator uses the band gap voltage as the
regulated voltage reference, this voltage reference is
permanently enabled in the PIC12HV752 devices.
The shunt regulator will still consume current when
below operating voltage range for the shunt regulator.
18.3 Design Considerations
For more information on using the shunt regulator and
managing current load, see Application Note AN1035,
“Designing with HV Microcontrollers” (DS01035).
Feedback
VDD
VSS
CBYPASS
RSER
VUNREG
ISUPPLY
ISHUNT
ILOAD
Device
RMAX =
(VUMIN - 5V)
1.05 • (4
MA + ILOAD)
RMIN =
(VUMAX - 5V)
0.95 • (50
MA)
Where:
R
MAX = maximum value of RSER (ohms)
R
MIN = minimum value of RSER (ohms)
VU
MIN = minimum value of VUNREG
VUMAX = maximum value of VUNREG
VDD = regulated voltage (5V nominal)
I
LOAD = maximum expected load current in mA
including I/O pin currents and external
circuits connected to VDD.
1.05 = compensation for +5% tolerance of R
SER
0.95 = compensation for -5% tolerance of RSER