TC1043 Datasheet, PDF (6/20 Pages) Microchip Technology – Linear Building Block . Low Power Voltage Reference with Dual Op Amp, Dual Comparator, and Shutdown Mode

TC1043
4.0 TYPICAL APPLICATIONS
The TC1043 lends itself to a wide variety of applica-
tions, particularly in battery powered systems. It typi-
cally finds application in power management,
processor supervisory, and interface circuitry.
4.1 Wake-Up Timer
Many microcontrollers have a low power “sleep” mode
that significantly reduces their supply current. Typically,
the microcontroller is placed in this mode via a software
instruction, and returns to a fully enabled state upon
reception of an external signal (“wake-up”). The wake-
up signal is usually supplied by a hardware timer. Most
system applications demand that this timer have a long
duration (typically seconds or minutes), and consume
as little supply current as possible.
The circuit shown in Figure 4-1 is a wake-up timer
made from comparator CMPTR2. (CMPTR2 is used
because the wake-up timer must operate when SHDN
is active.) Capacitor C1 charges through R1 until a volt-
age equal to V_R is reached, at which point the WAKE-
UP is driven active. Upon wake-up, the microcontroller
resets the timer by forcing a logic low on a dedicated,
open drain I/O port pin. This discharges C1 through R4
(the value of R4 is chosen to limit the maximum current
sunk by the I/O port pin). With a 3V supply, the circuit
as shown consumes typically 6µA and furnishes a
nominal timer duration of 25 seconds.
4.2 Precision Battery Monitor
Figure 4-2 is a precision battery low/battery dead mon-
itoring circuit. Typically, the battery low output warns
the user that a battery dead condition is imminent. Bat-
tery dead typically initiates a forced shutdown to pre-
vent operation at low internal supply voltages (which
can cause unstable system operation).
The circuit of Figure 4-2 uses a single TC1043 (one op
amp is unused) and only six external resistors. AMP 1
is a simple buffer, while CMPTR1 and CMPTR2 provide
precision voltage detection using V_R as a reference.
Resistors R2 and R4 set the detection threshold for
BATTLOW, while resistors R1 and R3 set the detection
threshold for BATTFAIL. The component values shown
assert BATTLOW at 2.2V (typical) and BATTFAIL at
2.0V (typical). Total current consumed by this circuit is
typically 22µA at 3V. Resistors R5 and R6 provide hys-
teresis for comparators CMPTR1 and CMPTR2
respectively.
4.3 Dual LDO with Shutdown
Figure 4-3 shows a portion of a TC1043 configured as
a dual low dropout regulator with shutdown. AMP1 and
AMP2 are independent error amplifiers that use V_R as
a reference. Resistors RA₁, RB₁, RA₂ and RB₂ set the
feedback around the amplifiers and therefore deter-
mine the output voltage settings (please see equation
in the figure). RA₁, RB₁, RA₂ and RB₂ can have large
ohmic values (i.e. 100’s of kΩ) to minimize supply cur-
rent.
Using the 2N2222 output transistors as shown, these
regulators exhibit low dropout operation. For example,
with V_OUT = 3.0V, the typical dropout voltage is only
50mV at an output current of 50mA. The unused com-
parators can be used in conjunction with this circuit as
power-on reset or low voltage detectors for a complete
LDO solution at a very low installed cost.
4.4 External Hysteresis
Hysteresis can be set externally with two resistors
using positive feedback techniques (see Figure 4-3).
The design procedure for setting external comparator
hysteresis is as follows:
1. Choose the feedback resistor R_C. Since the
input bias current of the comparator is at most
100pA, the current through R_C can be set to
100nA (i.e. 1000 times the input bias current)
and retain excellent accuracy. The current
through R_C at the comparator’s trip point is V_R /
R_C where V_R is a stable reference voltage.
FIGURE 4-1:
WAKE-UP TIMER
R1
5M
C1
10µF
R4
V_DD
V_DD
Microcontroller
I/O*
– ^CMPTR2
+
WAKE-UP
V_R
TC1043
*Open Drain Port Pin
2. Determine the hysteresis voltage (V_HY) between
the upper and lower thresholds.
3. Calculate R_A as follows:
DS21347B-page 6
© 2002 Microchip Technology Inc.