TC1027 Datasheet, PDF (4/18 Pages) TelCom Semiconductor, Inc – LINEAR BUILDING BLOCK - QUAD LOW POWER COMPARATOR AND VOLTAGE REFERENCE

TC1027
3.0 DETAILED DESCRIPTION
The TC1027 is one of a series of very low-power, linear
building block products targeted at low-voltage, single-
supply applications. The TC1027 minimum operating
voltage is 1.8V, and typical supply current is only 18µA.
It combines four comparators and a voltage reference
in a single package.
3.1 Comparators
The TC1027 contains four comparators. The compara-
tor's input range extends beyond both supply voltages
by 200mV and the outputs will swing to within several
millivolts of the supplies depending on the load current
being driven.
The comparators exhibit propagation delay and supply
current which are largely independent of supply
voltage. The low input bias current and offset voltage
make them suitable for high impedance precision
applications.
3.2 Voltage Reference
A 2.0% tolerance, internally biased, 1.20V bandgap
voltage reference is included in the TC1027. It has a
push pull output capable of sourcing and sinking at
least 50µA.
GND (Pin 9) is connected to V_SS (Pin 14) through the
substrate of the integrated circuit. Large currents can
flow between GND and V_SS if the pins are not at the
same voltage.
4.0 TYPICAL APPLICATIONS
The TC1027 lends itself to a wide variety of
applications, particularly in battery-powered systems. It
Typically it finds application in power management,
processor supervisory and interface circuitry.
4.1 External Hysteresis (Comparator)
Hysteresis can be set externally with two resistors
using positive feedback techniques (see Figure 4-1).
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.
2. Determine the hysteresis voltage (V_HY) between
the upper and lower thresholds.
3. Calculate R_A as follows:
EQUATION 4-1:
R_A
=
R

C_
-V----^H----^Y-^
V_{D D}^
4. Choose the rising threshold voltage for V_SRC
(V_THR).
5. Calculate R_B as follows:
EQUATION 4-2:
R_B = ----------------------------1------------------------------


----V----^T----^H---^R------
V_R × R_A


–
---1----
R_A
–
---1----
R_C
6. Verify the threshold voltages with these
formulas:
V_SRC rising:
EQUATION 4-3:
V_THR
=
(V_R)(R_A)
 _--1-----
 _RA
+ ^ --1-----^ + ^ ---1----^
 _RB  _RC
V_SRC falling:
EQUATION 4-4:
V_THF = V_THR –
 ^R_-----^A_----^×_-----^V_----^D_----^D_--
 _RC 
DS21284B-page 4
© 2002 Microchip Technology Inc.