TC1043 datasheet(6/20 Pages) MICROCHIP | Linear Building Block . Low Power Voltage Reference with Dual Op Amp, Dual Comparator, and Shutdown Mode

6 page

TC1043

DS21347B-page 6

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

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.

Thecircuit shown inFigure4-1 is awake-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 VR 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 6

µA 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 VR 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 22

µA 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 VR 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 k

Ω) to minimize supply cur-

rent.

Using the 2N2222 output transistors as shown, these

regulators exhibit low dropout operation. For example,

with VOUT = 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:

Choose the feedback resistor RC.Since the

input bias current of the comparator is at most

100pA, the current through RC canbeset to

100nA (i.e. 1000 times the input bias current)

and retain excellent accuracy. The current

through RC at the comparator’s trip point is VR /

RC where VR is a stable reference voltage.

FIGURE 4-1:

WAKE-UP TIMER

Determine the hysteresis voltage (VHY) between

the upper and lower thresholds.

Calculate RA as follows:

WAKE-UP

10µF

VDD

I/O*

*Open Drain Port Pin

Microcontroller

CMPTR2

TC1043

–

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