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TC1043CEQR Scheda tecnica(PDF) 6 Page - Microchip Technology |
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TC1043CEQR Scheda tecnica(HTML) 6 Page - Microchip Technology |
6 / 20 page TC1043 DS21347B-page 6 © 2002 Microchip Technology Inc. 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. 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: 1. 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 2. Determine the hysteresis voltage (VHY) between the upper and lower thresholds. 3. Calculate RA as follows: WAKE-UP R1 5M R4 C1 10µF VDD VDD VR I/O* *Open Drain Port Pin Microcontroller CMPTR2 TC1043 + – |
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