Application Note AN8122-1 Advanced Communications and Sensing SX8122 Application Note 1V Timer (Next Gen 555) Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 1 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing 1. Introduction The SX8122 is a new kind of timer based on a precise clock and an analog to digital converter. Its simple architecture is making possible to have very predictible results even when operating a extremely low supply voltages. It can be compared to the 555 timer in the sense that it compares one input voltage to two present threshold voltages to set or reset an internal memory cell that controls a digital output and an open drain discharge pin. In addition to this old timer, the SX8122 controls a charge pin and has a burst output that can be used to generate higher voltages. To reduce the size, weight and cost of small appliances, the SX8122 is made to operate from a single AA or AAA cell. It monitors the supply voltage and indicates when it is above 1.4V, allowing for very simple NiMH or NiCd rechargeable devices. 2. Description The main element of the SX8122 is an 8-bit ADC with a fixed LSB of 7 mV. The SX8122 acquires the TRIGGER and VDD in a cyclic way with periodiciy of 200 us. If VDD voltage is above 1.8V, the acquisition is saturated to 1.8V. The result of the TRIGGER acquisition is compared with the VDD acquisition and if TRIGGER < VDD/3 then the SX8122 is set in “charge” mode, if TRIGGER > 2*VDD/3 then the SX8122 is reset in “discharge” mode. 2/3 VDD TRIGGER 1/3 VDD DISCHARGE R Q S Q VDD MOTOR 5 kHz VSS VDD VSS CHARGE VDD 1.4 V NIMH SX8122 Burst 41 kHz LED In “charge” mode, the DISCHARGE pin is open while the CHARGE and the MOTOR pins are connected to VDD. In “discharge” mode, the CHARGE pin is open while the DISCHARGE and the MOTOR pins are connected to VSS. The LED pin is generating a 41 kHz square wave signal during the “charge” mode. This signal is “on” for 2 us and “off” for 21 us, thus has a total cycle time of 23 us. It is connected to VSS during the “discharge” mode. The NiMH pin is connected to VDD when the VDD voltage is below 1.4 V, it is connected to VSS when the VDD voltage is above 1.4 V. It has a 25 - 30 mV hysteresis around this threshold. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 2 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing 2.1. Monostable Operation (One Shot) This is used to generate a single shot of a controlled width on the MOTOR pin or a burst on the LED pin. The monostable timer is done with only two external elements: one resistor and one capacitor. The capacitor is connected between the TRIGGER pin and VSS while the resistor is connected between the TRIGGER pin and VDD. R 2/3 VDD TRIGGER 1/3 VDD 1.2 V cell R Q S Q VDD MOTOR DISCHARGE VSS VDD VSS CHARGE button C VDD Vth NIMH SX8122 Burst LED The monostable is reset either by a button-switch or by another circuit with an open collector output that will short the TRIGGER pin to VSS. At that time, the MOTOR pin goes to VDD and the LED pin starts to output its square wave. As soon as the TRIGGER pin is released, the capacitor will charge through the resistor and when the TRIGGER pin level reaches 2/3*VDD then the MOTOR and the LED pins go back to VSS. The “on” time after the trigger is released (tramp) is directly proportional to R * C (if parasitics are negligible). t ramp = 1.1 ⋅ R ⋅ C VDD tsw tramp TRIGGER 2/3 VDD 1/3 VDD VSS SWITCH MOTOR LED Example: With R = 10 kOhm and C = 1 uF, one get a tramp of 11 ms. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 3 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing 2.2. Sawtooth Astable Operation This is used to generate a frequency. The sawtooth astable timer is done with only two external elements: one resistor and one capacitor. The capacitor is connected between the TRIGGER pin and VSS while the resistor is connected between the TRIGGER pin and VDD. Pin TRIGGER is also connected to pin DISCHARGE. R 2/3 VDD TRIGGER 1/3 VDD 1.2 V cell R Q S Q VDD MOTOR DISCHARGE VSS VDD VSS CHARGE C VDD Vth NIMH SX8122 Burst LED The resistor loads the capacitor until TRIGGER reaches 2/3*VDD; during that time, pin MOTOR is connected to VDD and pin LED sends its square wave. At the next ADC acquisition, TRIGGER will be evaluated above 2/3*VDD, therefore the DISCHARGE pin is connected to VSS, unloading the capacitor for one full ADC period (200 us). If the capacitor can be downloaded to VSS in this amount of time, then the cycle restarts on the next ADC acquisition. MOTOR and LED pins go to VSS during the discharge time. The ramp period tramp is the same as for the monostable timer, the reset period is 200 us (if the capacitor can be discharge in a single period). t period = 1.1 ⋅ R ⋅ C + 0.2ms VDD 2/3 VDD TRIGGER 1/3 VDD VSS MOTOR LED Example: With R = 10 kOhm and C = 1 uF, one get a tramp of 11 ms and a full cycle period of 11.2 ms. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 4 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing 2.3. Astable Operation with 50% duty cycle This is used to generate a square wave on MOTOR pin and a series of bursts on LED pin. The square astable timer is done with only two external elements: one resistor and one capacitor. The capacitor is connected between the TRIGGER pin and VSS while the resistor is connected between the TRIGGER pin and pins DISCHARGE and CHARGE. 2/3 VDD TRIGGER 1/3 VDD 1.2 V cell R R Q S Q VDD MOTOR DISCHARGE VSS VDD VSS CHARGE C VDD Vth NIMH SX8122 Burst LED Let’s start in “charge” mode. In this mode, the resistor is connected to VDD through the CHARGE pin. The resistor loads the capacitor until TRIGGER reaches 2/3*VDD; during that time, pin MOTOR is connected to VDD and pin LED sends its square wave. At the next ADC acquisition, TRIGGER will be evaluated above 2/3*VDD, therefore the chip switches to “discharge” mode: the CHARGE pin is disconnected from VDD and the DISCHARGE pin is connected to VSS, unloading the capacitor; MOTOR and LED pins go to VSS during the discharge time. When TRIGGER pins goes below 1/3*VDD, then the chip switches back to “charge” mode. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 5 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing The “on” time between two transitions (tramp) is directly proportional to R * C (if parasitics are negligible). This is half of the period of the full cycle. t period = 2 ⋅ 0.66 ⋅ R ⋅ C VDD 2/3 VDD TRIGGER t ramp = 0.66 ⋅ R ⋅ C 1/3 VDD VSS MOTOR LED Example: With R = 10 kOhm and C = 15 uF, one get a tramp of 100 ms and a full cycle period of 200 ms. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 6 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing 2.4. Astable Operation with programmable duty cycle This is used to generate an uneven square wave on MOTOR pin and a series of bursts on LED pin. The variable duty cycle astable timer (PWM) is done with only three external elements: two resistors and one capacitor. The capacitor is connected between the TRIGGER pin and VSS while one resistor is connected between the TRIGGER pin and the DISCHARGE pin and the other resistor between the TRIGGER pin and the CHARGE pin. 2/3 VDD TRIGGER Rd 1.2 V cell 1/3 VDD R Q S Q VDD MOTOR DISCHARGE Rc VSS VDD VSS CHARGE C VDD Vth NIMH SX8122 Burst LED Let’s start in “charge” mode. In this mode, the Rd resistor is connected to VDD. Rd loads the capacitor until TRIGGER reaches 2/3*VDD; during that time, pin MOTOR is connected to VDD and pin LED sends its square wave. At the next ADC acquisition, TRIGGER will be evaluated above 2/3*VDD, therefore the chip switches to “discharge” mode: the CHARGE pin is disconnected from VDD and the DISCHARGE pin is connected to VSS, unloading the capacitor through Rc; MOTOR and LED pins go to VSS during the discharge time. When TRIGGER pins goes below 1/3*VDD, then the chip switches back to “charge” mode Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 7 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing The “on” time between two transitions is directly proportional to R * C (if parasitics are negligible). This is once for going up and once for going down on the period of the full cycle t ch arg e = 0.66 ⋅ ( Rc ) ⋅ C t disch arg e = 0.66 ⋅ ( Rd ) ⋅ C t period = 0.66 ⋅ ( Rd + Rc ) ⋅ C VDD 2/3 VDD TRIGGER 1/3 VDD VSS MOTOR LED Example: With Rc = 5 kOhm, Rd = 10kOhm and C = 15 uF, one get a trampup of 50 ms, a trampdown of 100 ms and a full cycle period of 150 ms. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 8 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing 2.5. Higher Voltage Monitoring This is used to generate a square wave on LED when the input signal is lower than a preset value and stop it when its above another value. The square wave can be used to generate a higher voltage (booster). The basic principle is to use a resistive divider to reduce the higher voltage to the 1/3 * VDD - 2/3 * VDD range. As this is usually used for regulation, the hysteresis needs to be reduced from 50% of full scale. This is done by using a positive feedback with MOTOR pin voltage that is added to the input voltage through another resistive divider. Ri Rf input 2/3 VDD TRIGGER 1/3 VDD 1.2 V cell R Q S Q VDD MOTOR DISCHARGE Rs VSS VDD VSS CHARGE VDD Vth NIMH SX8122 Burst LED Ri/Rs are set so that the mean threshold voltage is as expected. Rf is set to limit the hysteresis. input Input threshold VDD 2/3 VDD 1/3 VDD TRIGGER VSS MOTOR LED This can be used to generate a controlled high voltage using the LED burst output to drive a booster: LED output is active as long as the input voltage is below the input threshold and stops when the input voltage is above the input threshold. The hysteresis is programmed. Revision 1.0/ August 26, 2010 ©2010 Semtech Corp. Page 9 www.semtech.com Application Note AN8122-1 Advanced Communications and Sensing © Semtech 2010 All rights reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. 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