1V Timer (Next Gen 555)

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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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Application Note
AN8122-1
Advanced Communications and Sensing
© Semtech 2010
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