From 555 to SX8122

Application Note
AN8122-2
Advanced Communications and Sensing
SX8122
Application Note
From 555 to SX8122
Revision 1.00/ August 26, 2010
©2010 Semtech Corp.
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Application Note
AN8122-2
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 predictable 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 periodicity 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 1.00/ August 26, 2010
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Application Note
AN8122-2
Advanced Communications and Sensing
2.1. Monostable (One Shot) and Sawtooth Operation
In monostable and sawtooth operation, SX8122 operates very similar to 555. Same resistor and capacitor values will give
the same results.
On the 555, the Trigger pin is used to start the cycle. The Threshold pin is used to read the transition level. The Discharge
pin is used to discharge the R*C element.
On the SX8122, the switch directly resets the R*C element. The Trigger pin is the one used to read the transition level.
t ramp = 1.1 ⋅ R ⋅ C
Example:
With R = 10 kOhm and C = 1 uF, one get a tramp of 11 ms.
2.2. Astable Operation with Programmable Duty Cycle
The astable operation of the SX8122 is more flexible than the one of the 555 as the SX8122 has separate charge/
discharge pins. SX8122 can easily have 50% duty cycles that are not achievable with simple settings on the 555.
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
t ch arg e = 0.66 ⋅ ( Rc ) ⋅ C
t disch arg e = 0.66 ⋅ ( Rd ) ⋅ C
t period = 0.66 ⋅ ( Rd + Rc ) ⋅ C
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 1.00/ August 26, 2010
©2010 Semtech Corp.
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Application Note
AN8122-2
Advanced Communications and Sensing
© Semtech 2010
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