AN835 Datasheet, PDF (3/4 Pages) Silicon Laboratories – COMBINING THREE VERY LOW SUPPLY CURRENT ANALOG ICS INTO A USB-POWERED THERMOSTAT

AN835
2.2. Discussion and Results
In order to design this circuit, here’s the step-by-step procedure:
1. Choose the ambient trip temperature to sense T in degree Celsius.
2. Compute the LM35’s output voltage, V_OUT, based on (1) as:
V_OUT = TC  10mV / C
Equation 1.
3. Calculate the gain, G, necessary to achieve a 1.252V voltage on the output of the TS1001 based on (2) as:
G = 1.252 V / V_OUT
Equation 2.
4. Calculate the feedback total resistance:
R_T = R4 + R5
Equation 3.
5. Solve for R4 and R5 by taking into account the non-inverting gain equation of the op-amp, Equation 3, and
the value of G from Step (3). The non-inverting equation is:
G = 1 + R5 / R4
Equation 4.
When the fan is energized, total current consumption of the circuit from the USB-supplied 5V is 200.14 mA. The
TS9001-1, TS6001, and the TS1001 altogether consume only 0.02% of the total power consumption. To further
minimize the current consumption, the feedback resistor values of the TS1001 can be increased. For instance, if
the feedback resistance is doubled, the current on the feedback is halved. This results in a reduction of supply
current by 2.5 µA. In this particular case, with a total resistance of 248.3 k and an output voltage of 1.254 V on the
output of the TS1001, the current on the feedback is 5 µA. At a maximum output voltage of 2.5 V, the current is
10.1 µA.
In this circuit, the input-referred hysteresis of the TS9001-1 is 3mV; hence, the rising threshold or VTHR is at
1.254V, which corresponds to a 26.4ºC temperature. The falling threshold or VTHF is at 1.251V, which corresponds
to a 26.3 ºC temperature. However, if one wishes to turn off the fan at a lower temperature relative to its turn off
temperature, additional hysteresis can be added to the comparator. Please refer to the Applications Information
section of the TS9001-1 product datasheet for a detailed explanation on how to increase comparator hysteresis.
The MOSFET selected should be able to handle the fan maximum current specification. Since the output of the
TS9001-1 switches to a full-scale 5V, the MOSFET does not need to be an ultra-low threshold voltage type.
In order to maintain an accurate gain G and accurate trip points or VTHR and VTHF, metal-film resistors with 1%
tolerance were used in the circuit and are recommended.
3. Conclusion
The combination of these three low-supply precision analog IC’s produces a USB-powered thermostat/fan
controller with < 0.8 ºC error. In summary, the TS9001, the TS1001, and the TS6001-2.5 are excellent choices for
this application as well as other temperature sensing applications. For additional information, see documentation
on the TS1001 Op Amp, TS6001 Voltage Reference, and TS9001 Analog Comparator + Reference, or contact
Silicon Labs.
Rev. 1.0
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