SHT20P datasheet(6/10 Pages) ETC2

6 / 10 page

Datasheet SHT20P

www.sensirion.com

Version 4 – May 2014

6/10

Parameter

min

max

Units

VDD to VSS

-0.3

Digital IO Pins (SDA, SCL)

to VSS

-0.3

VDD + 0.3

Input Current on any Pin

-100

100

Table 3 Electrical absolute maximum ratings

ESD immunity is qualified according to JEDEC JESD22-

A114 method (Human Body Model at

4kV), JEDEC

JESD22-A115 method (Machine Model

200V) and ESDA

ESD-STM5.3.1-1999 and AEC-Q100-011 (Charged

Device Model, 750V corner pins, 500V other pins). Latch-

up immunity is provided at a force current of

100mA with

Tamb = 125°C according to JEDEC JESD78. For exposure

beyond named limits the sensor needs additional

protection circuit.

3.2

Input / Output Characteristics

The electrical characteristics such as power consumption,

low and high level input and output voltages depend on

the supply voltage. For proper communication with the

sensor it is essential to make sure that signal design is

strictly within the limits given in Table 4.

Parameter

min

typ

max Units

Output Low Voltage, VOL

Output High Voltage, VOH

VDD

Output Sink Current, IOL

µA

Table 4 DC characteristics of output pad. VDD = 2.1 V to 3.6 V,

T = -40 °C to 125 °C, unless otherwise noted.

4 Communication with Sensor

4.1

Start up Sensor

As a first step, the sensor is powered up to VDD (between

2.1V and 3.6V). After power-up, the sensor needs at most

150ms for reaching idle state. During that time SDA is in

undefined state. Then the sensor starts measuring and

providing data on PWM bit-stream.

4.2

PWM Specification

Pulse Width Modulation runs on a constant frequency and

the measured information is provided as duty cycle on that

frequency – see Figure 10. Such information is measured

humidity for SCL pulled high and temperature for SCL

pulled low.

Figure 10 PWM signal. Base frequency runs constantly at

approximately 120 Hz, hence tF is about 8.3ms. The signal is

provided on tPW as a ratio of tF.

The measured data – either humidity or temperature – is

provided as ratio of tPW and tF. tPW shall always be given as

ratio of tF to make it independent of variations of the base

frequency.

5 Conversion of Signal Output

Resolution is set to 10 bit relative humidity and 12 bit

temperature reading and cannot be changed. The sensor

reading is linear and hence it can be converted to a

physical value by an easy linear equation.

5.1

Relative Humidity Conversion

With the relative humidity signal output the relative

humidity RH is obtained by the following formula (result in

%RH):

125









The physical value RH given above corresponds to

the relative humidity above liquid water according to

World Meteorological Organization (WMO). For

relative humidity values above ice RHi the values need to

be transformed as from relative humidity above water RHw

at a certain temperature t follows – compare also

Application Note “Introduction to Humidity”:



































exp

Units are %RH for relative humidity and °C for

temperature. The corresponding coefficients are defined

as follows: βw = 17.62, λw = 243.12°C, βi = 22.46, λi =

272.62°C.

5.2

Temperature Conversion

The temperature T is calculated by inserting the ratio of

tPW and tF into the following formula (result in °C):

175.72

46.85









SDA

VDD

tPW

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