SHT21P SENSIRION, SHT21P Datasheet - Page 6

• Manufacturer Part Number: SHT21P
• Description: SENSOR, HUMIDITY & TEMP
• Manufacturer: SENSIRION

Specifications of SHT21P

• Humidity Range: 0 - 100% RH
• Sensor Output: Analogue - PWM Interface
• Sensor Terminals: SMD
• Response Time: 8s
• Supply Voltage Range: 2.1V To 3.6V
• Accuracy: 2% RH
• Supply Voltage: 3VDC
• Termination: RoHS Compliant
• Capacitance: 0µF
• Rohs Compliant: Yes
• Sensing Accuracy: 2%
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 2 - 1 Year

Datasheet SHT21P
Parameter
VDD to VSS
Digital IO Pins (SDA, SCL)
to VSS
Input Current on any Pin
Table 3 Electrical absolute maximum ratings
ESD immunity is qualified according to JEDEC JESD22-
A114E method (Human Body Model at 4kV), JEDEC
JESD22-A115A method (Machine Model 200V) and
ESDA
(Charged Device Model, 750V corner pins, 500V other
pins). Latch-up immunity is provided at a force current of
100mA with T
For exposure beyond named limits the sensor needs
additional protection circuit.
4.2
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
Output Low Voltage, VOL
Output High Voltage, VOH
Output Sink Current, IOL
Table 4 DC characteristics of output pad. VDD = 2.1 V to 3.6 V,
T = -40 °C to 125 °C, unless otherwise noted.
5 Communication with Sensor
5.1
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.
5.2
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.
www.sensirion.com
Input / Output Characteristics
Start up Sensor
PWM Specification
ESD-STM5.3.1-1999
amb
= 125°C according to JEDEC JESD78.
-100
-0.3
-0.3
min
min
and
VDD + 0.3
VDD
max
100
typ
0
5
AEC-Q100-011
max Units
40
Units
mA
Version 1.1 – May 2010
V
V
µA
V
V
Figure 10 PWM signal. Base frequency runs constantly at
approximately 120 Hz, hence t
provided on t
The measured data – either humidity or temperature – is
provided as ratio of t
ratio of t
frequency.
6 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.
6.1
With the relative humidity signal output the relative
humidity RH is obtained by the following formula (result in
%RH):
The physical value RH given above corresponds to
the relative humidity above liquid water according to
World Meteorological Organization (WMO).
relative humidity values above ice RH
be transformed as from relative humidity above water RH
at a certain temperature t follows – compare also
Application Note “Introduction to Humidity”:
Units are %RH for relative humidity and °C for
temperature. The corresponding coefficients are defined
as follows: β
272.62°C.
6.2
The temperature T is calculated by inserting the ratio of
t
T
RH
RH
PW
SDA

and t
i



46.85
Relative Humidity Conversion
Temperature Conversion

RH
F
F
6
to make it independent of variations of the base
into the following formula (result in °C):

w
PW
125
w


exp
as a ratio of t
175.72
= 17.62, λ


 

t
PW
t
F
λ
PW
β
w
w
and t



t
t
t
PW
t
F
F
w

 

.
F
= 243.12°C, β
F
. t
t
P
is about 8.3ms. The signal is
exp
PW
shall always be given as

 

t
λ
PW
β
i
i
i

the values need to

t
t

 

i
= 22.46, λ
VDD
For
6/9
i
0V
=
w