TDA21801 Datasheet, PDF (12/30 Pages) Infineon Technologies AG – Low Cost Fan Speed Controller

6.2 Oscillator
The technology of oscillator is similar to the industrial standard 555-timer, with control voltage to
increase frequency by a factor of up to 3.4. The external timing resistor, R_VCO and capacitor, C_VCO set
the minimum frequency of Voltage Controlled Oscillator. The minimum Fan Speed can be adjusted in
the range from 750 to 4000 rpm. With tolerance of 3% at rated speed of 1322 rpm, V_DD=12V and 25°C
(without external components’ tolerance), RC-VCO is switched to continuous VDD-output-voltage with
open collector output stage if OTP-condition is detected at NTC input. Typical source current up to
VDD-5V is 1mA. For this function the anode of a diode can be connected to RC-VCO and the cathode
with the 5V input of an external Power-Good circuit. In the case of no OTP the oscillator works between
VSP and VSPH and so the diode will not conduct due to lower voltage at Anode compared to Cathode
(=5V-input of PowerGood circuit).
R
C
Fig. 4 OTP Application circuit
FanControl
Page 12 of 30
DS-FanControl-TDA21801