Philips Semiconductors

Product data

P82CF201

Low power, low price dual fan manager

2002 Nov 18

3

su01717

RC CONTROL

RESISTANCE

MEASUREMENT

FAULT

DETECTION

WATCHDOG

RESET

PWM 1 & 2

FAULT

FAN1DRIVE

FAN2DRIVE

Figure 1. Functional Block Diagram.

FUNCTIONAL DESCRIPTION

PWM Description

The PWM controls the fan speed by comparing the thermistor

resistance to a reference resistance. This ratio translates into the

duty cycle of the PWM.

The PWM controls the fan speed proportionally to the thermistor

resistance. The 30Hz PWM duty cycle has resistance control range

from 10K to 1.75 K

Ω (typical for NTC thermistor) for 30% to 98%

output duty cycle. The PWM pins drive a low cost PNP transistor to

give a good drive into the N-channel MOSFET as the low side

power switch element in the system. Example of drive circuits will be

shown Figure 4. This output has asymmetric complementary drive.

Since the system relies on PWM rather than linear power control,

the dissipation in the power switch is kept to a minimum. Generally,

very small devices (TO-92 or SOT package) will suffice.

500 mS Delay Turn-On Description

When temperatures rise above 25

°C and THRESHOLD is tied to

GND (NTC thermistor at 10K or less) for both fans, there is a turn-on

delay of 500 mS for the second fan. This reduces the inrush current

and suppresses acoustic noise.

Thermistor Measurement Description

A resistor can be measured using a comparator, which compares

the RC time of a known reference resistance with the RC time of an

unknown resistor value. Since the same capacitor and internal

voltage reference are used throughout, it results in a simple

calculation.

((RunknownChargingTime*Rreference)/RreferenceChargingTime)

The RC circuits are charged in sequence, until they reach an

internal voltage reference of 1.23 V.

The RC charging time is measured in the 256uSec - 65535uSec.

This range is divided up in 256 time increments. Values of R and C

have to be chosen so that it will be within that window.

The RC charging time of the reference resistor and thermistor are

measured. The thermistor value is calculated by comparing the RC

time of the Reference resistor and RC time of the thermistor.