MIC502 datasheet(13/15 Pages) MICREL

13 / 15 page

Micrel, Inc.

MIC502

November 2006

M9999-112206

incorporated into the design then the operating voltage

(VDD = 5V vs. VDD = 12V) becomes a concern. The FanC

signal is derived from a 12V supply and is specified to

swing at least to 10.5V. A minimum implementation of

the FanC signal would provide the capability of asserting

full-speed operation of the fan; this is the case when

10.5V ≤ FanC ≤ 12V. This FanC signal can be applied

directly to the VT2 input of the MIC502, but only when its

VDD is 12V. If this signal is required when the MIC502

VDD = 5V a resistor divider is necessary to reduce this

input voltage so it does not exceed the MIC502 VDD

voltage. A good number is 4V (80%VDD).

Because of input leakage considerations, the impedance

of the resistive divider should be kept at ≤ 100kΩ. A

series resistor of 120kΩ driven by the FanC signal and a

100kΩ shunt resistor to ground make a good divider for

driving the VT2 input.

Transistor and Base-Drive Resistor Selection

The OUT motor-drive output, pin 7, is intended for

driving a medium-power device, such as an NPN

transistor. A rather ubiquitous transistor, the 2N2222A, is

capable of switching up to about 400mA. It is also

available as the PN2222A in a plastic TO-92 package.

Since 400mA is about the maximum current for most

popular computer power supply fans (with many drawing

substantially less current) and since the MIC502

provides a minimum of 10mA output current, the

PN2222A, with its minimum β of 40, is the chosen motor-

drive transistor.

The design consists solely of choosing the value RBASE in

Figures 7 and 8. To minimize on-chip power dissipation

in the MIC502, the value of RBASE should be determined

the

power

supply

voltage.

The

Electrical

Characteristics table specifies a minimum output current

of 10mA. However, different output voltage drops (VDD –

VOUT) exist for 5V vs.12V operation. The value RBASE

should be as high as possible for a given required

transistor base-drive current in order to reduce on-chip

power dissipation.

Referring to the “Typical Application” and to the

“Electrical Characteristics” table, the value for RBASE is

calculated as follows. For VDD = 5V systems, IOH of OUT

(pin 7) is guaranteed to be a minimum of 10mA with a

VOH of 2.4V.

RBASE then equals (2.4V – VBE) ÷ 10mA = 170Ω.

For VDD = 12V systems, RBASE = (3.4 – 0.7) ÷ 0.01 =

250Ω.

Overtemperature Fault Output

The /OTF output, pin 6, is an open-collector NPN output.

It is compatible with CMOS and TTL logic and is

intended for alerting a system about an overtemperature

condition or triggering a power supply crowbar circuit. If

VDD for the MIC502 is 5V the output should not be pulled

to a higher voltage. This output can sink up to 2mA and

remain compatible with the TTL logic-low level.

Timing Capacitors vs. PWM Frequency

The recommended CF (see first page) is 0.1µF for

operation at a PWM frequency of 30Hz. This frequency

is factory trimmed within ±3Hz using a 0.1% accurate

capacitor. If it is desired to operate at a different

frequency, the new value for CF is calculated as follows:

, where C is in µF and f is in Hz

The composition, voltage rating, ESR, etc., parameters

of the capacitor are not critical. However, if tight control

of frequency vs. temperature is an issue, the

temperature coefficient may become a consideration.

VT1

VS LP

GND

VDD

OUT

OTF

VT2

100k

56k

33k

RBASE

Overtemperature

Fault Output

MIC 5 0 2

NLX FanC

Signal Input

Yate Loon

YD80SM-12

or similar fan

0.1µF

180

100k

47k

Keystone Thermonics

RL2010-54.1K-138-D1

or similar

120k

12V

Figure 7. Typical 5V VDD Application Circuit

VT1

VS LP

GND

VDD

OUT

OT F

VT2

100k

56k

33k

12V

RBASE

Overtemperature

Fault Output

MIC502

NLX FanC

Signal Input

Yate Loon

YD80SM-12

or similar fan

0.1µF

280

4.7k

47k

Keystone Thermonics

RL2010-54.1K-138-D1

or similar

Figure 8. Typical 12V VDD Application Circuit

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