MIC502 Datasheet, PDF (11/11 Pages) Micrel Semiconductor – Fan Management IC Advance Information

MIC502
The design consists soley of choosing the value R_BASE in
Figures 7 and 8. To minimize on-chip power dissipation in the
MIC502, the value of R_BASE should be determined by the
power supply voltage. The Electrical Characteristics table
specifies a minimum output current of 10mA. However,
different output voltage drops (V_DD – V_OUT) exist for 5V vs.
12V operation. The value R_BASE 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 R_BASE is calculated as
follows. For V_DD = 5V systems, I_OH of OUT (pin 7) is
guaranteed to be a minimum of 10mA with a V_OH of 2.4V.
R_BASE then equals (2.4V – V_BE) ÷ 10mA = 170Ω.
For V_DD = 12V systems, R_BASE = (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
Micrel
alerting a system about an overtemperature condition or
triggering a power supply crowbar circuit. If V_DD for the
MIC502 is 5V the output should not be pulled to a higher
voltage. This output can sink up to 2mA and remain compat-
ible with the TTL logic-low level.
Timing Capacitors vs. PWM Frequency
The recommended C_F (see first page) is 0.1µF for opertaion
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
C_F is calculated as follows:
C
=
3
f
,
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.
Keystone Thermonics
RL2010-54.1K-138-D1
or similar
T1
R1
100k
R3 R2
56k 33k
R4
56k
C_F
0.1µF
NLX FanC
Signal Input
5V
12V
MIC502
1 _{VT1 VDD} 8
2 _CF
OUT ⁷
3 _{VSLP OTF} 6
4 _{GND VT2} 5
Yate Loon
YD80SM-12
47k
or similar fan
R_BASE
Q1
180Ω
Overtemperature
Fault Output
120k 100k
Figure 7. Typical 5V V_DD Application Circuit
Keystone Thermonics
RL2010-54.1K-138-D1
or similar
12V 5V
T1
R1
100k
R3 R2
56k 33k
R4
56k
C_F
0.1µF
MIC502
1 _{VT1 VDD} 8
2 _CF
OUT ⁷
3 _{VSLP OTF} 6
4 _{GND VT2} 5
47k
R_BASE
280Ω
Yate Loon
YD80SM-12
or similar fan
Q1
Overtemperature
Fault Output
4.7k
NLX FanC
Signal Input
Figure 8. Typical 12V V_DD Application Circuit
May 1999
173
MIC502