MIC502

Micrel

MIC502

170

May 1999

Normal Operation

Normal operation consists of the PWM operating to control

the speed of the fan according to V

this otherwise indefinite behavior can be caused by any of

three conditions: V

condition; V

shutdown and reset; or both V

activating sleep mode. Each of these exceptions is treated as

follows:

Overtemp?

?

NORMAL

OPERATION

?

NO

OUT Duty Cycle

Proportional to

YES

Assert /OTF while

Reset?

?

POWER ON

SLEEP

YES

NO

YES

NO

Figure 4. Normal System Behavior

•

Overtemperature: If the system temperature rises

typically 7% above the 100% duty-cycle operating point,

/OTF will be activated to indicate an overtemperature

fault. (V

tially independent of other operations—the PWM

continues its normal behavior; with V

output duty-cycle will be 100%. If V

the overtemperature condition is cleared and /OTF is no

longer asserted. It is assumed that in most systems, the

/OTF output will initiate power supply shutdown.

•

Shutdown/Reset: If V

ate, asynchronous shutdown occurs. While in shutdown

mode, OUT is off (low), and /OTF is unconditionally

inactive (high/floating). If V

V

from a power-up condition. The state of /OTF is cleared,

a startup interval is triggered, and normal fan operation

begins.

•

Sleep: If V

sleep mode. All internal functions cease unless V

V

/OTF output is unconditionally inactive (high/floating)

and the PWM is disabled during sleep. (OUT will float.)

Sleep Mode

During normal operation, if V

device will go into sleep mode and fan operation will stop. The

MIC502 will exit sleep mode when V

by the hysteresis voltage, V

operation will resume. The resumption of normal operation

upon exiting sleep is indistinguishable from a power-on reset.

(See “Sleep: Normal Operation,” above.)

Disable PWM

Reset Released

?

SLEEP

Reset Initiated

?

YES

YES

Wake Up?

?

POWER ON

NO

NO

NO

YES

Figure 5. Sleep-Mode Behavior