MAX6620
Quad Linear Fan-Speed Controller
13
Maxim Integrated
ter’s bit D4 can be used to cause a fan failure to force
the remaining fan speeds to 100%.
Watchdog
The MAX6620 includes an optional I
2
C watchdog func-
tion that monitors the I
2
C bus for transactions. When the
watchdog function is enabled, all fans will be forced to
full speed if no I
2
C transactions occur within a selected
period (2s, 6s, or 10s).
Spin-Up
When a fan is not spinning, and a voltage less than the
nominal fan-supply voltage is applied to its power-
supply terminals, it may fail to start spinning. To over-
come this, the full nominal supply voltage may be
applied to the fan terminals for a short time before a
lower voltage is applied. This “spin-up” period allows
the fan to overcome inertia and begin operating. Spin-
up is controlled using the Fan_ Configuration registers.
Spin-up can be disabled, or it can cause the fan to be
driven with the full supply voltage until it produces two
tachometer pulses, up to a maximum of 0.5s, 1s, or 2s
when the fan is started.
POR Options
Three inputs allow set up of the MAX6620’s behavior at
power-up. These inputs are sampled when power is
first applied to the MAX6620:
• WD_START. Connect WD_START to V
CC
to enable,
or to ground to disable, the watchdog function. When
enabled using WD_START, the timeout period is 10s.
After power is applied, the watchdog function may be
enabled or disabled through the global configuration
register.
• SPINUP_START. At power-up, spin-up operation is
controlled by the SPINUP_START pin, which can be
connected to ground (spin-up disabled), V
CC
(spin-
up for a maximum of 1s), or unconnected (spin-up for
a maximum of 0.5s).
• DAC_START. This input controls the fan drive volt-
age (for all four fans) at power-up. When connected
to ground, the initial fan drive voltage will be 0V.
When connected to V
CC
, the initial fan drive voltage
will be full scale. When unconnected, the initial fan
drive voltage will be 75% of V
FAN
.
