LTC1840

10

1840f

if the BLAST pin is high. If neither blasts nor an active serial

interface watchdog timer are desired, this pin should be

tied to ground. If timer operation is desired without having

a blast occur at power-up, the pin should be pulled above

1V after the part’s supply has ramped up. The blast state

is cleared by writing to the fault register.

Current Output DAC Interface to Switching Regulator

The output of a current DAC is used to control the output

voltage of a switching regulator that powers a fan, which

determines the rotational speed of the fan. The resistor

divider from the output of the regulator to the feedback pin

to ground should be ratioed to give the minimum desired

voltage from the fan, which corresponds to the minimum

fan speed. The size of the resistor from the output to the

feedback pin is then chosen by dividing the difference

between the maximum and minimum desired fan voltages

by the nominal maximum current output of the DAC, which

is 100

µA. The value of the resistor from the feedback pin

to ground is then derived from the divider ratio and the

resistor value just calculated.

For example, if the feedback pin of the regulator is at 1.25V

with respect to ground and the minimum desired fan

voltage is 5V, the top resistor in the divider should be

(5V – 1.25V)/1.25V = 3 times larger than the resistor from

the feedback node to ground. If the maximum desired fan

voltage is 12V, the top resistor value is then (12V – 5V)/

100

µA = 69.8k, and the bottom resistor is 69.8k/3 = 23.2k.

See Figure 1.

If the feedback pin voltage of a regulator is lower than the

1.1V compliance voltage of either of the LTC1840’s cur-

rent output DACs, the resistor from the regulator output to

the feedback pin can be divided into two resistors, giving

the DAC more room to operate. See Figure 2.

If more than one fan is controlled by one regulator output,

small differences in the actual rotational speeds of the fans

may result in audible beat frequencies, which can be very

annoying. To avoid this problem, the actual voltages

applied to the fans can be varied by adding resistors or

diodes in series with some of the fans, resulting in larger

differences between their rotational speeds and less

noticeable beating. See Figure 3.

GPIO Operation

The GPIO circuits feature N-channel MOSFET open drain

pull-downs that can drive LEDs and readback circuitry to

allow the logic states of the GPIO pins to be accessed

through the serial interface. The circuits that read the logic

states of the pins have standard CMOS thresholds. The

user must take care to minimize the power dissipation in

the pull-downs. LEDs should have series resistors added

to limit current and to limit the voltage drop across the

internal pull-down if their forward drop is less than about

sink 10mA at 0.7V drop to drive LEDs. A series resistor is

usually required to limit LED current and the LTC1840

internal power dissipation. See Table 3 for resistor values.

Table 3. Recommended LED Resistor Values

Recommended

LED Current (mA)

Series Resistor (

Ω)

11k

3k

3

270

910

5

120

510

10

30

240

Note: LED forward voltage drop assumed to be 2V.

FAULT Operation

Normally, the FAULT pin internal pull-down is only en-

abled if one of the fault bits in the fault register is high. But

it is also enabled if the part is shut down by the POR block

corresponding fault register bit.

BLAST and Serial Interface Watchdog Timer

Operation

The BLAST pin is used to force the DAC output currents to

full value instantaneously and also to gate the operation of

the serial interface watchdog timer. A blast will occur if the

BLAST pin is high when the part comes out of POR or if

there is a high to low transition on BLAST after POR. The

threshold of the BLAST pin is about 1V, independent of

a fault condition if the part has not been addressed via the

serial interface for about a minute and a half, is only active

OPERATIO