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DS1780
OPERATION - Analog Output
The DS1780 has a single analog output from a unsigned 8-bit D/A which produces 0-1.25 volts; this is
amplified and scaled with external circuitry such as a op-amp and transistor to provide fan speed control.
This register is set to 0xFF on power-up, which produces full fan speed. The analog output register (19h)
is unaffected by any reset other than power-on.
This voltage must be scaled and have an output current of at least 250 mA which is needed to drive the
fans; Figure 6 is a simple circuit that can be used, and Table 9 suggest R1 and R2 to select gain.
Although it is recommended to connect the DS1780 analog output to a high impedance node such as that
in Figure 6, the output driver can source 2.0 mA (max) at V OUT = 1.25V while maintaining the error spec
of 5% of FSR over temperature and supply voltage. Stability is guaranteed for a load capacitance up to
100 pF. More capacitance could cause severe overshoots and possible oscillation.
FAN AMPLIFIER CIRCUIT EXAMPLE Figure 6
AMPLIFIER DESIGN EXAMPLES Table 9
Input
Output
Gain
R1
1,000
2,200
3,300
4,700
10,000
1.22
12
9.84
R2
9,000
19,439
29,159
41,530
88,361
OPERATION - NAND TREE Test
A NAND tree is provided in the DS1780 for Automated Test Equipment (ATE) board level connectivity
testing. If the user applies (V DD - 0.5V) to the NT IN input, the device will be in the NAND tree test mode.
A0/NT OUT will become the NAND tree output pin. To perform a NAND tree test all pins included in the
NAND tree (see Figure 7 below) should be driven high.
Beginning with A1 and working around the chip, each pin can be toggled and a resulting toggle can be
observed on A0/NT OUT .
Allow for a typical propagation delay of 100 ns.
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