MCP4821 datasheet(19/50 Pages) MICROCHIP

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DS20002249B-page 19

MCP4802/4812/4822

4.1.3

POWER-ON RESET CIRCUIT

The internal Power-on Reset (POR) circuit monitors the

power

supply

voltage

(VDD) during the device

operation. The circuit also ensures that the DAC

powers up with high output impedance (<SHDN> = 0,

typically 500 k

. The devices will continue to have a

high-impedance output until a valid write command is

received and the LDAC pin meets the input low

threshold.

If the power supply voltage is less than the POR

threshold (VPOR = 2.0V, typical), the DACs will be held

in their Reset state. The DACs will remain in that state

until VDD > VPOR and a subsequent write command is

received.

Figure 4-3 shows a typical power supply transient

pulse and the duration required to cause a reset to

occur, as well as the relationship between the duration

and trip voltage. A 0.1 µF decoupling capacitor,

mounted as close as possible to the VDD pin, can

provide additional transient immunity.

FIGURE 4-3:

Typical Transient Response.

4.1.4

SHUTDOWN MODE

The user can shut down each DAC channel selectively

using a software command (<SHDN> = 0). During

Shutdown mode, most of the internal circuits in the

channel that was shut down are turned off for power

savings. The internal reference is not affected by the

shutdown command. The serial interface also remains

active, thus allowing a write command to bring the

device out of the Shutdown mode. There will be no

analog output at the channel that was shut down and

the VOUT pin is internally switched to a known resistive

load (500 k

 typical. Figure 4-4 shows the analog

output stage during the Shutdown mode.

The device will remain in Shutdown mode until the

<SHDN> bit = 1 is latched into the device. When a

DAC channel is changed from Shutdown to Active

mode, the output settling time takes < 10 µs, but

greater than the standard active mode settling time

(4.5 µs).

FIGURE 4-4:

Output Stage for Shutdown

Mode.

Transients above the curve

will cause a reset

Transients below the curve

will NOT cause a reset

Time

Transient Duration

VPOR

VDD - VPOR

TA = +25°C

VDD - VPOR (V)

500 k



Power-Down

Control Circuit

Resistive

Load

VOUT

Amp

Resistive String DAC

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