MCP3551-ESLASHMS datasheet(15/30 Pages) MICROCHIP

15 / 30 page

DS21950B-page 15

MCP3551/3

4.5

Voltage Reference Input Pin

The MCP3551/3 devices accept a single-ended

external reference voltage, to be connected on the

VREF input pin. Internally, the reference voltage for the

ADC is a differential voltage with the non-inverting input

connected to the VREF pin and the inverting input

connected to the VSS pin. The value of the reference

voltage is VREF - VSS and the common mode of the

reference is always (VREF - VSS)/2.

The MCP3551/3 devices accept a single-ended

reference voltage from 0.1V to VDD. The converter

output noise is dominated by thermal noise that is

independent of the reference voltage. Therefore, the

output noise is not significantly improved by lowering

the reference voltage at the VREF input pin. However, a

reduced reference voltage will significantly improve the

INL

performance

since

the

INL

max

error

proportional to VREF

2 (see Figure 2-4).

The charge and discharge of the input capacitor create

dynamic input currents at the VREF input pin inversely

proportional to the sampling capacitor, which is a func-

tion of the input reference voltage. The typical value of

the single-ended input impedance is 2.4 M

Ω, with

VDD =VREF = 5V. The DC leakage current caused by

the ESD input diodes, though on the order of 1 nA

typically, can cause additional gain error proportional to

the source resistance at the VREF pin.

4.6

Power-On Reset (POR)

The MCP3551/3 devices contain an internal Power-On

Reset (POR) circuit that monitors power supply voltage

VDD during operation. This circuit ensures correct

device start-up at system power-up and power-down

events. The POR has built-in hysteresis and a timer to

give a high degree of immunity to potential ripple and

noise on the power supplies, as well as to allow proper

settling of the power supply during power-up. A 0.1 µF

decoupling capacitor should be mounted as close as

possible to the VDD pin, providing additional transient

immunity.

The threshold voltage is set at 2.2V, with a tolerance of

approximately ±5%. If the supply voltage falls below

this threshold, the MCP3551/3 devices will be held in a

reset condition or in Shutdown mode. When the part is

in Shutdown mode, the power consumption is less than

1 µA. The typical hysteresis value is around 200 mV in

order to prevent reset during brown-out or other

glitches on the power supply.

Once a power-up event has occurred, the device must

require additional time before a conversion can take

place. During this time, all internal analog circuitry must

settle before the first conversion can occur. An internal

timer counts 32 internal clock periods before the

internal oscillator can provide clock to the conversion

process. This allows all internal analog circuitry to

settle to their proper operating point. This timing is

typically less than 300 µs, which is negligible compared

to one conversion time (72.7 ms for the MCP3551).

Figure 4-4 illustrates the conditions for a power-up and

power-down event under typical start-up conditions.

FIGURE 4-4:

Power-On Reset Operation.

4.7

Shutdown Mode

When not internally converting, the two modes of

operation for the MCP3551/3 devices are

the

Shutdown and Sleep modes. During Shutdown mode,

all internal analog circuitry, including the POR, is turned

off and the device consumes less than 1 µA. When

exiting Shutdown mode, the device must require addi-

tional time before a conversion can take place. During

this time, all internal analog circuitry must settle before

the first conversion can occur. An internal timer counts

32 internal clock periods before the internal oscillator

can provide clock to the conversion process. This

allows all internal analog circuitry to settle to their

proper operating point. This timing is typically less than

300 µs, which is negligible compared to one conversion

time (72.7 ms for MCP3551).

4.8

Sleep Mode

During Sleep mode, the device is not converting and is

awaiting data retrieval; the internal analog circuitry is

still running and the device typically consumes 10 µA.

In order to restart a conversion while in Sleep mode,

toggling CS to a logic-high (placing the part in Shut-

down mode) and then back to a logic-low will restart the

conversion. Sleep can only be entered in Single

Conversion mode. Once a conversion is complete in

Single Conversion mode, the device automatically

enters Sleep mode.

VDD

2.2V

2.0V

Reset

Normal

Operation

Reset

Start-up

Time

300 µs

MCP3551-E/MS Datasheet(PDF) 15 Page - Microchip Technology

MCP3551-E/MS Datasheet(HTML) 15 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL