MCP960X datasheet(43/57 Pages) MICROCHIP

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DS20005426G-page 43

MCP960X/L0X/RL0X

EQUATION 6-1:

EFFECT OF

SELF-HEATING

At room temperature (TA = +25°C) with IDD = 2.5 mA

(maximum) and VDD = 3.3V, the self-heating due to

power dissipation, T, is 0.32°C for the MQFN package.

6.2.2

CONVERSION TIME vs. SELF-HEAT

Once the ADC completes digitization, the processor

initiates the data computation routine for tCALC, which

also increases IDD. During the 18-bit ADC conversion

time (3 SPS, Samples per Second), the increased

current lasts for approximately 5% of the one-second

period. The effect of self-heat for the total power

consumed per second, including the 5% tCALC period, is

negligible. However, as the ADC resolution is reduced

from 18-bit to 16-bit, the power consuming tCALC period

increases to 20% per second. This change in resolution

adds approximately 0.04°C (typical) temperature error

due to self-heat. Table 6-1 provides an estimate for

self-heat for all resolutions using Equation 6-1.

In order to reduce the effects of self-heat for lower

resolution settings, the Burst mode feature is

recommended to manage the effects of self-heat.

6.2.3

USING BURST MODE TO MANAGE

SELF-HEAT

The Burst mode feature is useful to manage power

dissipation while maintaining the device sensitivity to

changes in temperature (see Section 5.2.3 “Device

Configuration Register”). While the device is in

Low-Power or Shutdown mode, the host controller exe-

cutes Burst mode to sample temperature. The number

of temperature samples and the measurement resolu-

tion settings are selected while executing the command.

While in Burst mode, if the temperature data exceeds

the alert limits, the device asserts the corresponding

alert output. The alert outputs are used so the host con-

troller does not need to continually poll the latest

temperature data and potentially increase the

temperature error.

In addition, with some applications monitoring several

hundred degrees of temperature changes, 18-bit

resolution may not be necessary. In this case, a fewer

number of burst samples reducing the resolution

enables the user to monitor fast transient temperatures

at the burst intervals. The 12-bit ADC resolution

provides approximately 3°C resolution (for Type K) and

a new sample of temperature data is computed at

approximately 20 ms intervals. Therefore, the number

of Burst mode Samples per Second can be selected to

manage the effects of self-heat using these estimates.

The temperature conversion status during Burst mode

can also be momentarily polled (using bit 7 of

bursts are completed. The host controller may

terminate an on-going burst by executing a shutdown

command or resetting the Burst mode by sending

another burst command.

6.2.4

ALERT OUTPUTS

The alert outputs are intended to drive high-impedance

loads. Typically, the outputs are connected to a

microcontroller input pin. However, if the outputs are

used to drive indicators, such as LEDs or buzzers, then

a buffer circuit is recommended in order to minimize the

effects of self-heat due to the applied load (see

Figure 6-3).

FIGURE 6-3:

Alert Output Buffer for LED

Indicator.

TABLE 6-1:

ADC RESOLUTION vs.

SELF-HEAT

Resolution

SPS

(typ.)

tCALC Duration

per Second

T

18-bit

0.0096°C

16-bit

20%

0.0384°C

14-bit

80%

0.1536°C

12-bit

240

100%

0.1920°C

Note:

VDD = 3.3V and IDD = 1.5 mA (typical).

T



JA VDD IDD





Where:

TJ = Junction Temperature

TA = Ambient Temperature

JA = Package Thermal Resistance:

- Junction to Ambient

JC = Package Thermal Resistance:

Junction to Case

T



JC VDD IDD





T

TJ TA

–

Alert Output

NPN

Active-High

VDD

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