Datasheet

MCP9600
DS20005426C-page 38  2015-2017 Microchip Technology Inc.
EQUATION 6-1: EFFECT OF
SELF-HEATING
At room temperature (T
A
= +25°C) with I
DD
= 2.5 mA
(maximum) and V
DD
= 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 t
CALC
, which
also increases I
DD
. During the 18-bit ADC conversion
time (3 SPS, Samples per Second), the increased cur-
rent lasts for approximately 5% of the one-second period.
The effect of self-heat for the total power consumed per
second, including the 5% t
CALC
period, is negligible.
However, as the ADC resolution is reduced from 18-bit to
16-bit, the power consuming t
CALC
period increases to
20% per second. This change in resolution adds approx-
imately 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 master controller
executes Burst mode to sample temperature. The
number of temperature samples and the measurement
resolution settings are selected while executing the
command. While in Burst mode, if the temperature data
exceeds the alert limits, the device asserts the corre-
sponding alert output. The alert outputs are used so the
master controller 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
Register 5-6) to detect whether the on-going sample
bursts are completed. The master 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 micro-
controller 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.
TABLE 6-1: ADC RESOLUTION vs.
SELF-HEAT
Resolution
SPS
(typ.)
t
CALC
Duration
per Second
T

18 bit 3 5% 0.0096°C
16 bit 15 20% 0.0384°C
14 bit 60 80% 0.1536°C
12 bit 240 100% 0.1920°C
Note: V
DD
= 3.3V and I
DD
= 1.5 mA (typical).
T


JA
V
DD
I
DD
 =
Where:
T
J
= Junction Temperature
T
A
= Ambient Temperature

JA
= Package Thermal Resistance:
- Junction to Ambient

JC
= Package Thermal Resistance:
Junction to Case
T


JC
V
DD
I
DD
 =
T

T
J
T
A
–=
Alert Output
NPN
Active-High
V
DD