Rainbow Electronics AT42QT1040 - Manual (Page 7)

7
9524A–AT42–03/09
AT42QT1040
3.5 Power Supply
See Section 5.2 on page 13 for the power supply range. If the power supply fluctuates slowly
with temperature, the device tracks and compensates for these changes automatically with only
minor changes in sensitivity. If the supply voltage drifts or shifts quickly, the drift compensation
mechanism is not able to keep up, causing sensitivity anomalies or false detections.
The usual power supply considerations with QT parts apply to the device. The power should be
clean and come from a separate regulator if possible. However, this device is designed to
minimize the effects of unstable power, and except in extreme conditions should not require a
separate Low Dropout (LDO) regulator.
See under Figure 1.2 on page 3 for suggested regulator manufacturers.
It is assumed that a larger bypass capacitor (for example, 1 µF) is somewhere else in the power
circuit; for example, near the regulator.
To assist with transient regulator stability problems, the QT1040 waits 500 µs any time it wakes
up from a sleep state (that is, in Sleep mode) before acquiring, to allow Vdd to fully stabilize.
3.6 VQFN Package Restrictions
The central pad on the underside of the VQFN chip should be connected to ground. Do not run
any tracks underneath the body of the chip, only ground.
Figure 3-1 shows an example of
good/bad tracking.
Figure 3-1. Examples of Good and Bad Tracking
Caution: A regulator IC shared with other logic can result in erratic operation and is
not advised.
A single ceramic 0.1 µF bypass capacitor, with short traces, should be placed very
close to the power pins of the IC. Failure to do so can result in device oscillation,
high current consumption, erratic operation, and so on.
Example of GOOD tracking Example of BAD tracking