WM8738
Production Data
In a real application the designer is unlikely to have control of the relative power up sequence of
AVDD and DVDD. Using the POR circuit to monitor CAP ensures a reasonable delay between
applying power to the device and Device Ready.
Figure 3 and Figure 4 show typical power up scenarios in a real system. Both AVDD and DVDD must
be established and CAP must have reached the threshold Vporr before the device is ready and can be
written to. Any writes to the device before Device Ready will be ignored.
Figure 3 shows DVDD powering up before AVDD. Figure 4 shows AVDD powering up before DVDD.
In both cases, the time from applying power to Device Ready is dominated by the charge time of
CAP.
A 10uF cap is recommended for decoupling on CAP. The charge time for CAP will dominate the
time required for the device to become ready after power is applied. The time required for VMIDADC
to reach the threshold is a function of the CAP resistor string and the decoupling capacitor. The
Resistor string has a typical equivalent resistance of 50kΩ (+/-20%). Assuming a 10uF capacitor, the
time required for CAP to reach threshold of 1V is approx 110ms.
w
PD Rev 4.4 August 2006
11