WM8775SEDSSLASHR datasheet(13/39 Pages) WOLFSON

13 / 39 page

Production Data

WM8775

PD Rev 4.1, June 2006

Figure 10 Typical Power up Sequence where AVDD is Powered before DVDD

Typical POR Operation (typical values, not tested)

SYMBOL

MIN

TYP

MAX

UNIT

Vpora

0.5

0.7

1.0

Vporr

0.5

0.7

1.1

Vpora_off

1.0

1.4

2.0

Vpord_off

0.6

0.8

1.0

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 VMID ensures a reasonable delay between

applying power to the device and Device Ready.

Figure 9 and Figure 10 show typical power up scenarios in a real system. Both AVDD and DVDD

must be established and VMID 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 9 shows DVDD powering up before AVDD. Figure 10 shows AVDD powering up before

DVDD. In both cases, the time from applying power to Device Ready is dominated by the charge

time of VMID.

A 10uF cap is recommended for decoupling on VMID. The charge time for VMID will dominate the

time required for the device to become ready after power is applied. The time required for VMID to

reach the threshold is a function of the VMID resistor string and the decoupling capacitor. The

Resistor string has an typical equivalent resistance of 50k

Ω (+/-20%). Assuming a 10uF capacitor,

the time required for VMID to reach threshold of 1V is approx 110ms.

WM8775SEDS/R Fiches technique(PDF) 13 Page - Wolfson Microelectronics plc