AM79C02

Power-On Reset and Power Sequencing for the Am79C02/03/031 DSLAC™ Devices Application Note The purpose of this application note is to consider the behavior of the Dual Subscriber Line AudioProcessing Circuit (DSLAC™) device on a linecard during power-up. It is necessary to understand the behavior of the DSLAC device during power-up to ensure a successful linecard design. This application note discusses the following topics: s Power-on reset circuit operation s Warm start s Sequence for the application of signals and power rails The DSLAC Power-On Reset Circuit All DSLAC devices have an internal circuit for generating a power-on reset when power is first applied to the VCC rail. The Am79C02 device also has an external hardware reset pin, but this is not present on the Am79C03 and the Am79C031 devices. Power-On Reset Circuit Operation A simplified circuit diagram for the power-on reset of the DSLAC devices is shown in Figure 1. VCC T3 VCC VCC T2 C1 Figure 1. Simplified Power-On Reset Circuit The reset signal is designed to be Low at initial powerup and become High shortly after VCC reaches its normal operating voltage. Assume the VCC rail has been at 0 V for some time so that capacitors C1 and C2 are completely uncharged and then VCC is applied. It can be seen that initially the gate of the n-channel transistor T1 will still have 0 V applied to it (due to the uncharged state of C1) and will therefore be turned off. C2 is also uncharged, but since one plate of this capacitor...