USB97C223 and USB97C243 Capacitor Selection for Internal Regulator Output Pins

SMSC AN 10.8

Page 3

Rev. 12-16-03

APPLICATION NOTE

Overview

The USB97C223 and USB97C243 have two internal 1.8V regulators used to provide the digital and analog 1.8V

supplies. The output pins of the regulator have very specific capacitor requirements that are described below.

It is important that the customer designs their product with the specifications set forth in this document.

Requirements

Table 1 lists the capacitor requirements on the regulator output pins.

Table 1 – Regulator Output Pins

USB97C223

PIN NUMBER

PIN NAME

PIN DESCRIPTION

CAPACITOR REQUIREMENT

98

VDDP

Analog Regulator

Output

10uF +/- 20 % with < 0.65 Ohm ESR

91

VDDCORE

Digital Regulator

Output

10uF +/- 20 % with < 0.65 Ohm ESR

USB97C243

50

VDDP

Analog Regulator

Output

10uF +/- 20 % with < 0.65 Ohm ESR

43

VDDCORE (closest

to VREG)

Digital Regulator

Output

10uF +/- 20 % with < 0.65 Ohm ESR

Figure 1 and Figure 2 show an example of the degradation in performance of the USB97C223 and USB97C243

as the ESR of the capacitor on the output of the regulators is increased to 0.65 Ohm.

Figure 1 - 10uF Capacitor with 0.1 Ohm ESR

Figure 2 - 10uF Capacitor with 0.65 Ohm ESR