916

I

(

BUS

mA)

3

914

910

912

4

5

6

7

V

(V)

STC

8

1

2

R

= 25

RID

V

RIDD

I

STC

= 25

V

RIDD

I

+ I

STC_use

IC1

TSS521

www.ti.com

SLAS980 – SEPTEMBER 2013

Slave Supply, 3.3 V

The TSS521 has an internal 3.3-V voltage regulator. The output power of this voltage regulator is supplied by the

storage capacitor CSTC at the STC pin. The storage capacitor CSTC at the STC pin is charged with constant

current ISTC_use from the current source CS1. The maximum capacitor voltage is limited to REF1. The charge

current ISTC has to be defined by an external resistor at pin RIDD.

The adjustment resistor RRIDD can be calculated using Equation 2.

(2)

Where,

ISTC = current from current source CS1

ISTC_use = charge current for support capacitor

ICI = internal current

VRIDD = voltage on pin RIDD

RRIDD = value of adjustment resistor

The voltage level of the storage capacitor CSTC is monitored with comparator TC1. Once the voltage VSTC

reaches VVDD_on, the switch SVDD connects the stabilized voltage VVDD to pin VDD. VDD is turned off if the

voltage VSTC drops below the VVDD_off level.

Voltage variations on the capacitor CSTC create bus current changes (see Figure 4).

Figure 4. Single Mode Bus Load

At a bus fault the shut down time of VDD (toff) in which data storage can be performed depends on the system

current IVDD and the value of capacitor CSTC. See Figure 5, which shows a correlation between the shutdown of

the bus voltage VBUS and VDD_off and toff for dimensioning the capacitor.

The output VS is meant for slave systems that are driven by the bus energy, as well as from a battery should the

bus line voltage fail. The switching of VS is synchronized with VDD and is controlled by the comparator TC1. An

external transistor at the output VS allows switching from the Meter-Bus remote supply to battery.

Copyright © 2013, Texas Instruments Incorporated

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