ST AN2132 APPLICATION NOTE

AN2132

Obsolete Product(s) - Obsolete Product(s)

Application note

STLC3075 very low single supply SLIC

for WLL application in flyback configuration

Introduction

The STLC3075 is a SLIC device specially designed for WLL (Wireless Local Loop) and ISDN terminal adapters.

This document contains a description of the device functions in flyback configuration, and provides some application hints. The device data sheet is an essential complement to this application note, providing important reference information that will simplify understanding of the content.

19-Feb-2007 Rev 3 1/26

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Content AN2132

Content

1 Wireless local loop system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 TQFP 10 mm x 10 mm x 1.4 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1 VBAT voltage generat ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Operati on in off-hook condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3 VPOS characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.4 Start-up and DC-DC converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.5 Suggested transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.6 Input curr ent limitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.7 VPOS current capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.7.1 With USA REN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.7.2 With European REN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.8 RSENSE setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.9 Trapezoidal ringing signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.10 Ringer load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.10.1 With European REN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.10.2 With USA REN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.11 Efficiency and powe r dissipation in flyback configuration . . . . . . . . . . . . . 14

3.12 Micro interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.13 Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.14 Ring trip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.15 PCB precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.16 Ground configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.17 Capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.18 On-hook transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.19 Phone detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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3.20 ESD immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.21 Setting resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.22 Longitudinal balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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AN2132 Content

3.23 TTX filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.24 Gain settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.25 Complex impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4 Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Appendix A STLC3075 in application with VPOS > 12 V. . . . . . . . . . . . . . . . . . . 24

Appendix B STLC3075 for USB suspended current specification. . . . . . . . . . . 25

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Wireless local loop system AN2132

WLL

SLIC

WLL SLIC

1 Wireless local loop system

Figure 1. Wireless central office to premises diagram

WLL SLIC

☎

WLL SLIC

WLL

SLIC

☎

Central

office

Local loop

SLIC

Base station

transceiver

Final connection

by radio link

wireless

local loop

☎

The main characteristics of this device consist in the possibility to:

● operate with a single supply voltage in Fly-Back or Buck-Boost config urati on (see

AN2118 for information on Buck-Boost configuration)

● operate in Fly-Back configuration with a single supply voltage VPOS in a range from

+4.5 V to +12 V

● generate negative battery voltage

● generate a ring signal (trapezoidal wave form)

PC00335

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AN2132 Packaging

2 Packaging

STLC3075 is housed in standard TQFP package plastic with copper lead frame. No copper slugs protrude from the plastic body. STLC3075 uses the “standard” package option.

The thermal resistances, shown in junction and the ambient still air, and are calculated or measured in ° C/W.

Table 1. Thermal resistance versus package size

Symbol Parameter Value Unit

Rth j-amb

Thermal resistance junction ambie nt (Full plastic TQFP on single layer board)

Thermal resistance junction ambie nt (Full plastic TQFP on four layer board)

Table 1

and

2.1 TQFP 10 mm x 10 mm x 1.4 mm

Theta (j-a) on boards, in still air

Figure 2. Thermal resistance versus board structure

Figure 2

, are considered between the

70 ° C/W

45 ° C/W

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Application information AN2132

3 Application information

Figure 3. Typical application schematic

CVPOS

VPOS

GATE

RSENSE

VBAT

CLK

TIP

RING CSVR CREV

RTH RLIM IREF

RF1

RF2

CSVR

RTH

N-ch

SENSE

RSF

CSF

CVB

VF CZ

RREF

CLK

TIP

RING

CREV

RLIM

CONTROL

INTERFACE

TTX CLOCK

DET

D0 D1 D2 PD

CCOMP

CFL

VDD

RDD

RLV

ZAC

RRX

AGND

RS ZAC ZAC1

GAIN SET

DET D0 D1 D2 PD CKTTX

CTTX1

CTTX2 FTTX

RTTX

BGND

CAC

CVCC

STLC3075

ILTF

SYSTEM GND

SUGGESTED GROUND LAY-OUT

AGND

BGND

PGND

RTTX

CTTX

CAC

CRD

D04TL625A

3.1 VBAT voltage generation

When operated with a positive supply voltage VPOS and a correctly set clock signal (typically 125 kHz), the SLIC generates a VBAT voltage for the active and ring operations.

The VBAT voltage lev el, with a 10% spread, is defined by the voltage divider RF1 / RF2 and can be set by choosing an RF1 value from a recommended set of values (see

Table 2. VBAT voltage values (VPOS = 4.5V)

RF1 (KΩ) VBAT (Active mode) VBAT (Ring mode)

270 -46.1V -64.4V 285 -48.4V -67,8V 300 -51.9V -71.7V 315 -54.3V -75.2V

Table 2

330 -56.3V -78.2V

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AN2132 Application information

These values are referred to the device in active mode, on-hook condition (IL = 0mA) and in ring mode without load.

The VBAT value must be chosen taking into account the absolute maximum ratings of the device (VBTOT = 90 V). VBTOT = (VBAT + VPOS) = 90 V must not be exceeded.

When ring mode is selected through the control interface, the VBAT voltage is increased by an internal circuit from it’s active level to a predetermined value for ring mode. These two voltage levels (VBAT active and VBAT ring) are hence correlated. When one is set, (ring or active), the other is also set at the same time.

3.2 Operation in off-hook condition

A major feature of this device is that when changing from on-hook to off-hook conditions (IL >0 mA), the VBAT voltage is automatically adjusted depending on the loop resistance and on the programmed current limitation value (ILIM).

It should be noted that the device is optimized to operate on short loop applications (RLOOP ≤ 500 Ω) in order to obtain the co rrec t ri ng-trip detection.

In these conditions, with line current reaching the programmed constant current feed value (ILIM), the STLC3075 works like a current generator with a fixed DC current.

A fixed voltage drop, 4 V on TIP/GND and approximately 6 V on RING/VBAT, assures the DC functionality and the proper swing for the AC signal.

When the line is set off-hook, the STLC3075 automatically adjusts the generated battery voltage (VBAT) to feed the line with a fixed DC current (programmable via RLIM), and so optimizes power dissipation.

Considering maximum and minimum values for RLOOP ranging from 500 to100 Ω, and with fixed parameters ILIM = 25 mA and 2Rp = 100 Ω, the battery voltage (VBA T) will be equal to:

1. VBAT = 25 mA x (500+100) + 10 V = - 25 V

2. VBAT = 25 mA x (100+100) + 10 V = - 15 V A correctly set current threshold (typically 9 mA), programmable by external resistor RTH,

allows the correct on/off hook transition function. During the off-hook dynamic transition, the CAC capacitor is charged. The line current

regulator system senses the current flowing into RD and reduces the ILOOP current to the programmed ILIM value, set by RLIM.

The settling time of the ILIM current is about 150 ms, and it is a function of the CAC splitter capacitor (min . value allowed is 22µF).

3.3 VPOS characteristics

The input voltage VPOS can change slowly within the data sheet range (4.5 V - 12 V) without any effect on the VBAT voltage.

The STLC3075 can continue to operate correctly even if the VPOS voltage occasionally goes below 4.5 V (instantaneous value, not steady-state). The only limitation is the minimum voltage required on the external PMOS to keep it in a linear area.

Fast transients, ripples and spikes on the supply voltage V a reduced amplitude, depending upon the voltage supply rejection of the device.

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will appear on TIP/RING with

POS

Obsolete Product(s) - Obsolete Product(s)

Application information AN2132

Bench measurements on SVRR give -35 dB @ f = 50 Hz and -47 dB @ f = 4 kHz, using the test circuit configuration with the device in active mode, loaded with an RLOOP = 500 Ω, and ILIM = 25 mA.

3.4 Start-up and DC-DC converter

In order to prevent problems during start-up, an internal circuit turns-on the gate of the MOSFET only when VPOS reaches 4 V and turns it off for VPOS lower than 3 V.

For VPOS voltage higher than 4 V the DC/DC converter power-on is controlled by a soft start circuit embedded on the devices.

Figure 4. DC-DC converter circuit

VPOS

Low/high

duty cycle

comparator

125 kHz

clock

Maximum

duty cycle

comparator

(see figure VPOS current

capability circuit)

PWM

comparator

Logic

Ramp

gen.

VREF

Current

limiter

Switch

driver

CSF

RSF

RSENSE

VBAT

RF1

RF2

PC00337

The DC/DC converter works in flyback condition using a two step process.

● During the ON-time of the MOSFET, energy is taken from the input and stored in the

primary winding of the flyback transformer. On the secondary side, the diode is reverse biased, thus the load is being supplied by the energy stored in the output bulk capacitor.

● As soon as the power-mos turns off, the primary circuit is open and the energy stored in

the primary is transferred to the secondary by magnetic coupling. The diode is forward biased, and the stored energy is delivered to the output capacitor and then on the load.

The dots on the transformer must be in accordance with the voltage, so that during the ONtime of the MOSFET they indicate the positive side with respect to the other one of the transformer. During MOSFET OFF-time they indicate the negative.

The MOSFET must be chosen with the correct Vds voltage rating, considering also the voltage reflected back (Vr) to the primary through the turns ratio n.

The reflected voltage (Vr) must be added to the input voltage VPOS giving out a much higher voltage on the drain of the Mosfet (VBAT / n ) + VPO S.

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