PBL 386 15/1

13

Figure 12. Chart describing current in Vbat and Vbat2.

Figure 13. Chart describing Power dissipation with different Vbat2.

0

5

10

15

20

25

30

∞ Ω

10000

Ω

7500

Ω

5000

Ω

2500

Ω

1000

Ω

0

Ω

mA

0

100

200

300

400

500

600

700

800

28 V

25 V

22 V

∞ Ω

10000

Ω

7500

Ω

5000

Ω

2500

Ω

1000

Ω

0

Ω

mW

Battery Switch

To reduce short loop power dissipation, a

second battery voltage, Off-hook, must be

connected to the device via an external

diode at terminal VBAT2. The SLIC auto-

matically switches between the two battery

supply voltages without need for external

control. The silent battery switching to VBAT

occurs when the line current is below 5.5

mA. This means that the current in the On-

hook battery is limited to 6 mA. To calculate

the switching voltage use this formula (See

formula C in figure 17):

V

If metering is used see section Metering

Applications down below.

Connect the terminal VBAT2 to the

second power supply via the diode D

figure 15. A diode D

VB and the VB2 power supply, see figure

15, will make sure that the SLIC continues

to work on the second battery even if the

first battery voltage disappears.

The current commute between the differ-

ent batteries as shown in figure 12, note

that some current is sourced from VB (typ.

0.5 mA, internal bias current) when the line

current is sourced from VB2. The next chart

(figure 13) is showing what power dissipa-

tion the SLIC is using with different batter-

ies and variation of the line.

Metering Applications, TTX

It is very easy to use PBL 386 15/1 in

metering applications; simply connect a

suitable resistor (R

in series with a

capacitor (C

metering source. Capacitor C

all DC-voltages that may be superimposed

on the metering signal. The metering signal

gain can be calculated from the equation:

G

V

TR

=

V

TTX

Z

T

.

Z

LTTX

R

TTX

Z

α

RSN

where:

V

TTX

is the wanted metering voltage

between the TIP and RING terminals

Z

or 16 kHz metering signal,

G

i e 0.5.

It is possible to mix voice voltage and

metering voltage up to 2.5 Vpeak (1.7 Vrms),

using AOV. Use following formula to

calculate the switching voltage of the Battery

Switch to get enough signal space.

V

where:

V

V

TTX

is the the metering voltage in peak.