MC33121

MOTOROLA

7

PIN FUNCTION DESCRIPTION

Sb l

Pin

Di

i

Symbol

DIP

PLCC

Description

VCC

20

28

Connect to noise–free battery ground. Carries loop current and some bias currents.

EP

19

27

Connect to the emitter of the PNP pass transistor.

BP

18

26

Connect to the base of the PNP pass transistor.

CP

17

24

Connect to TIP through a current limiting protection resistor (RC). CP is the noninverting

input to the internal transmit amplifier (Figure 28). Input impedance is 31 k

Ω.

TSI

16

23

Sense input. Connect to TIP through a current limiting protection resistor (RS) which also

sets the longitudinal impedance. Input impedance is

[100 Ω to VCC.

VDD

15

22

Connect to a + 5.0 V,

± 10% supply, referenced to digital ground. Powers logic section and

provides some bias currents for the loop current drivers.

VDG

14

20

Digital Ground. Reference for ST1, ST2 and VDD. Connect to system digital ground.

ST1

13

18

Status Output (TTL/CMOS). Indicates hook switch status — high when on–hook, low when

off–hook, and pulse dialing information. Used with ST2 to indicate fault conditions.

ST2/PDI

12

17

Status output and an input (TTL/CMOS). As an output, ST2 can indicate hook status — Low

when on–hook, high when off–hook. Used with ST1 to indicate fault conditions. As an input,

it can be taken low (when off–hook) to deny subscriber loop current.

TXO

11

16

Transmit voltage output. Amplitude is

[1/3 that across CP and CN. Nominally capable of

800

µA output current. DC referenced to VAG.

RXI

10

14

Receive current input. Current at this pin is multiplied by 102 at EP and EN to generate loop

current. RXI is a virtual ground at VAG level. Current flow is out of this pin.

VAG

9

13

Analog ground, reference for TXO and RXI. Connect to system analog ground. Current flow

is into this pin.

RFO

8

12

A resistor from this pin and RXI sets the maximum loop current and DC feed resistance.

Minimum resistor value is 3.3 k (see Figures 5 to 7).

CF

7

10

A low leakage capacitor between this pin and VAG provides DC and AC signal separation. A

series resistor is required for battery supply turn–on/off transient protection (Figure 4).

VQB

6

8

Quiet Battery. A capacitor between VQB and VCC filters noise and ripple from VEE, providing

a quiet battery source for the speech amplifiers. A series resistor is required for battery

supply turn–on/off transient protection (Figure 4).

RSI

5

7

Sense input. Connect to RING through a current limiting protection resistor which also sets

the longitudinal impedance. Input impedance is

[100 Ω to VQB.

CN

4

6

Connect to RING through a current limiting protection resistor. CN is the inverting input to

the internal transmit amplifier (Figure 28). Input impedance is 31 k

Ω.

BN

3

4

Connect to the base of the NPN pass transistor.

EN

2

3

Connect to the emitter of the NPN pass transistor.

VEE

1

2

Connect to battery voltage (– 21.6 V to – 42 V).

(Pins 1, 5, 9, 11, 15, 19, 21, and 25 are not internally connected on the PLCC package.)