ST3917A datasheet(4/16 Pages) STMICROELECTRONICS

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LED (Output, Pin 15)

When the MU/MFI pin is connected to either VDD

or GND, the LED connected to the LED pin, which

functions as a keyboard illumination or off-hook

indicator, will light up when the telephone is off-

hook.

When the MU/MFI pin is connected to any row pins,

the LED connected to LED pin functions as a tone

mode indicator.

From minimum operating line current up to 20mA,

ILN-ICC is sourced into the LED with a maximum

current limit of 18mA. For line current more than

20mA, this sourced current is limited at 18mA

(typical).

ILINE (Pin 16)

A recommended external resistor of 20

Ω is con-

nected between ILINE and GND. Changing this

resistor value will influence the microphone gain,

DTMF gain, sidetone, maximum output swing on

LN and the DC characteristics, especially in the low

voltage region.

LN (Pin 17)

LN is the positive line terminal of the device.

REG (Pin 18)

The internal voltage regulator has to be decoupled

by a capacitor from REG to GND. The DC charac-

teristics can be changed with an external resistor

connected between LN and REG or between REG

and ILINE.

GTX, MIC-, MIC+ (Pins 19, 20, 21)

The device has a symmetrical microphone inputs.

The amplification from microphone inputs to LN is

51dB at 15mA line current and it can be adjusted

between 43 and 51dB. The amplification is propor-

tional to the external resistor connected between

GTX and REG.

GDTMF (Pin 22)

When the DTMF input is enabled, the microphone

inputs and the receive amplifier input will be muted

and the dialing tone will be sent on the line. The

voltage amplification from GDTMF to LN is 40dB.

Final output level on the LN can be adjusted via the

external resistor connected between GDTMF and

GND through a decoupling capacitor. A confidence

tone is sent to the earphone during tone dialing.

The attenuation of the confidence tone from LN to

RXOUT is -32dB typically. The level of the confi-

dence tone in the earphone can be increased by

adjusting the resistor connected between GDTMF

and GRX pins, the possible range is 20dB.

VDD (Pin 23)

VDD is the positive supply for the dialing circuit and

it must meet the maximum and minimum voltage

requirements.

MU/MFI (Input, Pin 24)

A logic low input to this pin will disable the transmit

amplifier of the speech circuit. MUTE efficiency is

greater than 60dB. An open circuit to this pin will

enable the transmit amplifier. In this case, LED is

used for keypad lighting.

A connection to any row will disable the transmit

mute function and the LED connected to the LED

pin is used for tone mode indication.

Table 3 : Logic of MU/MFI Pin Indicator

MU/MFI

Transmit

Muting

LED at Pin 15

OPEN

Active

Lighted Dial Indicator

GND

Muted

Lighted Dial Indicator

Any Row

Not Available

Tone Mode Indicator

PIN DESCRIPTION (continued)

ST3917A - ST3917B

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