mc145443dw Freescale Semiconductor, Inc, mc145443dw Datasheet - Page 8

• Manufacturer Part Number: mc145443dw
• Description: Single-chip 300-baud Modem
• Manufacturer: Freescale Semiconductor, Inc

CARRIER DETECT TIMING ADJUSTMENT
how long a received modem signal must be present above
the minimum threshold level before CD (Pin 3) goes low. The
C CDT capacitor also determines how long the CD pin stays
low after the received modem signal goes below the mini-
mum threshold. The CD pin is used to distinguish a strong
modem signal from random noise. The following equations
show the relationship between t CDL , the time in seconds re-
quired for CD to go low; t CDH , the time in seconds required
for CD to go high; and C CDT , the capacitor value in F.
Valid signal to CD response time: t CDL
Invalid signal to CD off time:
Example: t CDL
CARRIER DETECT THRESHOLD ADJUSTMENT
activate CD with a typical – 44 dBm (into 600
deactivate CD with a typical – 47 dBm signal applied to the
input of the hybrid circuit. The carrier detect threshold level
can be adjusted by applying an external voltage on CDA
(Pin 7). The following equations may be used to find the CDA
voltage required for a given threshold voltage. (V on and V off
are in Vrms.)
Example (Internally Set)
Example (Externally Set)
voltage of 1.2 V and an output impedance of 100 k . When
using the internal 1.2 V reference a 0.1 F capacitor should
be connected between this pin and V SS (see Figure 5).
TRANSMIT LEVEL ADJUSTMENT
value of resistor R TLA that is connected between TLA (Pin
MC145442 MC145443
8
The value of a capacitor, C CDT at CDT (Pin 4) determines
The carrier detect threshold is set by internal resistors to
V on = 4.9 mV
V off = 3.5 mV
V on = 7.7 mV
V off = 5.4 mV
The CDA pin has an approximate Thevenin equivalent
The power output at TxA (Pin 17) is determined by the
t CDH
APPLICATIONS INFORMATION
– 44 dBm: V CDA = 244
– 47 dBm: V CDA = 345
– 40 dBm: V CDA = 244
– 43 dBm: V CDA = 345
0.54
6.4
V CDA = 244
V CDA = 345
0.1 F
0.1 F
0.64 seconds
0.054 seconds
t CDH
V on
V off
6.4
0.54
3.5 mV = 1.2 V
5.4 mV = 1.9 V
4.9 mV = 1.2 V
7.7 mV = 1.9 V
C CDT
) signal and
C CDT
20) to V DD (Pin 6). Table 3 shows the R TLA values and the
corresponding power output for a 600
TxA is twice the value of that at ring and tip because TxA
feeds the signal through a 600
transformer (see Figure 7). When choosing resistor R TLA ,
keep in mind that – 9 dBm is the maximum output level al-
lowed from a modem onto the telephone line (in the U.S.). In
addition, keep in mind that maximizing the power output from
the modem optimizes the signal–to–noise ratio, improving
accurate data transmission.
THE LINE DRIVER
telephone line. Both the inverting and noninverting input to
the line driver are available for transmitting externally gener-
ated tones.
gives a fixed gain of 2 (R i = 50 k ). The average signal level
must be the same as V AG to maintain proper operation. This
pin should be connected to V AG if not used.
nect an external signal, such as a DTMF dialer, to the phone
line. When applying a signal to the DSI pin, the modulator
should be squelched by bringing SQT (Pin 14) to a logic high
level. DSI must be left open if not used.
er and allows adjustable gain with a series resistor R DSI (see
Figure 6). The voltage gain, A V , is determined by the
equation:
unity gain.
The line driver is a power amplifier used for driving the
Exl (Pin 18) is the noninverting input to the line driver and
The driver summing input (DSI, Pin 1) may be used to con-
In addition, the DSI pin is the inverting side of the line driv-
where R f
Example: A resistor value of 20 k
Output Transmit Level
(Typical into 600 )
Table 3. Transmit Level Adjust
20 k
A V = – (20 k /20 k ) = – 1
– 12 dBm
– 11 dBm
– 10 dBm
– 9 dBm
A V = –
R DSI
R f
resistor R Tx to a 600
for RDSI will provide
19.8 k
load. The voltage at
9.2 k
5.5 k
R TLA
MOTOROLA
line