Datasheet LT1739 (Analog Devices) - 8

LT1739
U U W U APPLICATIO S I FOR ATIO Logic Controlled Operating Current
Linear Technology for assistance in implementing a single supply design with operating current control. These The DSP controller in a typical xDSL application can have modes can be useful for overall system power manage- I/O pins assigned to provide logic control of the LT1739 ment when full power transmissions are not necessary. line driver operating current. As shown in Figure 4 one or two logic control inputs can set two or four different
Shutdown and Recovery
operating modes. The logic inputs add or subtract current to the SHDN input to set the operating current. The one The ultimate power saving action on a completely idle port logic input example selects the supply current to be either is to fully shut down the line driver by pulling the SHDN pin full power, 10mA per amplifier or just 2mA per amplifier, to within 0.4V of the SHDNREF potential. As shown in which significantly reduces the driver power consumption Figure 5 complete shutdown occurs in less than 10µs and, while maintaining less than 2Ω output impedance to more importantly, complete recovery from the shut down frequencies less than 1MHz. This low power mode retains state to full operation occurs in less than 2µs. The biasing termination impedance at the amplifier outputs and the circuitry in the LT1739 reacts very quickly to bring the line driving back termination resistors. With this termina- amplifiers back to normal operation. tion, while a DSL port is not transmitting data, it can still sense a received signal from the line across the back- termination resistors and respond accordingly. VSHDN SHDNREF = 0V The two logic input control provides two intermediate (approximately 7mA per amplifier and 5mA per amplifier) operating levels between full power and termination AMPLIFIER modes. For proper operation of the current control cir- OUTPUT cuitry, it is necessary that the SHDNREF pin be biased at least 2V more positive than V–. In single supply applica- tions where V– is at ground potential, special attention to the DC bias of the SHDNREF pin is required. Contact 1794 F05
Figure 5. Shutdown and Recovery Timing Two Control Inputs
12V OR VLOGIC
RESISTOR VALUES (k
Ω
)
R
Power Dissipation and Heat Management R
V SHDN
SHDN TO VCC (12V) RSHDN TO VLOGIC
LOGIC RC1
VLOGIC 3V 3.3V 5V 3V 3.3V 5V
VC1 SHDN
RSHDN
40.2 43.2 60.4 4.99 6.81 19.6 R xDSL applications require the line driver to dissipate a 0V C0 V 2k
R
C0
C1
11.5 13.0 21.5 8.66 10.7 20.5 significant amount of power and heat compared to other
RCO
19.1 22.1 36.5 14.3 17.8 34.0
V V C1 C0 SUPPLY CURRENT PER AMPLIFIER (mA)
components in the system. The large peak to RMS varia-
H H
10 10 10 10 10 10
H L
7 7 7 7 7 7 SHDNREF tions of DMT and CAP ADSL signals require high supply
L H
5 5 5 5 5 5 voltages to prevent clipping, and the use of a step-up
L L
2 2 2 2 2 2 transformer to couple the signal to the telephone line can
One Control Input
12V OR VLOGIC require high peak current levels. These requirements
RESISTOR VALUES (k
Ω
)
result in the driver package having to dissipate significant
R
R
SHDN TO VCC (12V) RSHDN TO VLOGIC
V SHDN LOGIC RC
VLOGIC 3V 3.3V 5V 3V 3.3V 5V
0V VC SHDN amounts of power. Several multiport cards inserted into
RSHDN
40.2 43.2 60.4 4.99 6.81 19.6
R
2k
C
7.32 8.25 13.7 5.49 6.65 12.7 a rack in an enclosed central office box can add up to
VC SUPPLY CURRENT PER AMPLIFIER (mA)
many, many watts of power dissipation in an elevated
H
10 10 10 10 10 10
L
2 2 2 2 2 2 ambient temperature environment. The LT1739 has built- 1739 F04 SHDNREF in thermal shutdown circuitry that will protect the ampli- fiers if operated at excessive temperatures, however data
Figure 4. Providing Logic Input Control of Operating Current
transmissions will be seriously impaired. It is important in 1739fas, sn1739 8