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DRV1101U Datasheet(PDF) 5 Page - Texas Instruments |
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DRV1101U Datasheet(HTML) 5 Page - Texas Instruments |
5 / 8 page 5 ® DRV1101 To calculate the amplifier requirements for a DSL applica- tion: 1. Determine the average power that must be delivered to the line. The amplifier must deliver twice this power to account for the power dissipated in the series impedance matching resistors. Therefore, add 3dB to the line power. This is the average power delivered at the output of the amplifier. For ADSL G.Lite (as of June 1998), the aver- age line power is 10dBm. Adding 3dB results in an average power at the amplifier output of 13dBm. 2. Next add the power crest factor needed for the line code used. The power crest factor for ADSL is 15dB which means that the peak power (PPEAK) needed at the ampli- fier output is 28dBm (13dBm +15dB). 28dBm is 631mW. 3. The DRV1101 peak output voltage is calculated by the formula: VPEAK = (PPEAK • RL)1/2 where RL is the load impedance that the DRV1101 must drive. For ADSL Lite, using the circuit shown in Figure 3, VPEAK = (PPEAK • RL)1/2 = (.631W x 17Ω)1/2 = 3.3V. The peak-to-peak voltage out of the DRV1101 is 2 x 3.3V = 6.6V. 4. The transformer turns ratio can be changed to keep the required output voltage and current within the range of the DRV1101. The line impedance (RLINE) is 100Ω for ADSL. The impedance that is reflected to the DRV1101 side of the transformer is RLINE/(turns ratio)2. For best power transfer, the total of the impedance matching resistors should equal the reflected impedance. Thus, for the circuit shown in Figure 3, the reflected impedance is 100 Ω/(3.4)2 = 8.6 Ω. With two impedance matching resistors of 4Ω each and about 0.5 Ω transformer resistance, the total load impedance is about (8.6 Ω + 4Ω + 4Ω + 0.5Ω) = 17Ω. FIGURE 3. Typical Digital Subscriber Line Application. OUTPUT PROTECTION Figure 3 also shows overvoltage and short circuit protection elements that are commonly included in DSL applications. Overvoltage suppressors include diodes or MOV’s. The outputs of the DRV1101 can be momentarily shorted to ground or to the supply without damage. The outputs are not, however, designed for a continuous short to ground or the supply. POWER DISSIPATION AND THERMAL ANALYSIS The total internal power dissipation of the DRV1101 is the sum of a fixed overhead power that is independent of the load plus the power dissipated internally to deliver the average load power. The total internal power dissipation determines the internal temperature rise when in operation. For DSL applications with high crest factors, such as ADSL, the average load power delivered is much lower than the peak power required. For practical purposes, this means that internal temperature rise is not an issue for the DRV1101 in high-crest factor DSL applications. With a +5V supply, the DRV1101’s typical fixed overhead current of 22mA (out of total no-load supply current of 29mA) creates a fixed overhead power dissipation of 110mW. The load dependent power dissipation of the DRV1101 when delivering an output voltage Vrms to a load RL is: P = (VDD – Vrms) • (Vrms/RL) The internal power dissipation will reach a maximum when Vrms is equal to VDD/2. For a sinusoidal output, this corresponds to an output Vp-p = 1.41 • VDD. As an example, compute the power and junction temperature under a worst case condition with VDD = +5V and Vrms = 2.5V into a 20 Ω differential load. The total internal power dissipation would be: (110mW) + (5V – 2.5V) • (2.5V/20 Ω) = 423mW In+ In– Out+ +5V DRV1101 GND Out– 4 Ω 4 Ω Protection Circuits Impedance Matching Resistors 1:3.3 Transformer Line Impedance 100 Ω Fixed Load Related |
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