AN45 SILABS | Alldatasheet
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of the dc-dc converter architectures. minimum required for any given mode of operation. Figure 1. Linefeed Power Diagram circuit and load circuit in basic blocks of circuitry. and is defined as 1 REN (ringing equivalence number).
2 Rev. 0.5 Considering the resistance of 26 gauge telephone wire, which is 0.45 Ω per feet, this equation becomes the following: Equation 1 The required VBAT is equal to VTR_PK plus VCMR, which is the voltage drop across the linefeed circuit. The VCMR voltage is set by the indirect Register 40 and recommended to be 1.5 V for most applications. Equation 2 The worst-case peak current for NREN load is when the load is connected with a short loop of negligible line resistance. This yields an average current equation: Equation 3 The total output power required during ringing is equal to the power consumed in the load plus the power consumed in the sensing resistors and the external transistors of the linefeed circuitry. This leakage current has a magnitude of 2.5 mA. Equation 4 In the Off-Hook State In a few special cases, the power consumed during off-hook is higher than the power consumed during ringing. It is important to check for this and design the power supply to handle the larger power requirement. Most designers can skip this section unless their designs support long line and/or the brief on-hook voltage measurement (for caller ID 2 and 2.5) with TRACK = 0 (bit 0 of the direct Register 66). The output current equations in the off-hook active state are as follows: Equation 5 where I LIM is the current limit set by Register 71, and IBJTBIAS is the bipolar biasing current set by the direct Register 65. There are two power equations for different track settings. For TRACK = 1, V BAT is allowed to track the line resistance to minimize power consumption. The power equation for this mode is as follows: Equation 6 where V CM is set by the direct Register 73, and V OV is set by Register 66. RLOOPMAX is the maximum total loop resistance (RLINE + Phone’s RDC + RS) where RS is the internal series resistance. For TRACK = 0, V BAT can ramp up quickly to support the brief on-hook voltage measurement feature, the power equation is as follows: Equation 7 where V BATL is set by the direct Register 75. If the off-hook power consumption is greater than the power during ringing, the dc-dc converter should be designed based on the off-hook current and off-hook V BAT. However, the requirement for the switching components (Q7 and Q8 or M1) should still be based on the VBAT value during ringing. Power Input Requirement The input power is equal to the output power plus the wasted power during the power conversion process. The efficiency of the Si321 x dc-dc converter is mainly dependent on the inductor loss (copper and magnetic loss) and the switching loss. For worst-case estimation, the efficiency is assumed to be 60% for the BJT/inductor solution (the actual efficiency is between 63% and 73%) and 75% for the MOSFET/transformer solution (the actual efficiency is between 75% and 83%.) Solving for I IN: Equation 8 The input voltage to the dc-dc converter could drop quickly (depending on the source impedance) as the VTR _PK VRINGrms 2× ×= VBAT VTR _PK VCMR+= IPK VTR _PK IAVG IPK π---×
2 NREN V TR _PK××
POUT VBAT IAVG 0.0025+()×= IBAT ILIM IBJTBIAS
0.6 V 80 I LIM IBJTBIAS+()+
++= POFFHOOK IBAT VCM VOV ILIM RLOOPMAX×++()= POFFHOOK IBAT VBATL×= PO PIN Power Efficiency× IIN VIN Power Efficiency××== IIN PO
and low-cost inductor magnetic material.
- Calculate I PK based on Equation 12 (assumed 60%
- Calculate the inductance, L, based on Equation 13
128 kHz to obtain the desired inductance value.
- Calculate the period, T, for Fs and the corresponding value
for direct Register 92 in hexadecimal.
- Calculate the maximum off-time and the corresponding
value for direct Register 93 in hexadecimal. from the DCFF pin of the Si321x to turn Q7 off faster. 3 mA from the base current during on-time. Table 1. Switching Transistor Q7
since the base current is not delivered to the load. transistor off quickly, which further decreases efficiency. PK. The overdrive factor of 1.3 is sufficient. control the base drive current. filtering in most applications. stability. See CFILT and RFILT in Figure 2. undervoltage and overcurrent mechanism. Figure 5. Protection Sense Circuitry
0.8 V DC
Rev. 0.5 7 R19 should be calculated with a 20% lower value in VDC to prevent premature low-voltage lock-out. If the voltage lock-out is activated too of ten or if the Si321x goes in and out of low-voltage lock-out and creates an oscillation-like condition at the input voltage, it indicates that the input power source has high impedance and should be replaced with a better power source. However, the values of R 18, R19, and R20 should be checked against the intended low-voltage lock-out before any conclusion is made about the input power source. Overcurrent Protection Overcurrent protection is implemented via the SDCL pin. (See Figure 5.) The circuit is designed to produce equal current flow from V DC to both the SDCL and SDCH pins with zero curren t flow through Q7. (R20 is set to be equal to R19 and the value of R18 is small.) When current flows through Q7, it generates a voltage drop across R18 and reduces the current flow into the SDCL pin. When the current flow into the SDCL pin is 10.5 µA lower compared to the current flow into the SDCH pin, it triggers the overcurrent protection, and the Si321x ends the current PWM cycle to prevent excessive current flow through Q7. The overload current should be set 20% above the maximum inductor current to prevent current shut down prematurely. Equation 21 A fuse or other power overload circuit should be placed between the V DC power supply and each input of the ProSLIC dc-dc converter circuit (one per ProSLIC solution) to protect the switching components (Q7 or M1) from potential electrical overstress in the event of a hardware fault condition. For more information concerning fuse selection, please contact Silicon Labs. Output Overvoltage Protection It is possible for the dc-dc converter to generate excessively high voltage beyond the voltage rating of external components. To prevent damage to these components, a transistor (Q9) is added to limit the VBAT to a desired level. Resistors R28 and R29 are connected between VCC and VBAT as a biasing circuit for the transistor, Q9. When VBAT approaches the predetermined voltage level set by R28 and R29, Q9 is turned on and takes current from R20 away from pin SDCL and, consequentially, triggers the Si321x to end its current PWM cycle. Q9 can be any NPN low voltage (12 V or higher) general-purpose transistor (2N2222 is recommended). The equations for R28 and R29 are as follows: where VBE = .55 V. where V CLAMP is the clamping voltage for VBAT. VCLAMP should be set to a voltage less than the voltage rating of the external components and higher than the maximum VBAT to be generated for a given application. Design Example Suppose that the system requ ires 5REN of loading on 1680 ft. of line length with a ringing signal 45 V RMS at the phone. The fast voltage measurement feature is not supported, and the system prefers optimization for power saving. The system has regulated 5 V as the main supply voltage for the Si321x and an unregulated 12 V dc with a .75 A current rating. Step 1: Define the Output Requirement Calculate V TR_PK from Equation 1: From Equation 2: From Equation 3: The output power equation becomes R18 10.5 µA 4.5 k Ω R19,20+ R28 VCC VBE+() R29 VCLAMP VTR _PK VRINGrms 2× ×= VTR _PK 45 2× × 76.5== VBAT VTR _PK VCMR+ 76.5 1.5+ 78 V== = IAVG IPK π---× 257 6 . 5×× 2.9 W=
8 Rev. 0.5 Step 2: Selecting Output Power Requirement Set up for power optimizati on in the active off-hook mode: From Equation 5: From track 1 Equation 6: Conclusion: The 2.9 W ringing power is the worst-case power requirement because the active off-hook power requirement is much lower (.46 W). The ringing power is used for the design of the dc-dc converter. Step 3: Define Input Requirement for the 12 V DC From Equation 8: Experiments with the unregulated 12 V source showed that the actual V DC voltage drops down to 10 V at input current equal to .36 A. The adjusted V DC and I IN is as follows: The total current drew on the 12 V unregulated supply is .48 A, which is well withi n the 12 V unregulated maximum specification of .75 A. Step 4: Selecting the Power Inductor Calculate the IPK using Equation 12: Calculate the L1 Inductance from Equation 13: (This frequency was selected to round up the inductor value to 100 µH.) From Equation 14: From Equation 15: Step 5: Selecting the Q7 Switching Transistor Transistor requirement: The Zetex FZT955 bipolar transistor meets all of the above requirements and its HFE gain at I PK = 1.14 A is 100. Let I R16 = 3 mA for adequate Q7 base capacitor discharge. From Equation 16: ILIM 20 mA Register 65()= IBJTBIAS 4 mA Register 66()= Track 1 Register 71()= VCM 3V R e g i s t e r7 3()= VOV 9 V Register 66()= IBAT ILIM IBJTBIAS 0.6 80 I LIM IBJTBIAS+()+() 5100⁄++= 24.5 mA= POFFHOOK IBAT VCM VOV ILIM RLINE×++()= IIN POUT IIN POUT VDC 10 V= IPK 2POUT VBAT VDC+() L 2POUT EFF IPK Fs 89.5 kHz= T 1 Period Register 92 11.2 µS Delay Register 93 tOFFMAX 21 15 H== VCEO VBAT VDC+ 78 10+ 88 V==> VEBO VCC 8V=> VCBO VBAT VCC VDC++ 78 5 10++ 93 V==> ICMAX IPK 1.14 A=> Speed : fT 100 MHz> R16 0.6
Figure 6. R16, R17, Q7, and Q8 are eliminated. The M1 directly drive the M1 MOSFET using the DCFF pinout.
1.3 I MAX×
Table 2. Component Voltage Rating
Figure 6. Transformer DC-DC Converter NP is the number of turn of the primary winding. NS is the number of turn of the secondary winding. the DCFF pin, logic level MOSFET must be used. Table 3 lists the requirement for the switch MOSFET.
12 Rev. 0.5 Document Change List Revision 0.4 to Revision 0.5 " Equation 6 ! Changed RLINE to RLOOPMAX. " “In the Off-Hook State” ! Updated text. " “Step 1: Define the Output Requirement” ! Added +160 to first equation. " “Step 2: Selecting Output Power Requirement” ! Added +160 to last equation. ! Changed =.3822 to .46 W. ! Changed .312 to .46 in last paragraph. " Added SLIC series resistance. " VCMR adjustment. " Changed Si3210 to Si321x throughout.
Rev. 0.5 13 Notes:
14 Rev. 0.5 Contact Information Silicon Laboratories Inc.
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