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AN47 S i321 X L I N E F E E D P O W E R M O N I T O R I N G A N D P R O T E C T I O N Introduction The Silicon Laboratories’ ProSLIC products are designed to continuously monitor the power dissipated in each of the six external bipolar transistors in the linefeed circuit. These power measurement results are available to the user in software registers and are also used by the ProSLIC to protect linefeed transistors from damage due to overpower conditions. Using proper power threshold and thermal low-pass filter settings, the ProSLIC will either alert the user or automatically transition the open state in the event of an overpower condition. The thermal resistance (RTHJA) of the transistor is improved when it is mounted on a PCB board. This improvement depends on the PCB size, the material it is made of, and the amount of the copper surface on the PCB board. Figure 1 illustrates how the board material, available board area, and the amount of copper present influence the thermal resistance of the transistors. This chart can be obtained from the transistor manufacturer if not included in the transistor data sheet. As the dissipated power in linefeed transistors increases, so does the junction temperature of the transistor die. The maximum admissible junction temperature must not be exceeded because this could damage or destroy the transistor die. In the Si321x, the measured power consumed in each of the transistors is compared to the power threshold values in the corresponding indirect registers. If the power in any external transistor exceeds the programmed threshold (after passing through a user-programmable low pass filter which will be explained in the next section), a power alarm is triggered to indicate line fault condition. Unless the auto-open feature is disabled (direct register 67, bit 0), the ProSLIC automatically goes into the open state. The value of the power threshold is calculated based on the characteristic of the transistors used. Transistor manufacturers provide this information in terms of thermal resistance for each transistor package. The relationship between the maximum junction temperature and the maximum power that can be dissipated by the transistor package is defined in the following equation: T JMAX = T AMB + P MAX × R THJA Thermal Resistance (°C/W) Power Threshold 450 300 FR4 Min. Copper SRBP Min. Copper SRBP Max. Copper 150 FR4 Max. Copper 0 0.01 0.1 1 10 P.C.B. Area (Sq.Ins.) Thermal Resistance v P.C.B. Area Figure 1. SOT23 In practice, the transistors are normally mounted on a PCB with several square inches area, but for illustration purposes consider a model in which the transistor package is mounted on 1-inch square of FR4 PCB with 0.25-inch square of copper surface. This 1-inch square PCB model and the thermal resistance vs. PCB area charts provide the practical thermal resistances for the following transistor packages: SOT23: RTHJA = 200 °C/W The thermal resistance can also be obtained from the transient thermal resistance curve with D = 1 as shown in Figure 2 and Figure 3. SOT89: RTHJA = 82.5 °C/W SOT223: RTHJA = 62.5 °C/W where TJMAX is the maximum junction temperature (usually 150 °C), TAMB is the ambient temperature (70 °C for commercial rating), and PMAX is the maximum power allowance on the transistor package. RTHJA is the junction to ambient thermal resistance of the transistor package. Rev. 0.2 9/02 Copyright © 2002 by Silicon Laboratories AN47-DS02 A N47 Transient thermal resistance for a Zetex 1.5W SOT89 device mounted on a 15 mm x 15 mm ceramic substrate From equation 2: 7 1.28 PPT12 or PPT56 =  -----------------  × 2 = 5389 ( d ) = 0x150D  0.0304 7 1.28 PPT34 =  -----------------  × 2 = 0xB0CB| limited = 0x6E7E  0.0362 Because the range of the power threshold is from 0 to 0x7FFF, using 0x6E7E is recommended. While the maximum power threshold for these transistors falls outside the range of Indirect Register 33, 0x6E7E is a conservative estimate of a threshold above the maximum power Q3 and Q4 would be dissipating in a normal application. Thermal Low Pass Filter Figure 2. SOT89 While the power threshold coefficient sets the absolute maximum dc power that the transistor can handle for an indefinite period of time, it only provides a static maximum dc trip point. In the Si321x circuit application, the transistors are subjected to complex power dissipation, which is comprised of dc biasing current and ac signaling. The ac part of the power dissipation may be limited to short times and with repeated pulse (ringing). A static maximum power threshold setting does not provide an adequate model for real operating conditions. In conjunction with the power threshold setting, the Si321x also provides the thermal low pass filter setting which models the operating condition more accurately. Calculation of the thermal low pass filter is based on the characteristic of the transistor package. The heating process of the transistor package is an exponential phenomenon which can be described by the following equation: Transient thermal resistance for a Zetex 2W SOT223 device mounted on a 1 “ x 1 ” p.c.b. Figure 3. SOT223 The equation to calculate the power threshold value for the transistors pair is as follows: P MAX 7 PPTxx =  ------------------------------  × 2 ( decimal )  Resolution T ( t ) = T DC ( 1 – e –t ⁄ τ ) Where TDC is the final temperature and, τ is the thermal time constant. Thermal resistance (θ) may replace the temperature (T) in this equation since they both represent the temperature of the transistor package. θ ( t ) = θ DC ( 1 – e –t ⁄ τ Where Resolution = 30.4 mW for Q1, Q2 and Q5, Q6 pairs = 3.62 mW for Q3, Q4 pair The following calculations are examples of the power threshold coefficient for the SOT223 package: From equation 1: ( T JMAX – T AMB ) ( 150 ° – 70 ° ) P MAX = ------------------------------------------ = -------------------------------- = 1.28W 62.5 ° C ⁄ W R THJA ) Equation 1 Where θDC is the dc thermal resistance. The Si321x implements this transfer function by allowing the setting of the thermal constants (τ) to the registers. Figure 4 shows the heating and cooling of the transistor package. Power is applied to the transistor and heats it up during t1. It is allowed to cool during t2. 2 Rev. 0.2 A N47 P P MAX t1 T T MAX t2 t 2 = 4 t1 Figure 4. Transistor Package Heating and Cooling The θEFF is defined as the thermal resistance of the transistor package at t = τ (one time constant). θ EFF = θ ( τ ) = θ DC ( 1 – e –τ ⁄ τ ) = θ DC ( 1 – e ) = 0.63 θ DC –1 The thermal period (tP) can then can be found on the Transient Thermal Resistance graph using the θEFF value and the D = .2 curve. This estimation process is graphically illustrated in Figure 5. Below is the calculation example of the power threshold coefficient for the SOT223 package. From Figure 2: θDC = 62.5 (D = 1 line) From Equation 2: θEFF = .63θDC = 39.4 Using θEFF = 39.4 to find the thermal period (tP) in Figure 3, using D = .2, curve: tP = 15 s τ = 0.2t P = 3 s Equation 2 The cooling process of the transistor is also an exponential process which can be described by the following equation: θ ( t ) = θ DC × e –t ⁄ τ When t = 4τ the θDC (initial condition) is decayed to almost zero. θ ( 4 τ ) = θ DC × e –4τ ⁄ τ = 0.18 θ DC Equation 3 The thermal time constant (τ) can be estimated by calculating the θEFF with Equation 2 and the θDC data from the Transient Thermal Resistance curves. The equation for calculating the thermal LPF register is given in the Si321x data sheet: 4096 3 Thermal LPF register =  ------------  × 2  800 τ 3 4096 =  -----------------  × 2 = 13.7  800 ( 3 ) = 0x0E (hex) Rev. 0.2 3 A N47 θ J-A θ DC D=1 θ EFF ( 63% of θ DC ) D = .2 tP t Figure 5. Thermal Time Constant (τ) Estimation Table 1. Power Coefficients for Some Transistor Packages Indirect Register 32 (Q1/Q2 Power Threshold) 33 (Q3/Q4 Power Threshold) 34 (Q5/Q6 Power Threshold) 37 (Q1/Q2 Power LPF) 38 (Q3/Q4 Power LPF) 39 (Q5/Q6 Power LPF) SOT23 0x0700 0x373F — 0x008C 0x008C — SOT89 0x0FF4 0x6E7E* 0x0FF4 0x0012 0x0012 0x0012 SOT223 0x150D 0x6E7E* 0x150D 0x000E 0x000E 0x000E *Note: While the maximum power threshold for these transistors falls outside the range of Indirect Register 33, 0x6E7E is a conservative estimate of a threshold above the maximum power Q3 and Q4 would be dissipating in a normal application. Power Dissipation in the Si3201 The Si3201 is a line-side IC that replaces the discrete transistors in the Si321x schematic. Because the Si3201 circuitry differs from that of the discrete components, it is difficult to compute a maximum power threshold per transistor. Silicon Laboratories recommends SOT89 register settings (as shown in Table 1) when using the Si3201 linefeed IC. 4 Rev. 0.2 A N47 Document Change List Revision 0.1 to Revision 0.2 Changed power threshold values for SOT89/223 transistors Corrected calculations Added information for the Si3201 Rev. 0.2 5 A N47 Contact Information Silicon Laboratories Inc. 4635 Boston Lane Austin, TX 78735 Tel: 1+(512) 416-8500 Fax: 1+(512) 416-9669 Toll Free: 1+(877) 444-3032 Email: productinfo@silabs.com Internet: www.silabs.com The information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice. Silicon Laboratories assumes no responsibility for errors and omissions, and disclaims responsibility for any consequences resulting from the use of information included herein. Additionally, Silicon Laboratories assumes no responsibility for the functioning of undescribed features or parameters. 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Silicon Laboratories and Silicon Labs are trademarks of Silicon Laboratories Inc. Other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders. 6 Rev. 0.2