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| LR745 High Input Voltage SMPS Start-up Circuit Ordering Information Maximum Input Voltage 450V Order Number / Package TO-92 LR745N3 TO-243AA* LR745N8 Product marking for TO-243AA: LR7 where = 2-week alpha date code *Same as SOT-89. Product supplied on 2000 piece carrier tape reels. Features 25V to 450V operating input voltage range Compatible with industry standard PWM ICs. See application notes AN-H28 and AN-H29. Output current limiting For PWM ICs with start-up threshold voltage of 13.9V to 18.8V Very low power consumption after start-up General Description The Supertex LR7 is a high input voltage SMPS start-up circuit. The LR7 is ideally suited for use with industry standard low voltage PWM ICs having start thresholds of 13.9V to 18.8V. It allows the PWM ICs to be operated from rectified 120V or 240VAC lines, and eliminates the use of power resistors often used for this purpose. The internal circuitry of the LR7 allows the PWM ICs to operate at a VCC voltage below their start threshold voltage after start-up. The auxiliary voltage can be less than the start threshold voltage, which allows for improved efficiency. Current from the high voltage line is drawn only during the startup period. After start-up, the internal high voltage line is disconnected from the IC thereby reducing the continuous power dissipation to a minimum. Applications Notebook and Laptop computers Telecommunication power supplies Battery chargers Motor controller Pin Configuration Absolute Maximum Ratings Input Voltage Output Voltage Operating and Storage Temperature Soldering Temperature* *Distance of 1.6mm from case for 10 seconds 450V 25V -55C to 150C 300C 1 2 3 TAB TO-243AA (SOT-89) 123 TO-92 VIN TO-92 TO-243AA 1 1 GND 2 2, TAB VOUT 3 3 For detailed circuit and application information, please refer to application notes AN-H28 and AN-H29. 11/12/01 Supertex Inc. does not recommend the use of its products in life support applications and will not knowingly sell its products for use in such applications unless it receives an adequate "products liability indemnification insurance agreement." Supertex does not assume responsibility for use of devices described and limits its liability to the replacement of devices determined to be defective due to workmanship. No responsibility is assumed for possible omissions or inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications, refer to the Supertex website: http://www.supertex.com. For complete liability information on all Supertex products, refer to the most current databook or to the Legal/Disclaimer page on the Supertex website. 1 LR745 Electrical Characteristics Test conditions unless otherwise specified: TA = 25C; VIN = 450V Symbol VOUT IOUT VIN IINQ VOFF VRESET IOFF VAUX IAUX Output Voltage VOUT over Temperature Output Current Limiting Operating Input Voltage Range Input Quiescent Current Output Turn OFF Voltage VOFF Over Temperature Output Reset Voltage VRESET Over Temperature VIN Off-State Leakage Current External Voltage Applied to VOUT Input Current to VOUT 12.6 12.3 6.3 6 13.25 13.25 7 7 Parameter Min 18.8 18.5 2 25 3 Typ Max 24 24.3 4 450 500 13.9 14.2 7.7 8 75 22 500 Unit V V mA V A V V V V A V A VAUX = 22V TA = -40C to +85C VIN = 400V TA = -40C to +85C VIN = 400V, IOUT = 0 IOUT = 0 IOUT = 0, TA = -40C to +85C Conditions Block Diagram VIN R4 M1 + M2 23V - 2 to 4 mA VOUT VREF R1 Reset + comp1 - R2 Vz VOUT R3 QR D Clock Clk GND + comp2 - 2 LR745 Block Diagram Detailed Description The Supertex LR7 is a high voltage switch mode power supply start-up circuit, which has 3 terminals: VIN, GND, and VOUT. An input voltage range of 25VDC to 450VDC can be applied directly at the input VIN pin. The output voltage, VOUT, is monitored by the 2 comparators, comp1 and comp2. An internal reference, VREF, and resistor divider R1, R2, and R3 set the nominal VOUT trip points of 7.0V for comp1 and 13.25V for comp2. When a voltage is applied on VIN, VOUT will start to ramp up from 0V. When VOUT is less than 7.0V, the output of comp1 will be at a logic high state keeping the D flip flop in a reset state. The output of the D flip flop, Q, will be at logic low keeping transistor M2 off. The data input for the D flip flop, D, is internally connected to a logic high. As VOUT becomes greater than 7.0V, comp1 will change to a logic low state. VOUT will continue to increase, and the constant current source of typically 3mA output will charge an external storage capacitor. As VOUT reaches above 13.25V, the output of comp2, will then switch from a logic high to a logic low state. The D flip flop's output does not change state since its clock input is designed to trigger only on a rising edge, logic low to logic high transition. When there is no load connected to the output, the output voltage will continue to increase until it reaches 21.5V which is the zener voltage minus the threshold voltage of transistor M1. The zener voltage is typically 23V and the threshold voltage of M1 is typically 1.5V. The zener diode is biased by resistor R4. VOUT will start to decrease when it is connected to an external load greater than the internal constant current source, which is the case when the PWM IC starts up. When VOUT falls below 13.25V, the output of comp2 will switch from a logic low to a logic high. The output of comp2 will clock in a logic 1 into the D flip flop causing the D flip flop's output, Q, to switch from a logic low to a logic high. Transistor M2 will then be turned on pulling the gate of transistor M1 to ground thereby turning transistor M1 off. Transistor M1 will remain off as long as VOUT is greater than 7.0V. Once VOUT decreases below 7.0V, comp1 will reset the D flip flop, thereby turning transistor M2 off and transistor M1 back on. Typical Application Figure 1 shows a simplified typical configuration of a switch mode power supply, SMPS, using the Supertex LR7 in the startup circuit. The LR7's VOUT terminal is connected to the VCC line of a PWM IC, Unitrode part #UC3844. An auxiliary winding on the transformer is used to generate a VCC voltage to power the PWM IC after start-up. The LR7 is used to supply power for the PWM IC only during start-up. After start-up, the LR7 turns off and the auxiliary winding is used to supply power for the PWM IC. Figure 2 shows the typical current and voltage waveforms at various stages from power up to operation powered by the auxiliary winding. Stage I Once a voltage is applied on VIN, the LR7 will start to charge the VCC capacitor, C1. The VCC voltage will start to increase at a rate limited by the internal current limiter of 3.0mA. The PWM IC is in its start-up condition and will typically draw 0.5mA from the VCC line. The VCC voltage will continue to increase until it reaches the PWM IC's start threshold voltage of typically 16V. Stage II Once VCC reaches 16V, the PWM IC is in its operating condition and will draw typically 20mA depending on the operating frequency and size of the switching MOSFET. The output of LR7, VOUT, is internally current limited to 3.0mA. The remaining 17mA will be supplied by C1 causing the VCC voltage decrease. When VCC decreases to 13.25V, the LR7 will turn off its output thereby reducing its input current from 3.0mA to 10's of microamperes. At this point, all 20mA will be supplied by C1. The PWM IC can now operate to a minimum VCC voltage of typically 10V. Once the switching MOSFET starts operating, the energy in the primary winding is transferred to the secondary outputs and the auxiliary winding, thereby building up VAUX. It is necessary to size the VCC storage capacitor, C1, such that VAUX increases to a voltage greater than 10V before VCC decreases to 10V. This allows VAUX to supply the required operating current for the PWM IC. (Continued on page 14-9) VIN IIN High Voltage IAUX VAUX D2 C2 LR7 GND VOUT VCC PWM IC UC3844 C1 Figure 1: Simplified SMPS using LR7 3 LR745 LR7 Start-up Waveforms Stage I 16.0 VOUT 13.5 (Volts) 12.0 8.0 4.0 0.0 t Stage II Stage III PWM IC Start Threshold Voltage LR7 VOFF Trip Point Auxiliary Supply Powers PMW IC 3.0 IIN (mA) 2.0 1.0 IIN 0mA t 0.0 12.0 VAUX (Volts) 8.0 4.0 VAUX = 12V t 0.0 30.0 IAUX = 20mA 20.0 IAUX (mA) 10.0 t 0.0 Figure 2 4 LR745 (Continued from page 14-7) If for some reason the auxiliary voltage does not reach 10V, VCC will continue to decrease. Once VCC goes below 10V, the PWM IC will return to its start-up condition. The PWM IC will now only draw 0.5mA. VCC will continue to decrease but at a much slower rate. Once VCC decrease below 7.0V, the LR7 will turn the output, VOUT, back on. VOUT will start charging C1 as described in Stage I. Stage III At this stage the LR7's output is turned off and the PWM IC is operating from the VAUX supply. The auxiliary voltage, VAUX, can be designed to vary anywhere between the minimum operating VCC voltage of the PWM IC (10V) to the maximum auxiliary voltage rating of the LR7 (22V). Consider for example, a PWM IC with a switching frequency of 100KHz, operating current of 20mA, start threshold of 16V, and a minimum operating voltage of 10V. If 100 clock cycles are required to charge the auxiliary voltage to 10V, the minimum value of C1 is calculated as follows: 1 x (100) x (20mA ) 100KHZ C1= (16V -10V) C1= 3.3F II. SMPS with wide minimum to maximum load An important point is that the LR7's output voltage, VOUT, must discharge to below the nominal VOFF trip point of 13.25V in order for its output to turn off. If the SMPS requires a wide minimum to maximum output load variation, it will be difficult to guarantee that VCC will fall below 13.25V under minimum load conditions. Consider an SMPS that is required to power small as well as large loads and is also required to power up quickly. Such as SMPS may power up too fast with a small load, not allowing the VCC voltage to fall below 13.25V. For such conditions, the circuit in Figure 3 is recommended. In Figure 3, the VREF pin of the UC3844 is used to bias the ground pin of the LR7. The VREF pin on the UC3844 is a 5.0V reference, which stays at 0V until the VCC voltage reaches the start threshold voltage. Once VCC reaches the start threshold voltage, VREF will switch digitally from 0V to 5.0V. During start-up, the LR7 will be on and VCC will start to increase up to 16V. Once VCC reaches 16V, the UC3844 will start to operate and VREF will increase from 0V to 5.0V. The LR7 will see an effective VOUT voltage of 11V (16V minus 5.0V) because the ground of the LR7 is now at 5.0V. The LR7 will immediately turn off its output VOUT without having to wait for the VCC voltage to decrease. The VREF switching from 0 to 5V during start is a common feature in most PWM ICs. Design Considerations I. Calculating the value for C1 Sizing the VCC capacitor, C1, is an important factor. Making C1 too large will cause the SMPS to power up too slowly. However, if too small, C1 will not allow the SMPS to power up due to insufficient charge in the capacitor to power the IC and MOSFET until the auxiliary supply is available. The value of C1 can be approximately by the following equation: 1 x (N) x (l) f C1 = (VSTART - VMIN ) where, f N I VSTART VMIN = = = = = switching frequency number of clock cycles required to charge VAUX to VMIN value PWM operating current PWM IC start threshold rating PWM IC minimum VCC operating voltage VIN LR7 GND VOUT VCC PWM IC UC3844 VREF C1 Figure 3: Using VREF for the LR7 Ground Voltage 11/12/01 (c)2001 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited. 5 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 * FAX: (408) 222-4895 www.supertex.com |
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