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LR745N8-G-P003 Datasheet(PDF) 5 Page - Microchip Technology |
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LR745N8-G-P003 Datasheet(HTML) 5 Page - Microchip Technology |
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5 / 15 page  2015 Microchip Technology Inc. DS20005394A-page 5 LR745 3.0 APPLICATION INFORMATION Figure 3-1 shows a simplified typical configuration of a switch mode power supply, SMPS, using LR745 in the start-up circuit. LR745’s VOUT terminal is connected to the VCC line of a PWM IC. An auxiliary winding on the transformer generates a VCC voltage to power the PWM IC after start-up. LR745 supplies power for the PWM IC only during start-up. After start-up, LR745 turns off and the auxiliary winding supplies power for the PWM IC. Figure 3-2 shows the typical current and voltage wave- forms at various stages from power-up to operation powered by the auxiliary winding. 3.1 Stage I Once a voltage is applied on VIN, LR745 starts to charge the VCC capacitor, C1. The VCC voltage starts 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, typically 16V. 3.2 Stage II Once VCC reaches 16V, the PWM IC is in its operating condition and will typically draw 20mA, depending on the operating frequency and size of the switching metal–oxide–semiconductor field-effect transistor (MOSFET). The output of LR745, 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, LR745 will turn off its output, thereby reducing its input current from 3.0mA to 10s of microamperes. At this point, all 20mA will be supplied by C1. The PWM IC can now operate to a min- imum VCC voltage, typically 10V. Once the switching MOSFET starts operating, the energy in the primary winding is transferred to the sec- ondary outputs and the auxiliary winding, thereby build- ing 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. 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 condi- tion. The PWM IC will now only draw 0.5mA. VCC will continue to decrease but at a much slower rate. Once VCC decreases below 7.0V, LR745 will turn the output, VOUT, back on. VOUT will start charging C1 as described in Stage I. 3.3 Stage III At this stage, LR745 output is turned off and the PWM IC is operating from the VAUX supply. The auxiliary volt- age, 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 LR745 (22V). FIGURE 3-1: SIMPLIFIED SMPS USING LR745 High Voltage V IN VOUT V CC I AUX V AUX C 1 C 2 D 2 I IN GND LR7 PWM IC |
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Similar Description - LR745N8-G-P003 |
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