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BD8158FVM Datasheet(PDF) 9 Page - Rohm |
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BD8158FVM Datasheet(HTML) 9 Page - Rohm |
9 / 18 page BD8152FVM, BD8158FVM Technical Note 9/17 www.rohm.com 2010.03 - Rev.B © 2010 ROHM Co., Ltd. All rights reserved. (4) Selecting the output rectification diode Schottky barrier diode is recommended as the rectification diode to use at the DC/DC converter output stage. Select the diode paying attention to the max. inductor current and max. output voltage. Max. Inductor current IINMAX + ∆IL < Rating current of diode Max. output voltage VOMAX < Rating voltage of diode Since each parameter has 30% to 40% of dispersion, be sure to design providing sufficient margins. (5) Design of the feedback resistor constant Refer to the following equation to set the feedback resistor. As the setting range, 10 kΩ to 330 kΩ is recommended. If the resistor is set to 10 kΩ or lower, it causes the reduction of power efficiency. If it is set to 330 kΩ or larger, the offset voltage becomes larger by the input bias current 0.4 µA (Typ.) in the internal error amp. Step-up (6) Setting the soft start time Soft start is required to prevent the coil current at the time of startup from increasing and the overshoot of the output voltage at the starting time. Fig.26 shows the relation between the capacitance and soft start time. Please refer to it to set the capacitance. As the capacitance, 0.001 µF to 0.1 µF is recommended. If the capacitance is set to 0.001 µF or lower, the overshooting may occur on the output voltage. If the capacitance is set to 0.1 µF or larger, the excessive back current flow may occur in the internal parasitic elements when the power is turned off and it may damage IC. When the capacitor to 0.1 µF or larger is used, be sure to insert a diode to VCC in series, or a bypass diode between the SS pin and VCC. Fig. 27 Bypass Diode Example When there is the startup relation (sequences) with other power supplies, be sure to use the high accuracy product (such as X5R). Soft start time may vary according to the input voltage, output voltage, loads, coils and output capacity. Be sure to verify the operation using the actual product. (7) Setting the ENB pin When the ENB pin is set to Hi, the internal circuit becomes active and the DC/DC converter starts operating. When it is set to Low, the shut down is activated and all circuits will be turned off. (8) Setting the frequency by FCLK It is possible to change the switching frequency by setting the FCLK pin to Hi or Low. When it is set to Low, the product operates at 600 kHz (Typ.). When it is set to Hi, the product operates at 1,200 kHz (Typ.). Vo = R8 + R9 1.245 [V] R9 + - Vo R8 R9 ERR Reference voltage 1.245 V FB 2 0.01 0.1 1 10 0.001 0.01 0.1 SS CAPACITANCE[uF] Fig. 25 Feedback Resistor Setting VCC Output pin Back current prevention diode Bypass diode Fig. 26 SS Pin Capacitance vs Delay Time |
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