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TPS51123 Datasheet(PDF) 24 Page - Texas Instruments |
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TPS51123 Datasheet(HTML) 24 Page - Texas Instruments |
24 / 37 page ( ) ( ) ( ) ( ) ( ) f OUT RIPPLE V 20 mV 1 D 20 mV L ESR 2 V I 2 V ´ ´ - ´ ´ = - ´ ( ) ( ) ( ) ( ) ( ) f OUT OUT IN max TRIP IND peak DS on IN max V 1 R L V V V I V ´ - ´ = + ´ ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) f f OUT OUT OUT OUT IN max IN max IND ripple IN max OUT max IN max 1 3 I I V V V V V V L V V ´ ´ ´ - ´ - ´ = = ´ ( ) OUT V 2.0 R1 R2 2.0 - = ´ TPS51123 SLUS890E – DECEMBER 2008 – REVISED JANUARY 2013 www.ti.com UVLO Protection TPS51123 has VREG5 under voltage lock out protection (UVLO). When the VREG5 voltage is lower than UVLO threshold voltage both switch mode power supplies are shut off. This is non-latch protection. When the VREG3 voltage is lower than (VO2 - 1 V), both switch mode power supplies are also shut off Thermal Shutdown TPS51123 monitors the temperature of itself. If the temperature exceeds the threshold value (typically 150°C), TPS51123 is shut off including LDOs. This is non-latch protection. External Parts Selection The external components selection is much simple in D-CAP™ Mode. 1. Determine output voltage The output voltage is programmed by the voltage-divider resistor, R1 and R2, as shown in Figure 36. R1 is connected between VFBx pin and the output, and R2 is connected betwen the VFBx pin and GND. Recommended R2 value is from 10 k Ω to 20 kΩ. Determine R1 using equation as below. (5) 2. Choose the Inductor The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum output current. Larger ripple current increases output ripple voltage and improves S/N ratio and helps stable operation. (6) The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak inductor current before saturation. The peak inductor current can be estimated as follows. (7) 3. Choose the Output Capacitor(s) Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine ESR to meet required ripple voltage. A quick approximation is as shown in Equation 8. This equation is based on that required output ripple slope is approximately 20 mV per TSW (switching period) in terms of VFB terminal voltage. where • D is the duty cycle • the required output ripple slope is approximately 20 mV per tSW (switching period) in terms of VFB terminal voltage (8) 4. Choose the Low-Side MOSFET It is highly recommended that the low-side MOSFET should have an integrated Schottky barrier diode, or an external Schottky barrier diode in parallel to achieve stable operation. 24 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated Product Folder Links: TPS51123 |
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