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TPS51275RUKT Datasheet(PDF) 11 Page - Texas Instruments |
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TPS51275RUKT Datasheet(HTML) 11 Page - Texas Instruments |
11 / 31 page = £ p ´ ´ SW 0 OUT f 1 f 2 ESR C 4 R1 R2 Voltage Divider + VFB + VREF PWM Control Logic and Divider L ESR COUT VC RLOAD IIND IOUT UDG-12111 IC Switching Modulator Output Capacitor VOUT TPS51275 TPS51275B TPS51275C VIN DRVH DRVL TPS51275, TPS51275B, TPS51275C www.ti.com SLUSB45B – JUNE 2012 – REVISED MARCH 2013 Light-Load Condition in Out-of-Audio™ Operation (TPS51275B) Out-of-Audio™ (OOA) light-load mode is a unique control feature that keeps the switching frequency above acoustic audible frequencies toward a virtual no-load condition. During Out-of-Audio™ operation, the OOA control circuit monitors the states of both high-side and low-side MOSFETs and forces them switching if both MOSFETs are off for more than 40 µs. When both high-side and low-side MOSFETs are off for 40 µs during a light-load condition, the operation mode is changed to FCCM. This mode change initiates one cycle of the low- side MOSFET and the high-side MOSFET turning on. Then, both MOSFETs stay turned off waiting for another 40 µs. Table 1. SKIP Mode Operation (TPS51275/B/C) SKIP MODE OPERATION TPS51275 Auto-skip TPS51275B OOA TPS51275C Auto-skip D-CAP™ Mode From small-signal loop analysis, a buck converter using D-CAP™ mode can be simplified as shown in Figure 1. Figure 1. Simplifying the Modulator The output voltage is compared with internal reference voltage after divider resistors, R1 and R2. The PWM comparator determines the timing to turn on the high-side MOSFET. The gain and speed of the comparator is high enough to keep the voltage at the beginning of each ON cycle substantially constant. For the loop stability, the 0dB frequency, ƒ0, defined in Equation 2 must be lower than 1/4 of the switching frequency. (2) As ƒ0 is determined solely by the output capacitor characteristics, the loop stability during D-CAP™ mode is determined by the capacitor chemistry. For example, specialty polymer capacitors have output capacitance in the order of several hundred micro-Farads and ESR in range of 10 milli-ohms. These yield an f0 value on the order of 100 kHz or less and the loop is stable. However, ceramic capacitors have ƒ0 at more than 700 kHz, which is not suitable for this operational mode. Copyright © 2012–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 |
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