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TPS51225C Fiches technique(PDF) 11 Page - Texas Instruments |
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TPS51225C Fiches technique(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-12010 IC Switching Modulator Output Capacitor VOUT TPS51225 TPS51225B TPS51225C VIN DRVH DRVL TPS51225, TPS51225B, TPS51225C www.ti.com SLUSAV0A – JANUARY 2012 – REVISED JUNE 2012 Light-Load Condition in Out-of-Audio™ Operation (TPS51225B) 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 MOSFETs and forces them to transition into the ON state if both of 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 the low-side MOSFET on and pulls down the output voltage. Then, the high-side MOSFET is turned on and stops switching again. Table 1. SKIP Mode Operation (TPS51225/B/C) SKIP MODE OPERATION TPS51225 Auto-skip TPS51225B OOA TPS51225C 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 onthe 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, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s) :TPS51225 TPS51225B TPS51225C |
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