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TPS40140 Datasheet(PDF) 37 Page - Texas Instruments |
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TPS40140 Datasheet(HTML) 37 Page - Texas Instruments |
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37 / 68 page  æ ö = + ç ÷ è ø RIPPLE PEAK OC I I I 2 ( ) - = ´ ´ IN OUT OUT RIPPLE IN SW V V V 1 I L V f TPS40140 www.ti.com SLUS660I – SEPTEMBER 2005 – REVISED JANUARY 2015 In Figure 36, normal operation is occurring between t0 and t1 as shown by VOUT being at the regulated voltage, (C) and normal switching on the SW NODE (B) and COMP at its nominal level, (D). At t1, an overcurrent load is experienced. The increased current forces COMP to increase to the ILIM level as shown in (D). If the COMP voltage is above the ILIM voltage for 7 switching cycles, the controller enters a hiccup mode. During this time the controller is not switching and the switching MOSFETs are turned off. The TRKx voltage goes through 7 cycles of charging and discharging the soft-start capacitor. At the end of the 7 cycles the controller attempts another normal restart. If the fault has been cleared, the output voltage comes up to the regulation level as shown at time t3. If the fault has not cleared, the COMP voltage again rises above the ILIM voltage and the hiccup mode repeats. If the overcurrent condition exists for seven (7) PWM clock cycles the converter turns off the upper and lower MOSFETs and initiates a hiccup mode restart. In hiccup mode, the TRKx pin is periodically charged and discharged. After seven hiccup cycles, the controller attempts another soft-start cycle to restore normal operation. If the overload condition persists, the controller returns to the hiccup mode. This condition may continue indefinitely. 9.1.20 Calculating Overcurrent Protection Level In order to set the desired overcurrent (IOC), a few variables must be known. The input and output voltage, the output inductor value and it's DC resistance (DCR), as well as the switching frequency. Also known are the ramp voltage which is 0.5 V and the VSHARE voltage, VSH which is 1.8 V. See the list of variables and their values at the end of this section. The overcurrent set point is in terms of the DC output current, but the current sense circuit monitors the peak of the current. Therefore, the current ripple is needed and is calculated from the values of: • input voltage (VIN) • output voltage (VOUT) • switching frequency (fSW) • output inductance (L) The ripple current is given by Equation 11. (11) Equation 12 calculates the detected peak current and is used in Equation 14. (12) It is this IPEAK current that is detected by the current sense circuit. The two resistors needed to set the peak overcurrent protection threshold and their connection for each channel is shown in Figure 37. DESIGN HINT Resistor R2 may be connected to the output voltage, VOUT, or to the output of the differential amplifier, DIFFO, if used. Copyright © 2005–2015, Texas Instruments Incorporated Submit Documentation Feedback 37 Product Folder Links: TPS40140 |
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