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TPS53851 Datasheet(PDF) 24 Page - Texas Instruments |
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TPS53851 Datasheet(HTML) 24 Page - Texas Instruments |
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24 / 64 page  + + + + CSRTn CSn + _ PWM + + 0.5 V 0 V ERROR AMP COMP CSgain = 12.5 V SHR , 1.8 V I LIM 20 mA OVERCURRENT R1 R2 V OUT R SNS U1 U2 U3 U4 U5 U6 U7 RAMP V C + _ V e V OUT I OUT 5 . 12 ) R I ( V COMP SNS PEAK e ´ ´ + = SHR IN OUT e V V V RAMP V + ´ = 5 . 12 ) R I ( V V V RAMP COMP SNS PEAK SHR IN OUT ´ ´ + + ´ = TPS53851 SLUS985 – DECEMBER 2009 www.ti.com 5.23 CURRENT SENSE The current sensing and overcurrent detection architecture is shown in Figure 5-8. Figure 5-8. Output Current Sensing and Overcurrent Detection The output current, IOUT, flows through RSNS and develops a voltage, VC across it, representative of the output current. The voltage, VC, could also be derived from an R-C network in parallel with the output inductor. This voltage is amplified with a gain of 12.5 and then subtracted from the Error Amp output, COMP, to generate the Ve voltage. The Ve signal is compared to the slope-compensation RAMP signal to generate the PWM for the modulator. As the output current is increased, the amplified VC causes the Ve signal to decrease. In order to maintain the proper duty cycle (PWM), the COMP signal must increase. Therefore the magnitude of the COMP signal contains the output current information: (7) This is integral in the overcurrent detection as can be seen at comparator U7, comparing the ILIM voltage with COMP. In order to have the proper duty cycle at PWM, Ve is: (8) Combining equations: (9) This equation for COMP shows the reason for resistors R1 and R2 being tied to VSHR and VOUT respectively. 24 APPLICATION INFORMATION Copyright © 2009, Texas Instruments Incorporated Submit Documentation Feedback Product Folder Link(s): TPS53851 |
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