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TPS40180 Datasheet(PDF) 22 Page - Texas Instruments |
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TPS40180 Datasheet(HTML) 22 Page - Texas Instruments |
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22 / 52 page  www.ti.com Selecting Current Sense Network components CS CS L L R C R æ ö ´ = ç ÷ è ø (17) R CS L C CS R L UDG-07033 Inductor + _ V C ( ) ( ) f CS(max) IN OUT L SW G V 2 V L R ´ + ´ æ ö > ç ÷ è ø (18) TPS40180 SLVS753B – FEBRUARY 2007 – REVISED NOVEMBER 2007 Some consideration must be given to selecting the components that are used to sense current in the converter. If an R-C filter across the inductor is used, the R-C time constant should match the natural time constant of the inductor. Equation 17 and Figure 27 describe the relationship. Figure 27. Current Sense Network The amplitude of the VC voltage must also be considered. If the VC voltage is expected to rise above 60 mV at the desired overcurrent threshold, an attenuator should be used to keep the voltage to a maximum of 60 mV. To implement the divider, place a resistor in parallel with CCS. The time constant of the whole network should remain the same as the L/R time constant of the inductor. High ripple current applications can also cause problems under certain conditions. When the sensing network is matched to the inductor, the ripple voltage on the capacitor CCS is the same as the ripple voltage produced on the effective inductor resistance. If this ripple component is too great, sub-harmonic instability can result and the PWM exhibits excessive jitter or give a long pulse, short pulse type of output. To minimize this effect, the slope of the signal presented to the current sense amplifier must be less than a maximum value. This places a minimum limit on what the inductor L/R time constant can be for a given application as shown in Equation 18. If the chosen inductor and other application parameters fall outside these recommendations, it is necessary to attenuate the current feedback signal with an extra resistor. where • L is the inductance in H • R L is the equivalent series resistance of the inductor in Ω • G CS(max) is the maximum gain of the current sense amplifier, 13.75 • V IN is the input voltage in V • V OUT is the output voltage in V • f SW is the switching frequency in Hz 22 Submit Documentation Feedback Copyright © 2007, Texas Instruments Incorporated Product Folder Link(s): TPS40180 |
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