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LMZ12003 bảng dữ liệu(PDF) 9 Page - National Semiconductor (TI) |
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LMZ12003 bảng dữ liệu(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page charging until it reaches approximately 3.8V on the SS pin. Voltage levels between 0.8V and 3.8V have no effect on other circuit operation. Note that the following conditions will reset the soft-start capacitor by discharging the SS input to ground with an internal 200 μA current sink. • The enable input being “pulled low” • Thermal shutdown condition • Over-current fault • Internal Vcc UVLO (Approx 4V input to V IN) C O SELECTION None of the required C O output capacitance is contained with- in the module. At a minimum, the output capacitor must meet the worst case minimum ripple current rating of 0.5 * I LR P-P, as calculated in equation (19) below. Beyond that, additional capacitance will reduce output ripple so long as the ESR is low enough to permit it. A minimum value of 10 μF is generally required. Experimentation will be required if attempting to op- erate with a minimum value. Ceramic capacitors or other low ESR types are recommended. See AN-2024 for more detail. The following equation provides a good first pass approxima- tion of C O for load transient requirements: C O≥ISTEP*VFB*L*VIN/ (4*VO*(VIN—VO)*VOUT-TRAN)(6) Solving: C O≥ 3A*0.8V*6.8μH*12V / (4*3.3V*( 12V — 3.3V)*33mV) ≥ 52μF (7) The LMZ12003 demonstration and evaluation boards are populated with a 100 uF 6.3V X5R output capacitor. Locations for extra output capacitors are provided. C IN SELECTION The LMZ12003 module contains an internal 0.47 µF input ce- ramic capacitor. Additional input capacitance is required ex- ternal to the module to handle the input ripple current of the application. This input capacitance should be located in very close proximity to the module. Input capacitor selection is generally directed to satisfy the input ripple current require- ments rather than by capacitance value. Worst case input ripple current rating is dictated by the equation: I(C IN(RMS)) ≊ 1 /2 * IO * √ (D / 1-D) (8) where D ≊ V O / VIN (As a point of reference, the worst case ripple current will oc- cur when the module is presented with full load current and when V IN = 2 * VO). Recommended minimum input capacitance is 10uF X7R ce- ramic with a voltage rating at least 25% higher than the maximum applied input voltage for the application. It is also recommended that attention be paid to the voltage and tem- perature deratings of the capacitor selected. It should be noted that ripple current rating of ceramic capacitors may be missing from the capacitor data sheet and you may have to contact the capacitor manufacturer for this rating. If the system design requires a certain minimum value of input ripple voltage ΔV IN be maintained then the following equation may be used. C IN ≥ IO * D * (1–D) / fSW-CCM * ΔVIN(9) If ΔV IN is 1% of VIN for a 20V input to 3.3V output application this equals 200 mV and f SW = 400 kHz. C IN≥ 3A * 3.3V/20V * (1– 3.3V/20V) / (400000 * 0.200 V) ≥ 5.2μF Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input capacitance and parasitic inductance of the incoming supply lines. R ON RESISTOR SELECTION Many designs will begin with a desired switching frequency in mind. For that purpose the following equation can be used. f SW(CCM) ≊ VO / (1.3 * 10 -10 * R ON) (10) This can be rearranged as R ON ≊ VO / (1.3 * 10 -10 * f SW(CCM) (11) The selection of RON and f SW(CCM) must be confined by limi- tations in the on-time and off-time for the COT control section. The on-time of the LMZ12003 timer is determined by the re- sistor R ON and the input voltage VIN. It is calculated as follows: t ON = (1.3 * 10 -10 * R ON) / VIN (12) The inverse relationship of t ON and VIN gives a nearly constant switching frequency as VIN is varied. R ON should be selected such that the on-time at maximum V IN is greater than 150 ns. The on-timer has a limiter to ensure a minimum of 150 ns for t ON. This limits the maximum operating frequency, which is governed by the following equation: f SW(MAX) = VO / (VIN(MAX) * 150 nsec) (13) This equation can be used to select R ON if a certain operating frequency is desired so long as the minimum on-time of 150 ns is observed. The limit for R ON can be calculated as follows: R ON ≥ VIN(MAX) * 150 nsec / (1.3 * 10 -10) (14) If R ON calculated in (11) is less than the minimum value de- termined in (14) a lower frequency should be selected. Alter- natively, V IN(MAX) can also be limited in order to keep the frequency unchanged. Additionally note, the minimum off-time of 260 ns limits the maximum duty ratio. Larger R ON (lower FSW) should be se- lected in any application requiring large duty ratio. Discontinuous Conduction and Continuous Conduction Modes At light load the regulator will operate in discontinuous con- duction mode (DCM). With load currents above the critical conduction point, it will operate in continuous conduction mode (CCM). When operating in DCM the switching cycle begins at zero amps inductor current; increases up to a peak value, and then recedes back to zero before the end of the off-time. Note that during the period of time that inductor cur- rent is zero, all load current is supplied by the output capacitor. The next on-time period starts when the voltage on the at the FB pin falls below the internal reference. The switching fre- quency is lower in DCM and varies more with load current as compared to CCM. Conversion efficiency in DCM is main- tained since conduction and switching losses are reduced with the smaller load and lower switching frequency. Operat- ing frequency in DCM can be calculated as follows: f SW(DCM)≊VO*(VIN-1)*6.8μH*1.18*10 20*I O/(VIN–VO)*RON 2 (15) In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the off-time. The switching frequency remains relatively constant with load cur- rent and line voltage variations. The CCM operating frequen- cy can be calculated using equation 7 above. Following is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes. 9 www.national.com |
Số phần tương tự - LMZ12003_1 |
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Mô tả tương tự - LMZ12003_1 |
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