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LMZ14203TZ-ADJ 数据表(PDF) 10 Page - National Semiconductor (TI) |
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LMZ14203TZ-ADJ 数据表(HTML) 10 Page - National Semiconductor (TI) |
10 / 18 page These resistors should be chosen from values in the range of 1.0 kohm to 10.0 kohm. For V O = 0.8V the FB pin can be connected to the output di- rectly so long as an output preload resistor remains that draws more than 20uA. Converter operation requires this minimum load to create a small inductor ripple current and maintain proper regulation when no load is present. A feed-forward capacitor is placed in parallel with R FBT to im- prove load step transient response. Its value is usually deter- mined experimentally by load stepping between DCM and CCM conduction modes and adjusting for best transient re- sponse and minimum output ripple. A table of values for R FBT , RFBB , CFF and RON is included in the applications schematic. SOFT-START CAPACITOR SELECTION Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time to prevent overshoot. Upon turn-on, after all UVLO conditions have been passed, an internal 8uA current source begins charging the external soft-start capacitor. The soft-start time duration to reach steady state operation is given by the formula: t SS = VREF * CSS / Iss = 0.8V * CSS / 8uA (4) This equation can be rearranged as follows: C SS = tSS * 8 μA / 0.8V (5) Use of a 0.022 μF capacitor results in 2.2 msec soft-start du- ration. This is recommended as a minimum value. As the soft-start input exceeds 0.8V the output of the power stage will be in regulation. The soft-start capacitor continues 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*24V / (4*3.3V*( 24V — 3.3V)*33mV) ≥ 43μF (7) The LMZ14203 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 LMZ14203 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 24V input to 3.3V output application this equals 240 mV and f SW = 400 kHz. C IN≥ 3A * 3.3V/24V * (1– 3.3V/24V) / (400000 * 0.240 V) ≥ 3.7μF 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 LMZ14203 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 V IN is varied. RON 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- www.national.com 10 |
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类似说明 - LMZ14203TZ-ADJ |
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