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LTC1701ES5 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1701ES5 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LTC1701 Most LTC1701 circuits will be well served by either an MBR0520L or an MBRM120L. An MBR0520L is a good choice for IOUT(MAX) ≤ 500mA, as long as the output doesn’t need to sustain a continuous short. Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT/ VIN. To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maxi- mum RMS current must be used. The maximum RMS capacitor current is given by: II VV V V RMS MAX OUT IN OUT IN ≈ − () where the maximum average output current IMAX equals the peak current (1 Amp) minus half the peak-to-peak ripple current, IMAX = 1 – ∆IL/2. This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case is commonly used to design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours life- time. This makes it advisable to further derate the capaci- tor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet the size or height requirements of the design. An additional 0.1 µF to 1µF ceramic capacitor is also recom- mended on VIN for high frequency decoupling. Output Capacitor (COUT) Selection The selection of COUT is driven by the required ESR. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple ( ∆VOUT) is determined by: ∆≈ ∆ + V I ESR fC OUT L OUT 1 8 where f = operating frequency, COUT = output capacitance and ∆IL = ripple current in the inductor. With ∆IL = 0.4 IOUT(MAX) the output ripple will be less than 100mV with: ESRCOUT < 100mΩ Once the ESR requirements for COUT have been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. When the capacitance of COUT is made too small, the output ripple at low frequencies will be large enough to trip the ITH comparator. This causes Burst Mode operation to be activated when the LTC1701 would normally be in continuous mode operation. The effect can be improved at higher frequencies with lower inductor values. In surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirement of the application. Alumi- num electrolyte and dry tantulum capacitors are both available in surface mount configurations. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR(size) product of any aluminum elec- trolytic at a somewhat higher price. In the case of tanta- lum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS, AVX TPSV and KEMET T510 series of surface mount tantalums, avalable in case heights ranging from 2mm to 4mm. Other capacitor types include Nichicon PL series, Sanyo POSCAP and Panasonic SP. Ceramic Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempt- ing for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumen- tal in giving acceptable loop phase margin. Ceramic ca- pacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before ESR becomes effective. Also, ceramic caps are prone to temperature effects which requires the designer to check loop stability over the operating temperature range. For these reasons, most of the input and output capaci- tance should be composed of tantalum capacitors for stability combined with about 0.1 µF to 1µF of ceramic capacitors for high frequency decoupling. APPLICATIO S I FOR ATIO |
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