Datasheet LTC1701, LTC1701B (Analog Devices) - 7

LTC1701/LTC1701B
U U W U APPLICATIO S I FOR ATIO Input Capacitor (CIN) Selection
When the capacitance of COUT is made too small, the output ripple at low frequencies will be large enough to trip In continuous mode, the input current of the converter is the I a square wave with a duty cycle of approximately V TH comparator. This causes Burst Mode operation to OUT/ be activated when the LTC1701 would normally be in VIN. To prevent large voltage transients, a low equivalent continuous mode operation. The effect can be improved at series resistance (ESR) input capacitor sized for the maxi- higher frequencies with lower inductor values. mum RMS current must be used. The maximum RMS capacitor current is given by: In surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS V V ( −V ) current handling requirement of the application. Alumi- OUT IN OUT I ≈ I num electrolyte and dry tantulum capacitors are both RMS MAX VIN available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested where the maximum average output current IMAX equals for use in switching power supplies. An excellent choice is the peak current (1 Amp) minus half the peak-to-peak the AVX TPS, AVX TPSV and KEMET T510 series of ripple current, IMAX = 1 – ∆IL/2. surface mount tantalums, avalable in case heights ranging This formula has a maximum at VIN = 2VOUT, where IRMS from 2mm to 4mm. Other capacitor types include Nichicon = IOUT/2. This simple worst-case is commonly used to PL series, Sanyo POSCAP and Panasonic SP. design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple
Ceramic Capacitors
current ratings are often based on only 2000 hours life- Higher value, lower cost ceramic capacitors are now time. This makes it advisable to further derate the capaci- becoming available in smaller case sizes. These are tempt- tor, or choose a capacitor rated at a higher temperature ing for switching regulator use because of their very low than required. Several capacitors may also be paralleled to ESR. Unfortunately, the ESR is so low that it can cause meet the size or height requirements of the design. An loop stability problems. Solid tantalum capacitor ESR additional 0.1µF to 1µF ceramic capacitor is also recom- generates a loop “zero” at 5kHz to 50kHz that is instrumen- mended on VIN for high frequency decoupling. tal in giving acceptable loop phase margin. Ceramic ca- pacitors remain capacitive to beyond 300kHz and usually
Output Capacitor (COUT) Selection
resonate with their ESL before ESR becomes effective. The selection of COUT is driven by the required ESR. Also, ceramic caps are prone to temperature effects which Typically, once the ESR requirement is satisfied, the requires the designer to check loop stability over the capacitance is adequate for filtering. The output ripple operating temperature range. (∆VOUT) is determined by: For these reasons, most of the input and output capaci- tance should be composed of tantalum capacitors for ∆ 1 VOUT ≈ ∆  I ESR L +   stability combined with about 0.1µF to 1µF of ceramic 8fCOUT  capacitors for high frequency decoupling. Great care must where f = operating frequency, C be taken when using only ceramic input and output capaci- OUT = output capacitance and ∆I tors. The OPTI-LOOP compensation allows transient re- L = ripple current in the inductor. With ∆IL = 0.4 I sponse to be optimized for all types of output capacitors, OUT(MAX) the output ripple will be less than 100mV with: including low ESR ceramics. ESRCOUT < 100mΩ Once the ESR requirements for C
Setting the Output Voltage
OUT have been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) The LTC1701 develops a 1.25V reference voltage between requirement. the feedback pin, VFB, and the signal ground as shown in 7