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LTC1159CG Datasheet(PDF) 10 Page - Linear Technology |
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LTC1159CG Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1159 LTC1159-3.3/LTC1159-5 CIN and COUT Selection In continuous mode, the source current of the P-channel MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN Required IRMS ≈ IMAX [VOUT(VIN – VOUT)]1/2 VIN This formula has a maximum at VIN = 2VOUT, where IRMS = IMAX/2. This simple worst-case condition is com- monly used for 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 of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may be paralleled to meet size or height requirements in the design. An additional 0.1 µF ceramic capacitor may also be required on VIN for high frequency decoupling. The selection of COUT is driven by the required effective series resistance (ESR). The ESR of COUT must be less than twice the value of RSENSE for proper operation of the LTC1159: COUT Required ESR < 2RSENSE Optimum efficiency is obtained by making the ESR equal to RSENSE. Manufacturers such as Nichicon, Chemicon, and Sprague should be considered for high performance ca- pacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR for its size at a somewhat higher price. Once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. In surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirements of the application. Alumi- num electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. For example, if 200 µF/10V is called for in an application requiring 3mm height, two AVX 100 µF/10V(P/NTPSD107K010)couldbe used. Consult the manufacturer for other specific recom- mendations. At low supply voltages, a minimum value of COUT is suggested to prevent an abnormal low frequency operating mode (see Figure 4). When COUT is too small, the output ripple at low frequencies will be large enough to trip the voltage comparator. This causes the Burst Mode operation to be activated when the LTC1159 would normally be in continuous operation. The effect is most pronounced with low values of RSENSE and can be improved by operating at higher frequencies with lower values of L. The output remains in regulation at all times. Figure 4. Minimum Suggested COUT (VIN – VOUT) VOLTAGE (V) 0 600 800 1000 4 LTC1159 • TPC04 400 200 0 1 2 3 5 L = 50 µH RSENSE = 0.02Ω L = 25 µH RSENSE = 0.02Ω L = 50 µH RSENSE = 0.05Ω Load Transient Response Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ∆ILOAD • ESR, where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT until the regulator loop adapts to the current change and returns VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing which would indicate a stability problem. The ITH external components shown in the Figure 1 circuit will provide adequate compensation for most applications. A second, more severe transient is caused by switching in loads with large (>1 µF) supply bypass capacitors. The APPLICATIO S I FOR ATIO |
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