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LTC1159-3.3 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1159-3.3 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1159/LTC1159-3.3/LTC1159-5 APPLICATIO S I FOR ATIO N-Ch PD = VIN – VOUT VIN (IMAX)2 (1 + ∂N) RDS(ON) P-Ch PD = VOUT VIN (IMAX)2 (1 + ∂P) RDS(ON) + k(VIN)2 (IMAX) (CRSS) (f) where ∂ is the temperature dependency of RDS(ON) and k is a constant inversely related to the gate drive current. Both MOSFETs have I2R losses while the P-channel equation includes an additional term for transition losses, which are highest at high input voltages. For VIN < 20V the high current efficiency generally improves with larger MOSFETs, while for VIN > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actually provides higher effi- ciency. The N-channel MOSFET losses are the greatest at high input voltage or during a short circuit when the N- channel duty cycle is nearly 100%. The term (1 + ∂)isgenerallygivenforaMOSFETintheform of a normalized RDS(ON) vs Temperature curve, but ∂ = 0.007/°C can be used as an approximation for low voltage MOSFETs. CRSS is usually specified in the MOSFET electrical characteristics. The constant k = 5 can be used for the LTC1159 to estimate the relative contributions of the two terms in the P-channel dissipation equation. The Schottky diode D1 shown in Figure 1 only conducts during the dead time between the conduction of the two power MOSFETs. D1 prevents the body diode of the N-channel MOSFET from turning on and storing charge during the dead time, which could cost as much as 1% in efficiency (although there are no other harmful effects if D1 is omitted). Therefore, D1 should be selected for a forward voltage of less than 0.6V when conducting IMAX. 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/N TPSD107K010) could be 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 oper- ating 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 |
Similar Part No. - LTC1159-3.3 |
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Similar Description - LTC1159-3.3 |
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