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LTC1174HVCS8 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1174HVCS8 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LTC1174 LTC1174-3.3/LTC1174-5 S APPLICATI I FOR ATIO the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short-circuited. Under this condition the diode must safely handle IPEAK at close to 100% duty cycle. A fast switching diode must also be used to optimize efficiency. Schottky diodes are a good choice for low forward drop and fast switching times. Most LTC1174 circuits will be well served by either a 1N5818, a MBRS140T3 or a MBR0520L Schot- tky diode. Short-Circuit Protection The LTC1174 is protected from output short by its internal current limit. Depending on the condition of IPGM pin, the limit is either set to 340mA or 600mA. In addition, the off- time of the switch is increased to allow the inductor’s current to decay far enough to prevent any current build-up (see Figure 2). the maximum RMS current must be used. The CIN RMS current is given by: I IV V V V A RMS OUT OUT IN OUT IN RMS ≈ − () [] () 12 / This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2.Thissimpleworstcaseiscommonlyusedfordesign because even significant deviations do not offer much relief. Note that ripple current directly affects capacitor’s lifetime. DO NOT UNDERSPECIFY THIS COMPONENT. An additional 0.1 µF ceramic capacitor is also required on VIN for high frequency decoupling. COUT To avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. The worst case RMS ripple current in the output capacitor is given by: I I A mA RMS PEAK RMS ≈ () = 2 170 or 300mA Although the output voltage ripple is determined by the hysteresis of the voltage comparator, ESR of the output capacitor is also a concern. Too high of an ESR will create a higher ripple output voltage and at the same time cause the LTC1174 to sleep less often. This will affect the efficiency of the LTC1174. For a given technology, ESR is a direct function of the volume of the capacitor. Several small-sized capacitors can also be paralleled to obtain the same ESR as one large can. Manufacturers such as Nichicon, Chemicon and Sprague should be considered for high performance capacitors. The OS-CON semiconductor dielectric capaci- tor available from Sanyo has the lowest ESR for its size, at a higher price. Catch Diode Selection The catch diode carries load current during the off-time. The average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the diode conducts most of the time. As VIN approaches VOUT Low-Battery Detector The low-battery indicator senses the input voltage through an external resistive divider. This divided voltage connects to the “–” input of a voltage comparator (pin 3) which is compared with a 1.25V reference voltage. With the current going into pin 3 being negligible, the following expression is used for setting the trip limit: V R R LBTRIP =+ 125 1 4 3 . IPGM = VIN IPGM = 0 GND L = 100 µH VIN = 13.5V 20 µs/DIV 1174 F02 Figure 2. Inductor's Current with Output Shorted |
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