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LTC1429 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1429 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1429 APPLICATIONS INFORMATION tions can be obtained with larger filter capacitors or by using an LC output filter or higher FSYNC clock rate with a lower value (< 0.1 µF) of flying capacitor. Also see the section on Output Capacitor ESR. For applications requir- ing ripple below 1mV, see the LTC1550/LTC1551 data sheet. CAPACITOR SELECTION Capacitor Sizing The performance is dependent on the type of capacitors used. The LTC1429 requires bypass caps to ground for both the VCC and OUT pins. The input cap provides most of the LTC1429’s supply current while it is charging the flying caps. It should be mounted as close to the package as possible, its value should be equal to or larger than the flying cap in doubling mode and at least twice the value of the flying caps in tripling mode. Ceramic capacitors generally provide adequate performance; avoid using a tantalum capacitor as the input bypass unless there is at least a 0.1 µF ceramic cap in parallel with it. The charge pump caps are somewhat less critical, since their peak currents are limited by the switches inside the LTC1429. Most applications should use 0.1 µF as the flying cap value; conveniently, ceramic caps are the most common type of 0.1 µF cap and they work well here. Usually the easiest solution is to use the same type of capacitor for both the input bypass and flying caps. The output cap performs two functions; it provides output current to the load during half of the charge pump cycle and its value helps to set the output ripple voltage. For applications that are insensitive to output ripple, the output bypass cap can be as small as 1 µF. To achieve specified low output ripple, a 3.3 µF or greater output capacitor, high input clock rate (FSYNC) and lower value (< 0.1 µF)offlyingcapacitorshouldbeused.Largeroutput caps will reduce output ripple further, at the expense of turn on time. In an application where the maximum load current is well- defined and output ripple is critical or input peak currents need to be minimized, the flying capacitor values can be tailored to the application. Reducing the value of the flying capacitors reduces the amount of charge transferred with each clock cycle. The smaller capacitors draw smaller pulses of current out of VCC as well, limiting peak currents and reducing the demands on the input supply. Tables 1 and 2 show recommended values of flying capacitors vs maximum load capacity at FSYNC = 400kHz and 700kHz respectively. Table 1. Typical Max Load (mA) vs Flying Capacitor Value at TA = 25°C, VOUT = – 4V, FSYNC = 400kHz MAX LOAD (mA) MAX LOAD (mA) FLYING CAPACITOR VCC = 5V VCC = 3.3V VALUE ( µF) DOUBLER MODE TRIPLER MODE 0.1 22 20 0.047 16 15 0.033 8 11 0.022 4 5 0.01 1 3 Table 2. Typical Max Load (mA) vs Flying Capacitor Value at TA = 25°C, VOUT = –4V, FSYNC = 700kHz MAX LOAD (mA) MAX LOAD (mA) FLYING CAPACITOR VCC = 5V VCC = 3.3V VALUE ( µF) DOUBLER MODE TRIPLER MODE 0.1 18 25 0.047 17 22 0.033 14 20 0.022 12 17 0.01 3 9 Output Capacitor ESR Output capacitor the Equivalent Series Resistance (ESR) is another factor to consider. Excessive ESR in the output capacitor can fool the regulation loop into keeping the output artificially low by prematurely terminating the charg- ing cycle. As the charge pump switches to recharge the output, a brief surge of current flows from the flying caps to the output cap. This current surge can be as high as 100mA under full load conditions. A typical 3.3 µFtantalum capacitor has 1 Ω or 2Ω of ESR; 100mA × 2Ω = 200mV. If the output is within 200mV of the set point, this additional 200mV surge will trip the feedback comparator and termi- nate the charging cycle. The pulse dissipates quickly and the comparator returns to the correct state, but the RS latch will not allow the charge pump to respond until the next clock edge. This prevents the charge pump from |
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