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LTC3204B-5 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3204B-5 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC3204-3.3/LTC3204-5/ LTC3204B-3.3/LTC3204B-5 9 3204fa where fOSC is the oscillator frequency (typically 1.2MHz) and COUT is the value of output charge storage capacitor. Also, the value and style of the output capacitor can signifi- cantly affect the stability of the LTC3204-3.3/LTC3204-5/ LTC3204B-3.3/LTC3204B-5. As shown in the Block Diagram, the LTC3204-3.3/LTC3204-5/LTC3204B- 3.3/LTC3204B-5 use a linear control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output storage capacitor. This out- put capacitor also serves to form the dominant pole of the control loop. To prevent ringing or instability on the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5, it is important to maintain at least 1µF of capacitance over all conditions. ExcessiveESRontheoutputcapacitorcandegradetheloop stability of the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/ LTC3204B-5. The closed loop output resistance of the LTC3204-5 is designed to be 0.5Ω. For a 100mA load current change, the output voltage will change by about 50mV. If the output capacitor has 0.5Ω or more of ESR, the closed loop frequency response will cease to roll off in a simple one-pole fashion and poor load transient response or instability could result. Ceramic capacitors typicallyhaveexceptionalESRperformanceandcombined with a good board layout should yield very good stability and load transient performance. As the value of COUT controls the amount of output ripple, the value of CIN controls the amount of ripple present at the input pin (VIN). The input current to the LTC3204-3.3/ LTC3204-5/LTC3204B-3.3/LTC3204B-5 will be relatively constant during the input charging phase or the output charging phase but will drop to zero during the nonoverlap times. Since the nonoverlap time is small (~25ns), these missing notches will result in only a small perturbation on the input power supply line. Note that a higher ESR capacitor such as tantalum will have higher input noise due to the voltage drop in the ESR. Therefore, ceramic capacitors are again recommended for their exceptional ESR performance. Further input noise reduction can be achieved by powering the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5 through a very small series inductor as shown in Figure 3. A 10nH inductor will reject the fast current notches, thereby presenting a nearly constant current load to the input power supply. For economy, the 10nH inductor can be fabricated on the PC board with about 1cm (0.4") of PC board trace. Figure 3. 10nH Inductor Used for Additional Input Noise Reduction Flying Capacitor Selection Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capaci- tor since its voltage can reverse upon start-up of the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/LTC3204B-5. Low ESR ceramic capacitors should always be used for the flying capacitor. The flying capacitor controls the strength of the charge pump. In order to achieve the rated output current, it is necessary to have at least 1µF of capacitance for the fly- ing capacitor. For very light load applications, the flying capacitor may be reducedtosavespaceorcost.Fromthefirstorderapproxi- mation of ROL in the section “Effective Open-Loop Output Resistance,” the theoretical minimum output resistance of a voltage doubling charge pump can be expressed by the following equation: R V V I f C L MIN IN OUT OUT OSC FLY 0 2 1 ( ) – • = ≅ where fOSC is the switching frequency (1.2MHz) and CFLY is the value of the flying capacitor. The charge pump will typically be weaker than the theoretical limit due to additional switch resistance. However, for very light load applications, the above expression can be used as a guide- line in determining a starting capacitor value. LTC3204-3.3/ LTC3204-5 0.22µF 2.2µF VIN GND 1cm OF WIRE 10nH VIN 1 2 32005 F03 APPLICATIO S I FOR ATIO |
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Similar Description - LTC3204B-5 |
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