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LTC3205 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3205 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3205 10 3205f APPLICATIO S I FOR ATIO VIN VIN 1 µF 0.1 µF GND 3205 F06 LTC3205 10nH Figure 6. 10nH Inductor Used for Input Noise Reduction (Approximately 1cm of Wire) necessary for LD to load the data once it has shifted into the device. Command data is latched into the command register on the falling edge of the LD signal. The LTC3205 will begin to act on new command data as soon as LD goes low. Any general purpose microcontroller I/O line can be configured to control the LD pin if the microcontroller doesn’t provide this feature automatically. VIN, CPO Capacitor Selection The style and value of capacitors used with the LTC3205 determine several important parameters such as regulator control-loop stability, output ripple and charge pump strength. To reduce noise and ripple, it is recommended that low equivalent series resistance (ESR) multilayer ceramic capacitors be used on both VIN and CPO. Tanta- lum and aluminum capacitors are not recommended be- cause of their high ESR. The value of the capacitor on CPO directly controls the amount of output ripple for a given load current. Increasing the size of this capacitor will reduce the output ripple. The peak-to-peak output ripple is approximately given by the expression: V I fC RIPPLE OUT OSC OUT P-P ≅ 3• where fOSC is the LTC3205’s oscillator frequency (typically 800kHz) and COUT is the output charge storage capacitor on CPO. Both the style and value of the output capacitor can significantly affect the stability of the LTC3205. The LTC3205 uses 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 charge storage capacitor. The charge storage capacitor also serves to form the dominant pole for the control loop. To prevent ringing or instability, it is impor- tant for the output capacitor to maintain at least 0.6 µF of capacitance over all conditions. Likewise, excessive ESR on the output capacitor will tend to degrade the loop stability of the LTC3205. The closed-loop output resis- tance of the LTC3205 is designed to be 0.6 Ω. For a 100mA load current change, the error signal will change by about 60mV. If the output capacitor has 0.6 Ω or more of ESR, the closed-loop frequency response will cease to roll off in a simple one-pole fashion and poor load transient re- sponse or instability could result. Multilayer ceramic chip capacitors typically have exceptional ESR performance. MLCCs combined with a tight board layout will yield very good stability. 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 LTC3205 will be relatively constant while the charge pump is on either the input charging phase or the output charg- ing phase but will drop to zero during the clock 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 input current change times the ESR. Therefore, ceramic capacitors are again recommended for their ex- ceptional ESR performance. Input noise can be further reduced by powering the LTC3205 through a very small series inductor as shown in Figure 6. 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. Flying Capacitor Selection Warning: A polarized capacitor such as tantalum or alumi- num should never be used for the flying capacitors since their voltage can reverse upon start-up of the LTC3205. Ceramic capacitors should always be used for the flying capacitors. 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 0.7 µF of capacitance for each of the flying capacitors. Capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. For example, a ceramic capacitor made of X7R material will retain most of its capacitance from –40 °C to 85°C whereas a Z5U or Y5V style capacitor will |
Similar Part No. - LTC3205_15 |
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Similar Description - LTC3205_15 |
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