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LTC3704 数据表(PDF) 18 Page - Linear Technology |
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LTC3704 数据表(HTML) 18 Page - Linear Technology |
18 / 32 page LTC1871X 18 1871xf For more information www.linear.com/LTC1871X APPLICATIONS INFORMATION Boost Converter: Input Capacitor Selection The input capacitor of a boost converter is less critical than the output capacitor, due to the fact that the inductor is in series with the input and the input current waveform is continuous (see Figure 11b). The input voltage source impedance determines the size of the input capacitor, which is typically in the range of 10µF to 100µF. A low ESR capacitor is recommended, although it is not as critical as for the output capacitor. The RMS input capacitor ripple current for a boost con- verter is: IRMS(CIN) = 0.3• VIN(MIN) L • f •DMAX Please note that the input capacitor can see a very high surge current when a battery is suddenly connected to the input of the converter and solid tantalum capacitors can fail catastrophically under these conditions. Be sure to specify surge-tested capacitors! Burst Mode Operation and Considerations The choice of MOSFET RDS(ON) and inductor value also determines the load current at which the LTC1871X en- ters Burst Mode operation. When bursting, the controller clamps the peak inductor current to approximately: IBURST(PEAK) = 30mV RDS(ON) which represents about 20% of the maximum 150mV SENSE pin voltage. The corresponding average current dependsupontheamountofripplecurrent.Lowerinductor values (higher ∆IL) will reduce the load current at which Burst Mode operations begins, since it is the peak current that is being clamped. The output voltage ripple can increase during Burst Mode operation if ∆IL is substantially less than IBURST. This can occur if the input voltage is very low or if a very large inductor is chosen. At high duty cycles, a skipped cycle causes the inductor current to quickly decay to zero. However, because ∆IL is small, it takes multiple cycles for the current to ramp back up to IBURST(PEAK). Dur- ing this inductor charging interval, the output capacitor must supply the load current and a significant droop in the output voltage can occur. Generally, it is a good idea to choose a value of inductor ∆IL between 25% and 40% of IIN(MAX). The alternative is to either increase the value of the output capacitor or disable Burst Mode operation using the MODE/SYNC pin. Burst Mode operation can be defeated by connecting the MODE/SYNC pin to a high logic-level voltage (either with a control input or by connecting this pin to INTVCC). In this mode, the burst clamp is removed, and the chip can operateatconstantfrequencyfromcontinuousconduction mode (CCM) at full load, down into deep discontinuous conduction mode (DCM) at light load. Prior to skipping pulses at very light load (i.e., <5% of full load), the control- ler will operate with a minimum switch on-time in DCM. VIN L D SW 11a. Circuit Diagram 11b. Inductor and Input Currents COUT VOUT RL IIN IL 11c. Switch Current ISW tON 11d. Diode and Output Currents 11e. Output Voltage Ripple Waveform IO ID VOUT (AC) tOFF ΔVESR RINGING DUE TO TOTAL INDUCTANCE (BOARD + CAP) ΔVCOUT 1871x F11 Figure 11. Switching Waveforms for a Boost Converter |
类似零件编号 - LTC3704 |
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类似说明 - LTC3704 |
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