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ILC6380BP-25 Datasheet(PDF) 4 Page - Impala Linear Corporation |
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ILC6380BP-25 Datasheet(HTML) 4 Page - Impala Linear Corporation |
4 / 6 page SOT-89 Step-up Dual-Mode Switcher with Shutdown Impala Linear Corporation 4 (408) 574-3939 www.impalalinear.com Sept 1999 ILC6380/1 1.4 The ILC6380 performs boost DC-DC conversion by controlling the switch element shown in the circuit below When the switch is closed, current is built up through the inductor. When the switch opens, this current has to go somewhere and is forced through the diode to the output. As this on and off switch- ing continues, the output capacitor voltage builds up due to the charge it is storing from the inductor current. In this way, the out- put voltage gets boosted relative to the input. The ILC6380 moni- tors the voltage on the output capacitor to determine how much and how often to drive the switch. In general, the switching characteristic is determined by the output voltage desired and the current required by the load. Specifically the energy transfer is determined by the power stored in the coil during each switching cycle. PL = ƒ(tON, VIN) The ILC6380 and ILC6381 use a PWM or Pulse Width Modulation technique. The parts come in one of three fixed internal frequen- cies: 50, 100, or 180kHz. The switches are constantly driven at these frequencies. The control circuitry varies the power being delivered to the load by varying the on-time, or duty cycle, of the switch. Since more on-time translates to higher current build-up in the inductor, the maximum duty cycle of the switch determines the maximum load current that the device can support. The ILC6380 and ILC6381 both support up to 87% duty cycles, for maximum usable range of load currents. There are two key advantages of the PWM type controllers. First, because the controller automatically varies the duty cycle of the switch’s on-time in response to changing load conditions, the PWM controller will always have an optimized waveform for a steady-state load. This translates to very good efficiency at high currents and minimal ripple on the output. [Ripple is due to the out- put cap constantly accepting and storing the charge received from the inductor, and delivering charge as required by the load. The “pumping” action of the switch produces a sawtooth-shaped volt- age as seen by the output.] The other key advantage of the PWM type controllers is that the radiated noise due to the switching transients will always occur at the (fixed) switching frequency. Many applications do not care much about switching noise, but certain types of applications, especially communication equipment, need to minimize the high frequency interference within their system as much as is possible. Using a boost converter requires a certain amount of higher fre- quency noise to be generated; using a PWM converter makes that noise highly predictable; thus easier to filter out. Dual Mode Operation But there are downsides of PWM approaches, especially at very low currents. Because the PWM technique relies on constant switching and varying duty cycle to match the load conditions, there is some point where the load current gets too small to be handled efficiently. An actual switch consumes some finite amount of current to switch on and off; at very low currents this can be of the same magnitude as the load current itself, driving switching efficiencies down to 50% and below. The ILC6380 and ILC6381 overcome this limitation by automatically switching over to a PFM, or Pulse Frequency Modulation, technique at low currents. This technique conserves power loss by only switching the output if the current drain requires it. As shown in the diagram below, the wave- form actually skips pulses depending on the power needed by the output. [This technique is also called “pulse skipping” because of this characteristic.] In the ILC6380 and ILC6381, this switchover is internally set to be at the point where the PWM waveform hits approximately 10% duty cycle. So the PFM mode is running at 10% duty cycle at the rated frequency; for 100kHz part this means a constant on-time of 1msec. This not only is ideal for efficiency at these low currents, but a 10% duty cycle will have much better output ripple charac- teristics than a similarly configured PFM part, such as the ILC6390 and ILC6391. The Dual-Mode architecture was designed specifically for those applications, like communications, which need the spectral pre- dictability of a PWM-type DC-DC converter, yet which also needs the highest efficiencies possible, especially in Shutdown or Standby mode. [For other conversion techniques, please see the ILC6370/71 and ILC6390/91 datasheets.] Other Considerations The other limitation of PWM techniques is that, while the funda- mental switching frequency is easier to filter out since it’s constant, the higher order harmonics of PWM will be present and may have to be filtered out, as well. Any filtering requirements, though, will vary by application and by actual system design and layout, so generalizations in this area are difficult, at best. However, PWM control for boost DC-DC conversion is widely used, especially in audio-noise sensitive applications or applica- tions requiring strict filtering of the high frequency components. Impala’s products give very good efficiencies of 85% at 50mA out- put (5V product), 87% maximum duty cycles for high load condi- tions, while maintaining very low shutdown current levels of V SET V OUT Switch Waveform Functions and Operation |
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