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ILC6371AP-25 Arkusz danych(PDF) 5 Page - Impala Linear Corporation |
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ILC6371AP-25 Arkusz danych(HTML) 5 Page - Impala Linear Corporation |
5 / 8 page SOT-89 Step up Switching Regulator with Shutdown Impala Linear Corporation 5 (408) 574-3939 www.impalalinear.com July 1999 ILC6370/1 1.3 The ILC6370 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 ILC6370 mon- itors 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 ILC6370 and ILC6371 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 maxmim duty cycle of the switch determines the maximum load current that the device can support. The ILC6370 and ILC6371 both support up to 87% duty cycles, for maximum usable range of load currents. There are two key advantages of PWM type controllers. First, because the controller automatically varies the duty cycle of the switche’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 output cap constanty accepting and storing the charge recieved from the inductor, and delivering charge as required by the load. The “pumping” action of the switch produces a sawtooth-shaped voltage as seen by the output.] The other key advatage of the PWM type controllers is that the radiated noise due to the swtiching 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 minimze 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. There are downsides of PWM approaches, especially at very low currents. Because the PWM technique relies on constant switch- ing and varying duty cycle to match the load conditions, there is some point where the load current gets to small to be handled effi- ciently. If the ILC6370 had an ideal switch, this would not be such a problem. But an actual switch consumes some finite amount of current to switch on and off; at very low current this can be of the same magnitude as the load current itself, driving switching effi- ciencies down to 50% and below. 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 rquirements will vary by appli- cation and by actual system design and layout, so generalizations in this area are difficult, at best. [For other boost converter tech- niques, please see the ILC6380/81 and ILC6390/91 data sheets.] 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 operation), 87% maximum duty cycles for high load con- ditions, while maintaining very low shutdown current levels of 0.5µA. The only difference between the ILC6370 and ILC6371 parts is that the 6371 is configured to drive an external transistor as the switch element. Since larger transistors can be selected for this element, higher effective loads can be regulated. Start-up Mode The ILC6370 has an internal soft-start mode which suppresses ringing or overshoot on the output during start-up. The following diagram illustrates this start-up condition’s typical performance External Components and Layout Consideration The ILC6370 is designed to provide a complete DC-DC convertor solution with a minmum of external components. Ideally, only three externals are required: the inductor, a pass diode, and an output capacitor. The inductor needs to be of low DC Resistance type, typically 1 Ω value. Toroidal wound inductors have better field containment (less high frequency noise radiated out) but tend to be more expensive. Some manufacturers like Coilcraft have new bobbin-wound induc- tors with shielding included, which may be an ideal fit for these applications. Contact the manufacturer for more information. The inductor size needs to be in the range of 47µH to 1mH. In general, larger inductor sizes deliver less current, so the load cur- rent wil determine the inductor size used. V IN - Vf V OUT MIN T SOFT-START (~10msec) t = 0 Functions and Operation |
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