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ILC6391CM-25 датащи(PDF) 5 Page - Impala Linear Corporation |
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ILC6391CM-25 датащи(HTML) 5 Page - Impala Linear Corporation |
5 / 9 page SOT-89 Step-Up PFM Switcher with Auto-Load Sense Impala Linear Corporation 5 (408) 574-3939 www.impalalinear.com Feb 2001 ILC6390 1.1 The ILC6390 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 ILC6390 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 ILC6390 and ILC6391 use a PFM or Pulse Frequency Modulation technique. In this technique, the switch is always turned on for a fixed period of time, corresponding to a fixed switching frequency at a predefined duty cycle. For the ILC6390 this value is 3.55msec on time, corresponding to 55% duty cycle at 155kHz. Because the inductor value, capacitor size, and switch on-time and frequency are all fixed, the ILC6390 in essence delivers the same amount of power to the output during each switching cycle. This in turn creates a constant output voltage ramp which is dependent on the output load requirement. In this mode, the only difference between the PFM and PWM techniques is the duty cycle of the switch. Once the output voltage reaches the set point, the ILC6390 will shut off the switch oscillator and wait until the output voltage drops low again, at which point it will re-start the oscillator. As you can see in the diagram, the PFM boost converter actually skips pulses as a way of varying the amount of power being deliv- ered to the output. Because of this, PFM is sometimes called “Pulse Skipping Modulation.” The chief advantage of using a PFM technique is that, at low cur- rents, the switcher is able to maintain regulation without con- stantly driving a switch on and off. This power savings can be 5mA or more for the ILC6390 versus the ILC6370, and at very light loads this current difference can make a noticeable impact on overall efficiency. However, because the ILC6390 will skip pulses based on load current, the effective frequency of switching may well drop into the audio band. This means that the radiated noise of the ILC6390 may interfere with the audio channel of the system and additional filtering may be necessary. In addition, because the PFM on-time is fixed, it usually has higher output ripple voltage than the PWM switcher, which dynamically changes the on-time to match the load current requirements. [Ripple is due to the output cap constantly accepting and storing the charge received from the induc- tor, and delivering charge as required by the load. The “pumping” action of the switch produces a sawtooth- shaped voltage as seen by the output.] On the plus side, because pulses are skipped, overtone content of the frequency noise is lower than in a PWM configuration. The sum of these characteristics for PFM converters makes it the ideal choice for low-current or ultra-long runtime applications, where overall conversion efficiency at low currents is of primary concern. [For other conversion techniques, please see the ILC6370/71 and ILC6380/81 datasheets.] Dual-Step Mode The ILC6390 and ILC6391 have one other unique feature, that being to automatically switch to a second switching scheme in the presence of heavy output loading. As we mentioned, the stan- dard switching scheme for these parts is a 3.55msec, 155kHz, 55% duty cycle part. However, if the device detects that the out- put load increases beyond a set point (as seen by the voltage drop on the output capacitor), it switches in a 7.5msec, 100kHz, 75% duty cycle “turbo mode” specifically to keep up with the increased load demand. This switchover is seamless to the user, but will result in a change in the output ripple voltage characteris- tic of the DC-DC converter. PFM converters are widely used in portable consumer applica- tions not requiring a high current level and relatively unaffected by audio noise. Applications such as pagers and PDAs, which need to operate in stand-by for extended periods of time, gravitate toward the advantages of PFM since maximum run-time is a chief differentiating element. The ILC6390 addresses this low-current requirement, and additionally offers a “turbo” mode which main- tains output regulation in the presence of heavier-than-normal load currents, and maintains 0.5mA shutdown currents. The only difference between the ILC6390 and ILC6391 parts is that the 6391 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. V SET V OUT Switch Waveform |
Аналогичный номер детали - ILC6391CM-25 |
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Аналогичное описание - ILC6391CM-25 |
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