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ILC6382CIR-50 Datasheet(PDF) 6 Page - Impala Linear Corporation |
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ILC6382CIR-50 Datasheet(HTML) 6 Page - Impala Linear Corporation |
6 / 19 page 1-Cell to 3-Cell Boost with True Load Disconnect, 3.3V, 5V, or Adjustable Output Impala Linear Corporation 6 (408) 574-3939 www.impalalinear.com Oct 1999 ILC6382 1.5 The ILC6382 performs boost DC-DC conversion by control- ling the switch element as shown in the simplified circuit in figure 3 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 switching continues, the output capacitor voltage builds up due to the charge it is storing from the inductor current. In this way, the output voltage gets boost- ed relative to the input. 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) Synchronous Rectification The ILC6382 also uses a technique called "synchronous rectification" which removes the need for the external diode used in other circuits. The diode is replaced with a second switch or in the case of the ILC6382, an FET as shown in figure 4 below. The two switches now open and close in opposition to eachother, directing the flow of current to either charge the inductor or to feed the load. The ILC6382 monitors the volt- age on the output capacitor to determine how much and how often to drive the switches. PWM Mode Operation The ILC6382 uses a PWM or Pulse Width Modulation tech- nique. The switches are constantly driven at typically 300kHz. The control circuitry varies the power being deliv- ered to the load by varying the on-time, or duty cycle, of the switch SW1 (see fig. 5). 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. 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 opti- mized 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 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. 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 appli- cations 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 possible. Using a boost converter requires a certain amount of higher frequency noise to be generated; using a PWM converter makes that noise high- ly predictable thus easier to filter out. PFM Mode Operation 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 con- sumes 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 ILC6382 overcomes this limitation by automatically switching over to a PFM, or Pulse Frequency Modulation, technique at low currents. This technique con- serves power loss by only switching the output if the current drain requires it. As shown in the figure 5, the waveform actually skips pulses depending on the power needed by the output. This technique is also called "pulse skipping" because of this characteristic. V OUT POK LBO LB/SD SYNC GND L X V IN ILC6382 PWM/PFM CONTROLLER SHUTDOWN CONTROL V REF DELAY + + - - SW1 SW2 Figure 3: Basic Boost Circuit Figure 4: Simplified ILC6382 block diagram APPLICATIONS INFORMATION |
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