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LT1766EGN-5-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT1766EGN-5-PBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 30 page  LT1766/LT1766-5 8 1766fc PIN FUNCTIONS BLOCK DIAGRAM VIN (Pin 4): This is the collector of the on-chip power NPN switch. VIN powers the internal control circuitry when a voltage on the BIAS pin is not present. High dI/dt edges occur on this pin during switch turn on and off. Keep the path short from the VIN pin through the input bypass capacitor, through the catch diode back to SW. All trace inductance on this path will create a voltage spike at switch off, adding to the VCE voltage across the internal NPN. BOOST (Pin 6): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. Without this added voltage, the typical switch voltage loss would be about 1.5V. The additional BOOST voltage allows the switch to saturate and voltage loss approximates that of a 0.2Ω FET structure, but with much smaller die area. BIAS (Pin 10): The BIAS pin is used to improve efficiency when operating at higher input voltages and light load cur- rent. Connecting this pin to the regulated output voltage forces most of the internal circuitry to draw its operating current from the output voltage rather than the input supply. This architecture increases efficiency especially when the input voltage is much higher than the output. Minimum output voltage setting for this mode of operation is 3V. VC (Pin 11) The VC pin is the output of the error amplifier and the input of the peak switch current comparator. It is normally used for frequency compensation, but can also serve as a current clamp or control loop override. VC sits at about 0.9V for light loads and 2.1V at maximum load. It can be driven to ground to shut off the regulator, but if driven high, current must be limited to 4mA. FB/SENSE (Pin 12): The feedback pin is used to set the output voltage using an external voltage divider that gener- ates 1.22V at the pin for the desired output voltage. The 5V fixed output voltage parts have the divider included on the chip and the FB pin is used as a SENSE pin, connected directly to the 5V output. Three additional functions are performed by the FB pin. When the pin voltage drops below 0.6V, switch current limit is reduced and the external SYNC function is disabled. Below 0.8V, switching frequency is also reduced. See Feedback Pin Functions in Applications Information for details. SYNC (Pin 14): The SYNC pin is used to synchronize the internal oscillator to an external signal. It is directly logic compatible and can be driven with any signal between 10% and 90% duty cycle. The synchronizing range is equal to initial operating frequency up to 700kHz. See Synchroniz- ing in Applications Information for details. SHDN (Pin 15): The SHDN pin is used to turn off the regulator and to reduce input drain current to a few mi- croamperes. This pin has two thresholds: one at 2.38V to disable switching and a second at 0.4V to force complete micropower shutdown. The 2.38V threshold functions as an accurate undervoltage lockout (UVLO); sometimes used to prevent the regulator from delivering power until the input voltage has reached a predetermined level. If the SHDN pin functions are not required, the pin can either be left open (to allow an internal bias current to lift the pin to a default high state) or be forced high to a level not to exceed 6V. The LT1766 is a constant frequency, current mode buck converter. This means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. In addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. A switch cycle starts with an oscillator pulse which sets the RSflip-floptoturntheswitchon.When switch current reaches a level set by the inverting input of the comparator, the flip-flop is reset and the switch turns off. Output voltage control is obtained by using the output of the error amplifier to set the switch current trip point. This technique means that the error amplifier commands current to be delivered to the output rather than voltage. A voltage fed system will have low phase shift up to the resonant frequency of the inductor and output capacitor, then an abrupt 180° shift will occur. The current fed system |
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