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LT1377 Datasheet(PDF) 6 Page - Linear Technology |
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LT1377 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 24 page  6 LT1506 PIN FUNCTIONS FB/SENSE: The feedback pin is used to set output voltage using an external voltage divider that generates 2.42V at the pin with the desired output voltage. The fixed voltage (-3.3) parts have the divider included on the chip and the FB pin is used as a SENSE pin, connected directly to the 3.3V output. Three additional functions are performed by the FB pin. When the pin voltage drops below 1.7V, switch current limit is reduced. Below 1.5V the external sync function is disabled. Below 1V, switching frequency is also reduced. See Feedback Pin Function section in Applica- tions Information for details. BOOST: 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 0.07 Ω FET structure, but with much smaller die area. Efficiency improves from 75% for conventional bipolar designs to > 89% for these new parts. VIN: This is the collector of the on-chip power NPN switch. This pin powers the internal circuitry and internal regula- tor. At NPN switch on and off, high dI/dt edges occur on this pin. Keep the external bypass and catch diode close to this pin. All trace inductance on this path will create a voltage spike at switch off, adding to the VCE voltage across the internal NPN. GND: The GND pin connection needs consideration for two reasons. First, it acts as the reference for the regulated output, so load regulation will suffer if the “ground” end of the load is not at the same voltage as the GND pin of the IC. This condition will occur when load current or other currents flow through metal paths between the GND pin and the load ground point. Keep the ground path short between the GND pin and the load and use a ground plane when possible. The second consideration is EMI caused by GND pin current spikes. Internal capacitance between the VSW pin and the GND pin creates very narrow (<10ns) current spikes in the GND pin. If the GND pin is connected to system ground with a long metal trace, this trace may radiate excess EMI. Keep the path between the input bypass and the GND pin short. The GND pin of the SO-8 package is directly attached to the internal tab. This pin should be attached to a large copper area to improve thermal resistance. VSW: The switch pin is the emitter of the on-chip power NPN switch. This pin is driven up to the input pin voltage during switch on time. Inductor current drives the switch pin negative during switch off time. Negative voltage is clamped with the external catch diode. Maximum negative switch voltage allowed is – 0.8V. SYNC: The sync pin is used to synchronize the internal oscillator to an external signal. It is directly logic compat- ible and can be driven with any signal between 10% and 90% duty cycle. The synchronizing range is equal to initial operating frequency, up to 1MHz. This pin replaces SHDN on -SYNC option parts. See Synchronizing section in Applications Information for details. When not in use, this pin should be grounded. SHDN: The shutdown pin is used to turn off the regulator and to reduce input drain current to a few microamperes. Actually, this pin has two separate 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). This is sometimes used to prevent the regulator from operating until the input votlage has reached a predeter- mined level. VC: 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 do double duty as a current clamp or control loop override. This pin sits at about 1V for very light loads and 2V at maximum load. It can be driven to ground to shut off the regulator, but if driven high, current must be limited to 4mA. |
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