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LT1777 Datasheet(PDF) 6 Page - Linear Technology |
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LT1777 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 24 page  6 LT1777 PIN FUNCTIONS removed or supplied accordingly to limit dI/dt (see Appli- cations Information). VSW (Pin 6): This is the emitter node of the output switch and has large currents flowing through it. Keep the traces to the switching components as short as possible to minimize electromagnetic radiation and voltage spikes. SGND (Pin 7): This is the device signal ground pin. The internal reference and feedback amplifier are referred to it. Keep the ground path connection to the FB divider and the VC compensation capacitor free of large ground currents. VIN (Pin 10): This is the high voltage supply pin for the output switch. It also supplies power to the internal control circuitry during start-up conditions or if the VCC pin is left open. A high quality bypass capacitor which meets the input ripple current requirements is needed here (see Applications Information). SYNC (Pin 12): Pin to synchronize internal oscillator to external frequency reference. It is directly logic compat- ible and can be driven with any signal between 10% and 90% duty cycle. The sync function is internally disabled if the FB pin voltage is low enough to cause oscillator slowdown. If unused, this pin should be grounded. FB (Pin 13): This is the inverting input to the feedback amplifier. The noninverting input of this amplifier is inter- nally tied to the 1.24V reference. This pin also slows down the frequency of the internal oscillator when its voltage is abnormally low, e.g. 2/3 of normal or less. This feature helps maintain proper short-circuit protection. Coupling from high speed noise to this pin can cause irregular operation. (See Switch Node Considerations section.) VC (Pin 14): This is the control voltage pin which is the output of the feedback amplifier and the input of the current comparator. Frequency compensation of the over- all loop is effected by placing a capacitor (or in most cases a series R/C combination) between this node and ground. Coupling from high speed noise to this pin can cause irregular operation. (See Switch Node Considerations section.) GND (Pins 1, 8, 9, 16): These corner package pins are mechanically connected to the die paddle and thus aid in conducting away internally generated heat. As these are electrically connected to the die substrate, they must be held at ground potential. A direct connection to the local ground plane is recommended. NC (Pins 2, 11, 15): Package Pins 2, 11 and 15 are unconnected. SHDN (Pin 3): When pulled below the shutdown mode threshold, nominally 0.5V, this pin turns off the regulator and reduces VIN input current to a few tens of microam- peres (shutdown mode). When this pin is held above the shutdown mode threshold, but below the lockout threshold, the part will be opera- tional with the exception that output switching action will be inhibited (lockout mode). A user-adjustable undervolt- age lockout can be implemented by driving this pin from an external resistor divider to VIN. This action is logically “ANDed” with the internal UVLO, nominally set at 6.7V, such that minimum VIN can be increased above 6.7V, but not decreased (see Applications Information). If unused, this pin should be left open. However, the high impedance nature of this pin renders it susceptible to coupling from the VSW node, so a small capacitor to ground, typically 100pF or so is recommended when the pin is left open. VCC: (Pin 4): Pin to power the internal control circuitry from the switching supply output. Proper use of this pin enhances overall power supply efficiency. During start-up conditions, internal control circuitry is powered directly from VIN. If the output capacitor is located more than an inch from the VCC pin, a separate 0.1µF bypass capacitor to ground may be required right at the pin. VD (Pin 5): This pin is used in conjunction with a small external sense inductor to limit power path dI/dt. The sense inductor is placed between the VSW output node and the cathode of the freewheeling (power) diode, and the VD pin is connected to the diode. As the voltage across the inductor reaches ±2VBE, drive to the output transistor is |
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