Datasheet LT1766, LT1766-5 (Analog Devices) - 8

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