Datasheet LT1375, LT1376 (Analog Devices) - 7

LT1375/LT1376
U U U PIN FUNCTIONS VIN:
This is the collector of the on-chip power NPN switch. pin is used as a SENSE pin, connected directly to the 5V This pin powers the internal circuitry and internal regulator output. Two additional functions are performed by the FB when the BIAS pin is not present. At NPN switch on and off, pin. When the pin voltage drops below 1.7V, switch high dl/dt edges occur on this pin. Keep the external current limit is reduced. Below 1V, switching frequency is bypass and catch diode close to this pin. All trace induc- also reduced. See Feedback Pin Function section in Appli- tance on this path will create a voltage spike at switch off, cations Information for details. adding to the VCE voltage across the internal NPN.
VC:
The VC pin is the output of the error amplifier and the
BIAS (LT1376 Only):
The BIAS pin is used to improve input of the peak switch current comparator. It is normally efficiency when operating at higher input voltages and used for frequency compensation, but can do double duty light load current. Connecting this pin to the regulated as a current clamp or control loop override. This pin sits output voltage forces most of the internal circuitry to draw at about 1V for very light loads and 2V at maximum load. its operating current from the output voltage rather than It can be driven to ground to shut off the regulator, but if the input supply. This is a much more efficient way of driven high, current must be limited to 4mA. doing business if the input voltage is much higher than the
GND:
The GND pin connection needs consideration for output. Minimum output voltage setting for this mode of two reasons. First, it acts as the reference for the regulated operation is 3.3V. Efficiency improvement at VIN = 20V, output, so load regulation will suffer if the “ground” end of VOUT = 5V, and IOUT = 25mA is over 10%. the load is not at the same voltage as the GND pin of the
SYNC (LT1375 Only):
The SYNC pin is used to synchro- IC. This condition will occur when load current or other nize the internal oscillator to an external signal. It is directly currents flow through metal paths between the GND pin logic compatible and can be driven with any signal be- and the load ground point. Keep the ground path short tween 10% and 90% duty cycle. The synchronizing range between the GND pin and the load, and use a ground plane is equal to initial operating frequency, up to 900kHz. See when possible. The second consideration is EMI caused Synchronizing section in Applications Information for by GND pin current spikes. Internal capacitance between details. the VSW pin and the GND pin creates very narrow (<10ns) current spikes in the GND pin. If the GND pin is connected
FB/SENSE:
The feedback pin is used to set output voltage, to system ground with a long metal trace, this trace may using an external voltage divider that generates 2.42V at radiate excess EMI. Keep the path between the input the pin with the desired output voltage. The fixed voltage bypass and the GND pin short. (-5) parts have the divider included on the chip, and the FB
W BLOCK DIAGRAM
The LT1376 is a constant frequency, current mode buck amplifier commands current to be delivered to the output converter. This means that there is an internal clock and rather than voltage. A voltage fed system will have low two feedback loops that control the duty cycle of the power phase shift up to the resonant frequency of the inductor switch. In addition to the normal error amplifier, there is a and output capacitor, then an abrupt 180° shift will occur. current sense amplifier that monitors switch current on a The current fed system will have 90° phase shift at a much cycle-by-cycle basis. A switch cycle starts with an oscilla- lower frequency, but will not have the additional 90° shift tor pulse which sets the RS flip-flop to turn the switch on. until well beyond the LC resonant frequency. This makes When switch current reaches a level set by the inverting it much easier to frequency compensate the feedback loop input of the comparator, the flip-flop is reset and the and also gives much quicker transient response. switch turns off. Output voltage control is obtained by Most of the circuitry of the LT1376 operates from an using the output of the error amplifier to set the switch internal 2.9V bias line. The bias regulator normally draws current trip point. This technique means that the error power from the regulator input pin, but if the BIAS pin is 13756fd 7