Datasheet LT1375, LT1376 (Analog Devices) - 8

LT1375/LT1376
W BLOCK DIAGRAM
connected to an external voltage higher than 3V, bias than the input voltage, allowing switch to be saturated. power will be drawn from the external source (typically the This boosted voltage is generated with an external capaci- regulated output voltage). This will improve efficiency if tor and diode. Two comparators are connected to the the BIAS pin voltage is lower than regulator input voltage. shutdown pin. One has a 2.38V threshold for undervoltage lockout and the second has a 0.4V threshold for complete High switch efficiency is attained by using the BOOST pin shutdown. to provide a voltage to the switch driver which is higher 0.05Ω INPUT + – CURRENT 2.9V BIAS INTERNAL SENSE BIAS REGULATOR V AMPLIFIER CC VOLTAGE GAIN = 10 SLOPE COMP Σ BOOST 0.9V 500kHz SYNC OSCILLATOR S Q1 RS DRIVER POWER FLIP-FLOP CIRCUITRY SWITCH + R SHUTDOWN COMPARATOR – V + – SW CURRENT COMPARATOR 0.37V FREQUENCY SHDN SHIFT CIRCUIT 3.5µA FOLDBACK Q2 CURRENT + LIMIT CLAMP – FB – LOCKOUT + COMPARATOR ERROR AMPLIFIER 2.38V V g 2.42V C m = 2000µMho GND 1375/76 BD
Figure 1. Block Diagram U U W U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS
resistors and the FB pin is renamed SENSE, connected directly to the output. The feedback (FB) pin on the LT1376 is used to set output voltage and also to provide several overload protection The suggested value for the output divider resistor (see features. The first part of this section deals with selecting Figure 2) from FB to ground (R2) is 5k or less, and a resistors to set output voltage and the remaining part talks formula for R1 is shown below. The output voltage error about foldback frequency and current limiting created by caused by ignoring the input bias current on the FB pin is the FB pin. Please read both parts before committing to a less than 0.25% with R2 = 5k. A table of standard 1% final design. The fixed 5V LT1376-5 has internal divider values is shown in Table 1 for common output voltages. 13756fd 8