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LTC3405A-1.8 Datasheet(PDF) 6 Page - Linear Technology |
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LTC3405A-1.8 Datasheet(HTML) 6 Page - Linear Technology |
6 / 16 page 6 LTC3405A-1.5/LTC3405A-1.8 sn3405a1518 3405a1518fs FU CTIO AL DIAGRA OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC3405A Fixed output voltage series parts use a constant frequency, current mode step-down architec- ture. Their main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, the output voltage de- creases which causes a slight decrease in VFB relative to the 1.2V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. Comparator OVDET guards against transient overshoots > 7.8% by turning the main switch off and keeping it off until the fault is removed. Burst Mode Operation The LTC3405A series parts are capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply connect the MODE pin to GND. To disable Burst Mode operation and enable PWM pulse skipping mode, connect the MODE pin to VIN or drive it with a logic high (VMODE > 1.5V). In this mode, the efficiency is lower at light loads, but becomes comparable to Burst Mode operation when the output load exceeds 25mA. The advantage of pulse skipping mode is lower output ripple and less interference to audio circuitry. + – + – + – OVDET EA IRCMP + – ICOMP 5 1 RUN OSC SLOPE COMP OSC FREQ SHIFT 1.2V R1 LTC3405A-1.5 R1 = 110k R2 = 440k LTC3405A-1.8 R1 = 180k R2 = 360k R2 1.294V 1.2V REF SHUTDOWN OV 0.4V 0.65V SLEEP VIN VOUT 6 MODE EN BURST VIN S R RS LATCH SWITCHING LOGIC AND BLANKING CIRCUIT ANTI- SHOOT- THRU Q Q 5 Ω 4 SW 3 GND 3405A1518 BD 2 VFB |
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