Datasheet LTC3405A-1.375 (Analog Devices) - 6

LTC3405A-1.375
U U W FU CTIO AL DIAGRA
MODE 6 SLOPE COMP 0.65V OSC OSC FREQ 4 VIN V SHIFT – OUT 5 + – EN R1 SLEEP + 5 95k 0.9V Ω 0.4V + – + I – EA COMP BURST R2 V 180k FB S Q R Q SWITCHING V RS LATCH IN LOGIC AND ANTI- SHOOT- RUN BLANKING CIRCUIT THRU 3 SW 1 0.9V REF – OV OVDET 0.95V + + SHUTDOWN IRCMP– 2 GND 3405A1375 BD
U OPERATIO (Refer to Functional Diagram) Main Control Loop
Comparator OVDET guards against transient overshoots > 5.6% by turning the main switch off and keeping it off The LTC3405A-1.375 uses a constant frequency, current until the fault is removed. mode step-down architecture. The main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches
Burst Mode Operation
are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator The LTC3405A-1.375 is capable of Burst Mode operation sets the RS latch, and turned off when the current com- in which the internal power MOSFETs operate intermit- parator, I tently based on load demand. To enable Burst Mode COMP, resets the RS latch. The peak inductor current at which I operation, simply connect the MODE pin to GND. To COMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current disable Burst Mode operation and enable PWM pulse increases, the output voltage decreases which causes a skipping mode, connect the MODE pin to VIN or drive it slight decrease in V with a logic high (V FB relative to the 0.9V reference, which MODE > 1.5V). In this mode, the in turn, causes the EA amplifier’s output voltage to in- efficiency is lower at light loads, but becomes comparable crease until the average inductor current matches the new to Burst Mode operation when the output load exceeds load current. While the top MOSFET is off, the bottom 25mA. The advantage of pulse skipping mode is lower MOSFET is turned on until either the inductor current output ripple and less interference to audio circuitry. starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. 3405a1375f 6