Datasheet LTC3405A (Analog Devices) - 6

LTC3405A
U U W FU CTIO AL DIAGRA
MODE 6 SLOPE COMP 0.65V OSC OSC FREQ 4 VIN SHIFT – VFB + – EN 5 SLEEP + 0.8V 5Ω 0.4V + – + I – EA COMP BURST S Q R Q SWITCHING V RS LATCH IN LOGIC AND ANTI- SHOOT- RUN BLANKING CIRCUIT THRU 3 SW 1 0.8V REF – OV OVDET 0.85V + + SHUTDOWN IRCMP– 2 GND 3405A BD
U OPERATIO (Refer to Functional Diagram) Main Control Loop
comparator IRCMP, or the beginning of the next clock cycle. The LTC3405A uses a constant frequency, current mode step-down architecture. Both the main (P-channel Comparator OVDET guards against transient overshoots MOSFET) and synchronous (N-channel MOSFET) switches > 6.25% by turning the main switch off and keeping it off are internal. During normal operation, the internal top until the fault is removed. power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current com-
Burst Mode Operation
parator, ICOMP, resets the RS latch. The peak inductor The LTC3405A is capable of Burst Mode operation in current at which ICOMP resets the RS latch, is controlled by which the internal power MOSFETs operate intermittently the output of error amplifier EA. The VFB pin, described in based on load demand. To enable Burst Mode operation, the Pin Functions section, allows EA to receive an output simply connect the MODE pin to GND. To disable Burst feedback voltage from an external resistive divider. When Mode operation and enable PWM pulse skipping mode, the load current increases, it causes a slight decrease in connect the MODE pin to VIN or drive it with a logic high the feedback voltage relative to the 0.8V reference, which (VMODE > 1.5V). In this mode, the efficiency is lower at in turn, causes the EA amplifier’s output voltage to in- light loads, but becomes comparable to Burst Mode crease until the average inductor current matches the new operation when the output load exceeds 25mA. The ad- load current. While the top MOSFET is off, the bottom vantage of pulse skipping mode is lower output ripple and MOSFET is turned on until either the inductor current less interference to audio circuitry. starts to reverse, as indicated by the current reversal 3405afa 6