LTC3406B-1.2 UUWFU CTIO AL DIAGRA SLOPE COMP OSC OSC FREQ 4 VIN SHIFT – VOUT + 5 + 0.8V – + 5Ω 60k I FB – EA COMP 120k S Q R Q SWITCHING RS LATCH LOGIC VIN AND ANTI- BLANKING SHOOT- – OV CIRCUIT THRU 3 SW RUN OVDET 0.8V + ∆V + 1 OVL 0.8V REF + SHUTDOWN IRCMP– 2 GND 3406B12 BD UOPERATIO (Refer to Functional Diagram) V 2.2µH* IN VOUT as indicated by the current reversal comparator I 4 3 RCMP, or 2.7V VIN SW 1.2V TO 5.5V CIN** COUT† 600mA the beginning of the next clock cycle. The comparator 4.7µF 10µF LTC3406B-1.2 CER CER OVDET guards against transient overshoots >6.25% by 1 5 RUN 3406B12 F01 VOUT turning the main switch off and keeping it off until the fault GND 2 is removed. *MURATA LQH3C2R2M24 **TAIYO YUDEN JMK212BJ475MG Pulse Skipping Mode Operation †TAIYO YUDEN JMK316BJ106ML At light loads, the inductor current may reach zero or re- Figure 1. Typical Application verse on each pulse. The bottom MOSFET is turned off by Main Control Loop the current reversal comparator, IRCMP, and the switch The LTC3406B-1.2 uses a constant frequency, current voltage will ring. This is discontinuous mode operation, mode step-down architecture. Both the main (P-channel and is normal behavior for the switching regulator. At very MOSFET) and synchronous (N-channel MOSFET) switches light loads, the LTC3406B-1.2 will automatically skip pulses are internal. During normal operation, the internal top in pulse skipping mode operation to maintain output regu- power MOSFET is turned on each cycle when the oscillator lation. Refer to LTC3406-1.2 data sheet if Burst Mode op- sets the RS latch, and turned off when the current com- eration is preferred. parator, ICOMP, resets the RS latch. The peak inductor current at which I Short-Circuit Protection COMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current When the output is shorted to ground, the frequency of the increases, it causes a slight decrease in the feedback oscillator is reduced to about 210kHz, 1/7 the nominal voltage, FB, relative to the 0.8V reference, which in turn frequency. This frequency foldback ensures that the in- causes the EA amplifier’s output voltage to increase until ductor current has more time to decay, thereby preventing the average inductor current matches the new load cur- runaway. The oscillator’s frequency will progressively rent. While the top MOSFET is off, the bottom MOSFET is increase to 1.5MHz when VOUT rises above 0V. turned on until either the inductor current starts to reverse, sn3406b12 3406b12fs 6
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