Datasheet LTC3405A-1.5, LTC3405A-1.8 (Analog Devices) - 7

LTC3405A-1.5/LTC3405A-1.8
U OPERATIO (Refer to Functional Diagram)
When the converter is in Burst Mode operation, the peak 600 VOUT = 1.5V current of the inductor is set to approximately 100mA re- 500 gardless of the output load. Each burst event can last from VOUT = 1.8V a few cycles at light loads to almost continuously cycling 400 with short sleep intervals at moderate loads. In between 300 these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 200 20µA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage 100 MAXIMUM OUTPUT CURRENT (mA) droops, the EA amplifier’s output rises above the sleep 0 threshold signaling the BURST comparator to trip and turn 2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) the top MOSFET on. This process repeats at a rate that is 3405A1518 F02 dependent on the load demand.
Figure 2. Maximum Output Current vs Input Voltage Short-Circuit Protection
When the output is shorted to ground, the frequency of the
Slope Compensation and Inductor Peak Current
oscillator is reduced to about 210kHz, 1/7 the nominal Slope compensation provides stability in constant fre- frequency. This frequency foldback ensures that the in- quency architectures by preventing subharmonic oscilla- ductor current has more time to decay, thereby preventing tions at high duty cycles. It is accomplished internally by runaway. The oscillator’s frequency will progressively adding a compensating ramp to the inductor current increase to 1.5MHz when VOUT rises above 0V. signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current
Low Supply Operation
for duty cycles > 40%. However, the LTC3405A series The LTC3405A series parts will operate with input supply parts use a patent-pending scheme that counteracts this voltages as low as 2.5V, but the maximum allowable compensating ramp, which allows the maximum inductor output current is reduced at this low voltage. Figure 2 peak current to remain unaffected throughout all duty shows the reduction in the maximum output current as a cycles. function of input voltage for both fixed output voltages. 3405a1518fa 7