Datasheet LTC3405A (Analog Devices) - 7

LTC3405A
U OPERATIO (Refer to Functional Diagram)
When the converter is in Burst Mode operation, the peak be determined by the input voltage minus the voltage drop current of the inductor is set to approximately 100mA re- across the P-channel MOSFET and the inductor. gardless of the output load. Each burst event can last from Another important detail to remember is that at low input a few cycles at light loads to almost continuously cycling supply voltages, the RDS(ON) of the P-channel switch with short sleep intervals at moderate loads. In between increases (see Typical Performance Characteristics). There- these burst events, the power MOSFETs and any unneeded fore, the user should calculate the power dissipation when circuitry are turned off, reducing the quiescent current to the LTC3405A is used at 100% duty cycle with low input 20µA. In this sleep state, the load current is being supplied voltage (See Thermal Considerations in the Applications solely from the output capacitor. As the output voltage Information section). droops, the EA amplifier’s output rises above the sleep threshold signaling the BURST comparator to trip and turn
Low Supply Operation
the top MOSFET on. This process repeats at a rate that is The LTC3405A will operate with input supply voltages as dependent on the load demand. low as 2.5V, but the maximum allowable output current is
Short-Circuit Protection
reduced at this low voltage. Figure 2 shows the reduction in the maximum output current as a function of input When the output is shorted to ground, the frequency of the voltage for various output voltages. oscillator is reduced to about 210kHz, 1/7 the nominal frequency. This frequency foldback ensures that the in-
Slope Compensation and Inductor Peak Current
ductor current has more time to decay, thereby preventing Slope compensation provides stability in constant fre- runaway. The oscillator’s frequency will progressively quency architectures by preventing subharmonic oscilla- increase to 1.5MHz when VFB rises above 0V. tions at high duty cycles. It is accomplished internally by
Dropout Operation
adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this As the input supply voltage decreases to a value approach- results in a reduction of maximum inductor peak current ing the output voltage, the duty cycle increases toward the for duty cycles > 40%. However, the LTC3405A uses a maximum on-time. Further reduction of the supply voltage patent-pending scheme that counteracts this compensat- forces the main switch to remain on for more than one cycle ing ramp, which allows the maximum inductor peak until it reaches 100% duty cycle. The output voltage will then current to remain unaffected throughout all duty cycles. 600 VOUT = 1.8V 500 VOUT = 1.3V 400 VOUT = 2.5V 300 200 100 MAXIMUM OUTPUT CURRENT (mA) 02.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) 3405A F02
Figure 2. Maximum Output Current vs Input Voltage
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