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LTC3406 Datasheet(PDF) 7 Page - Linear Technology |
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LTC3406 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LTC3406 LTC3406-1.5/LTC3406-1.8 3406fa OPERATIO (Refer to Functional Diagram) Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, 1/7 the nominal frequency. This frequency foldback ensures that the in- ductor current has more time to decay, thereby preventing runaway. The oscillator’s frequency will progressively increase to 1.5MHz when VFB or VOUT rises above 0V. Dropout Operation As the input supply voltage decreases to a value approach- ing the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the LTC3406 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Informa- tion section). Low Supply Operation The LTC3406 will operate with input supply voltages as low as 2.5V, but the maximum allowable output current is reduced at this low voltage. Figure 2 shows the reduction in the maximum output current as a function of input voltage for various output voltages. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant fre- quency architectures by preventing subharmonic oscilla- tions at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current for duty cycles > 40%. However, the LTC3406 uses a patent-pending scheme that counteracts this compensat- ing ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. SUPPLY VOLTAGE (V) 2.5 1200 1000 800 600 400 200 0 3.0 3.5 4.0 4.5 3406 F02 5.0 5.5 VOUT = 1.8V VOUT = 1.5V VOUT = 2.5V Figure 2. Maximum Output Current vs Input Voltage |
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