Datasheet LTC3405A-1.375 (Analog Devices) - 7

LTC3405A-1.375
U OPERATIO (Refer to Functional Diagram)
When the converter is in Burst Mode operation, the peak frequency. This frequency foldback ensures that the current of the inductor is set to approximately 100mA re- inductor current has more time to decay, thereby prevent- gardless of the output load. Each burst event can last from ing runaway. The oscillator’s frequency will progressively a few cycles at light loads to almost continuously cycling increase to 1.5MHz when VOUT rises above 0V. with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded
Slope Compensation and Inductor Peak Current
circuitry are turned off, reducing the quiescent current to Slope compensation provides stability in constant fre- 20µA. In this sleep state, the load current is being supplied quency architectures by preventing subharmonic oscilla- solely from the output capacitor. As the output voltage tions at high duty cycles. It is accomplished internally by droops, the EA amplifier’s output rises above the sleep adding a compensating ramp to the inductor current threshold signaling the BURST comparator to trip and turn signal at duty cycles in excess of 40%. Normally, this the top MOSFET on. This process repeats at a rate that is results in a reduction of maximum inductor peak current dependent on the load demand. for duty cycles > 40%. However, the LTC3405A-1.375 uses a patented scheme that counteracts this compensat-
Short-Circuit Protection
ing ramp, which allows the maximum inductor peak When the output is shorted to ground, the frequency of the current to remain unaffected throughout all duty cycles. oscillator is reduced to about 210kHz, 1/7 the nominal
U U W U APPLICATIO S I FOR ATIO
The basic LTC3405A-1.375 application circuit is shown in The inductor value also has an effect on Burst Mode Figure 1. External component selection is driven by the operation. The transition to low current operation begins load requirement and begins with the selection of L when the inductor current peaks fall to approximately followed by CIN and COUT. 100mA. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause a dip in
Inductor Selection
efficiency in the upper range of low current operation. In For most applications, the inductor value will fall in the Burst Mode operation, lower inductance values will cause range of 2.2µH to 10µH. Its value is determined by the the burst frequency to increase. desired ripple current. Large value inductors lower ripple
Inductor Core Selection
current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple Different core materials and shapes will change the size/ current as shown in equation 1. A reasonable starting point current and price/current relationship of an inductor. Tor- for setting ripple current is ∆IL = 120mA (40% of 300mA). oid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost 1 ⎛ V ⎞ more than powdered iron core inductors with similar ∆I OUT L = V 1 ( OUT electrical characteristics. The choice of which style induc- f)(L) − ⎜ ⎟ ⎝ V (1) IN ⎠ tor to use often depends more on the price vs size requirements and any radiated field/EMI requirements The DC current rating of the inductor should be at least than on what the LTC3405A-1.375 requires to operate. equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 360mA rated inductor should be enough for most applications (300mA + 60mA). For better efficiency, choose a low DC-resistance inductor. 3405a1375f 7