LTC3405A-1.375_15 Datasheet, PDF (7/12 Pages) Linear Technology – 1.375V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

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