LTC3415 Datasheet, PDF (14/28 Pages) Linear Technology – 7A, PolyPhase Synchronous Step-Down Regulator

LTC3415
OPERATION
noise vs free-running constant frequency operation. Spread
spectrum operation is disabled if CLKIN is tied to ground
or if it’s driven by an external frequency synchronization
signal.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle will increase toward the maximum
on-time. Further reduction of the supply voltage forces the
P-channel power MOSFET 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 power MOSFET and
the inductor.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability by preventing sub-
harmonic oscillations. It works by internally adding a ramp
to the inductor current signal at duty cycles in excess of
30%. This causes the internal current comparator to trip
earlier. The I_TH clamp is also reached earlier than condi-
tions in which the duty cycle is below 30%. As a result,
the maximum inductor peak current is lower for higher
duty cycle operations.
To compensate for this loss in maximum inductor peak
current during high duty cycles, the LTC3415 uses a pat-
ented scheme that raises the I_TH clamp level (proportional
to the amount of slope compensation) when the duty cycle
is greater than 30%.
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest amount
of time that the LTC3415 is capable of turning the top
P-channel power MOSFET on and off again. It is determined
by the internal timing delays. The minimum on-time for
the LTC3415 is about 100ns. Low duty cycle and high
frequency applications may approach this minimum on-
time limit and care should be taken to ensure that:
t_ON(MIN)
<
V_OUT
(f • V_IN
)
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3415 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple current and ripple voltage will increase.
If an application can operate close to the minimum on-time
limit, an inductor must be chosen that has low enough
inductance to provide sufficient ripple amplitude to meet
the minimum on-time requirement. As a general rule, keep
the inductor ripple current equal or greater than 30% of
the I_OUT(MAX) at V_IN(MAX).
Output Margining
For a convenient system stress test on the LTC3415’s out-
put, the user can program the LTC3415’s output to ±5%,
±10% or ±15% of its normal operational voltage.
The MGN pin, when connected to a voltage between 0.5V
and SV_IN – 0.SV allows normal operation. When the MGN
pin is low, it forces negative margining, in which the output
voltage is below the regulation point. When MGN is high, the
output voltage is forced to above the regulation point. The
amount of output voltage margining is determined by the
BSEL pin. When BSEL is low, it’s 5%. When BSEL is high,
it’s 10%. When BSEL is V_IN/2, it’s 15%. When margining
is active, the internal output overvoltage and undervoltage
comparators are disabled and PGOOD remains high.
Output Power-Good
When the LTC3415’s output voltage is within a ±10%
window of the regulation point, which is reflected back
as a V_FB voltage in the range of 0.54V to 0.66V, the output
voltage is good and the PGOOD pin is pulled high with
the external resistor.
3415fa
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