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Rev. E, Sep 2005
ML3406
Operating
Main Control Loop
The ML3406 uses a constant frequency, current mode
step-down
architecture.
Both
the
main
(P-channel
MOSFET)
and
synchronous
(N-channel
MOSFET)
switches are internal. During normal operation, the internal
top power MOSFET is turned on each cycle when the
oscillator sets the RS latch, and turned off when the current
comparator, ICOMP, resets the RS latch. The peak inductor
current at which Icomp resets the RS latch, is controlled by
the output of error amplifier EA. When the load current
increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn ,
causes the EA amplifier’s output voltage to increase until
the average inductor current matches the new load current.
While the top MOSFET is off, the bottom MOSFET is turned
on until either the inductor current starts to reverse, as
indicated by the current reversal comparator IRCmp, or the
beginning of the next clock cycle.
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
inductor 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
approaching 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.
Therefore, the user should calculate the power dissipation
when the ML3406 is used at 100% duty cycle with low input
voltage.
Burst Mode Operation
The ML3406 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand.
In Burst Mode operation, the peak current of the inductor is
set to approximately 200mA regardless 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.
Slope Compensation and Inductor Peak
Current
Slope
compensation
provides
stability
in
constant
frequency
architectures
by
preventing
subharmonic
oscillations 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 ML3406 uses a patent -
pending scheme that counteracts this compensating ramp,
which allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
Low Supply Operation
The ML3406 will operate with input supply voltages as low
as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 4 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
Figure 4
The basic ML3406 application circuit is shown in External
component selection is driven by the load requirement and
begins with the selection of L followed by CIN and COUT.