LR8506 Datasheet, PDF (7/10 Pages) Leshan Radio Company – 1.5MHz 600mA Synchronous

„ OPERATION
MAIN CONTROL LOOP
The LR8506 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, I_COMP, resets the
RS latch. The peak inductor current at which
I_COMP 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 I_RCMP, or
the beginning of the next clock cycle. The
OVDET comparator controls output transient
overshoots by turning the main switch off and
keeping it off until the fault is removed.
MAXIMUM LOAD CURRENT
The LR8506 will operate with input
voltage as low as 2.5V, however, the
maximum load current decreases at lower
input due to large IR drop on the main switch
and synchronous rectifier. The slope
compensation signal reduces the peak
inductor current as a function of the duty
cycle to prevent sub-harmonic oscillations at
duty cycles greater than 50%.Conversely the
current limit increase as the duty cycle
decreases.
LESHAN RADIO COMPANY, LTD.
PULSE SKIPPING MODE
At light loads, the inductor current may
reach zero reverse on each pulse. The
bottom MOSFET is turned off by the current
reversal comparator, I_RCMP, and the switch
voltage will ring. This is discontinuous mode
operation, and is normal behavior for the
switching regulator. At very light loads, the
LR8506 will automatically skip pulses in
pulse skipping mode operation to maintain
output regulation.
SLOPE COMPENSATION
Slope compensation provides stability in
constant frequency architecture 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
50%. This slope compensated current mode
PWM control provides stable switching and
cycle-by-cycle current limit for excellent load
and line response.
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
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 inputs supply voltages, the R_DS(ON) of
the P-channel switch increases. Therefore,
the user should calculate the power
dissipation when the LR8506 is used at
100% duty cycle with low input voltage.
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