AME
AME5258
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Detailed Description
Main Control Loop
The AME5258 uses a constant frequency, current
modestep-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET)
switches are internal. During normal operation, the inter-
nal top power MOSFET is turned on each cycle when the
oscillator sets the RS latch, and turned off when the cur-
rent 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 com-
parator IRCMP, or the beginning of the next clock cycle.
The comparator OVDET guards against transient over-
shoots >7.8% by turning the main switch off and keeping
it off until the fault is removed.
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero or
reverse on each pulse. The bottom MOSFET is turned off
by the current reversal comparator, IRCMP, and the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator.
Short-Circuit Protection
When the output is shorted to ground, the frequency of
the oscillator is reduced to about 210kHz, 1/7 the nomi-
nal frequency. This frequency foldback ensures that the
inductor current has more time to decay, thereby pre-
venting runaway. The oscillator's frequency will progres-
sively increase to 1.5MHz when V
FB
or V
OUT
rises above
0V.
1.5MHz, 600mA
Synchronous Buck Converter
Dropout Operation
As the input supply voltage decreases to a value ap-
proaching 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 remem-
ber is that at low input supply voltages, the RDS(ON) of
the P-channel switch increases (see Typical Performance
Characteristics). Therefore, the user should calculate the
power dissipation when the AME5258 is used at 100%
duty cycle with low input Voltage.
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Application Information
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 1µH to 4.7µH. Its value is chosen based on
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 IL = 240mA (40% of 600mA).
∆
IL
=
V
OUT
1
⋅
V
OUT
(
1
−
)
f
⋅
L
V
IN
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 720mA rated
inductor should be enough for most applications (600mA+
120mA). For better efficiency, choose a low DC-resis-
tance inductor.
8
Rev.A.05
AMETHERM [ AMETHERM CIRCUIT PROTECTION THERMISTORS ]
