AME5283 Datasheet, PDF (7/20 Pages) Analog Microelectronics – Rectified Step-Down Converter

AME
AME5283
2A, 300KHz ~ 2MHz Synchronous
Rectified Step-Down Converter
n Detailed Description
Normal Operation
The AME5283 uses a user adjustable frequency, cur-
rent mode step-down architecture with internal MOSFET
switch. During normal operation, the internal high-side
(PMOS) switch is turned on each cycle when the oscilla
for sets the SR latch, and turned off when the comparator
resets the SR latch. The peak inductor current at which
comparator resets the SR latch is controlled by the out-
put of error amplifier EA. While the high-side switch is
off, the low-side switch turns on until either the inductor
current starts to reverse or the beginning of the next
switching cycle.
Dropout Operation
The output voltage is dropped from the input supply for
the voltage which across the high-side switch. 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 volt-
age forces the high-side switch to remain on for more
than one cycle until it reaches 100% duty cycle.
Power Good Output
The AME5283 power good output is an open-drain out-
put and requires a pull up resistor. When the output volt-
age is 12.5% above or 12.5% below its set voltage,
PGOOD will be pulled low. It is held low until the output
voltage returns to within the allowed tolerances once more.
During soft-start, PGOOD is actively held low and is only
allowed to transition high when soft-start is over and the
output voltage reaches 87.5% of its set voltage.
Soft-Start
The AME5283 contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timing
is programmed by the external capacitor between SS pin
and GND. The chip provides an internal charge current
for the external capacitor. If 10nF capacitor is used to set
the soft-start, the period will be 800uS(typ.).
Hiccup Mode
During hiccup mode, the AME5283 disables the high-
side MOSFET and begins a cool down period of 8000uS
. At the conclusion of this cool down period, the regula-
tor performs an external 800uS identical to the soft start
at turn-on. ( If 10nF capacitor is used to set the soft-start)
Under Voltage Protection
Under Voltage Protection will activate once the feed-
back voltage falls below 0.4V, the operating frequency is
switched to 1/10 of normal switching frequency and after
four-times hiccup mode counted, the internal high-side
power switch will be turned off, and latched, Unless Re-
start the power supply.
Over Current Protection
The AME5283 cycle-by-cycle limits the peak inductor
current to protect embedded switch from damage. Hence
the maximum output current (the average of inductor cur-
rent) is also limited. In case the load increases, the in-
ductor current is also increase. Whenever the current
limit level is reached, the output voltage can not be regu-
lated and starting to drop.
Over Temperature Protectiion
The in most applications the AME5283 does not dissi-
pate much heat due to high efficiency. But, in applica-
tions where the AME5283 is running at high ambient tem-
perature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximum junction temperature of the part. If the junc-
tion temperature reaches approximately 160^oC, the inter-
nal high-side power switch will be turned off and the SW
switch will become high impedance.
Rev. A.03
7