AME5282 Datasheet, PDF (8/21 Pages) Analog Microelectronics – Rectified Step-Down Converter

AME
AME5282
2A, 300KHz ~ 2MHz Synchronous
Rectified Step-Down Converter
n Detailed Description
Normal Operation
The AME5282 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-
tor sets the SR latch, and turned off when the compara-
tor resets the SR latch. The peak inductor current at
which comparator resets the SR latch is controlled by
the output of error amplifier EA. While the high-side switch
is off, the low-side switch turns on until either the induc-
tor 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.
Soft-Start
The AME5282 has a built-in digital soft-start to control
the output voltage rise and limit the current surge at the
start-up.
When the internal soft-start begins, and count 896
switching cycles, soft start is complete, the converter
enters steady state operation.
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 Temperature Protection
In most applications the AME5282 does not dissipate
much heat due to high efficiency. But, in applications
where the AME5282 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such
as in dropout, the heat dissipated may exceed the maxi-
mum junction temperature of the part. If the junction
temperature reaches approximately 160^oC, the internal
high-side power switch will be turned off and the SW
switch will become high impedance.
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 2.2µH 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 increase the ripple
current ∆I_L:
∆I _L
=
f
1
×
L
V_OU
T
¹
−
V_OUT
V_IN

Hiccup Mode
During hiccup mode, the AME5282 disables the high-
side MOSFET and begins a cool down period of 8320
switching cycles. At the conclusion of this cool down
period, the regulator performs an internal 896 cycle soft
start identical to the soft start at turn-on.
8
Rev. A.02