ame5251 AME, Inc., ame5251 Datasheet - Page 8

• Manufacturer Part Number: ame5251
• Description: Dual 1a, 1.5mhz Synchronous Step-down Converter
• Manufacturer: AME, Inc.

n Detailed Description
do decay, thereby preventing runaway. The oscillator’ s
frequency will progressively increase to 1.5MHz when V
or V
n Application Information
cal Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and followed by C
C
and operating frequency determine the ripple current. The
ripple current DIL increases with higher V
with higher inductance.
8
AME5251
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET)
switches are intermal. During normal operation, the in-
ternal top power MOSFET is turned on each cycle when
the oscillator sets the RS latch, and turned off when the
current comparator resets the RS latch. 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.
the oscillator is reduced to about 180KHz. This frequency
foldback ensures that the inductor current hsa more time
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.
OUT
The basic AME5251 application circuit is shown in Typi-
Inductor Selecton
For a given input and output voltage, the inductor value
Main Control Loop
AME5251 uses a constant frequency, current mode
Short-Circuit Protection
When the output is shorted to ground, the frequency of
Dropout Operation
As the input supply voltage decreases to a value ap-
OUT
.
rises abole 0V.
I
L
AME
f
1
L
V
OUT
1 (
V
V
OUT
IN
IN
)
and decreases
IN
and
FB
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, V
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V
part.
be at least equal to the maximum load current plus half
the ripple current to prevent core saturation. For better
efficiency, choose a low DC-resistance inductor.
ezoidal current at the source of the top MOSFET. To
prevent large voltage transients, a low ESR input
capacitorsized for the maximum RMS current must be
used. The maximum RMS capacitor current is given by:
IRMS=I
monly used for design because even significant devia-
tions do not offer much relief. Note that the capacitor
manufacturer ripple current ratings are often based on 2000
hours of life. This makes it advisable to further derate the
capacitor, or choose a capacitor rated at a higher tem-
perature than required.
series resistance(ESR) that is required to minimize volt-
age ripple and load step transients. The output ripple,
V
IL=0.4(lmax). The DC current rating of the inductor should
OUT
Higher values, lower cost ceramic capacitors are now
A reasonable starting point for setting ripple current is
C
The input capacitance, C
This formula has a maximum at V
The selection of C
Using Ceramic Input and Output Capacitors
Dual 1A, 1.5MHz Synchronous
IN
I
, is determined by:
RMS
and C
V
OUT
IN
OUT
. At best, this ringing can couple to the output
/2. This simple worst-case condition is com-
OUT
I
OUT
Selection
I
(
L
Step-Down Converter
max
ESR
OUT
)
is determined by the effective
IN
IN
V
large enough to damage the
V
is needed to filter the trap-
OUT
8
IN
f
1
C
OUT
V
V
IN
OUT
IN
=2V
OUT
1
Rev. A.02
, where