ame5252 AME, Inc., ame5252 Datasheet - Page 10

• Manufacturer Part Number: ame5252
• Description: Dual Synchronous, 600ma, 1.5mhz Step-down Dc/dc Converter
• Manufacturer: AME, Inc.

architecture. The operating frequency is set at 1.5MHz
and can be synchronized to an external oscillator. Both
channels share the same clock and run in-phase.
to the V
output voltage with a reference voltage of 0.6V and adjusts
the peak inductor current accordingly. Overvoltage and
undervoltage comparators will pull the PORB output low if
the output voltage is not within 8.5%. The PORB output
will go high after 262,144 clock cycles (about 175ms) of
achieving regulation.
nel MOSFET) is turned on at the beginning of a clock
cycle when the V
The current into the inductor and the load increases until
the current limit is reached. The switch turns off and en-
ergy stored in the inductor flows through the bottom switch
(N-channel MOSFET) into the load until the next clock
cycle.
compensated COMP voltage, which is the output of the
error amplifier. This amplifier compares the V
0.6V reference. When the load current increases, the V
voltage decreases slightly below the reference. This de-
crease causes the error amplifier to increase the COMP
voltage until the average inductor current matches the new
load current. The main control loop is shut down by pull-
ing the EN pin to ground.
the oscillator is reduced to about 210kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the in-
ductor current has more time to decay, thereby preventing
runaway. The oscillator's frequency will progressively in-
crease to 1.5MHz when V
10
AME5252
n Detailed Description
Main Control Loop
The peak inductor current is controlled by the internally
During normal operation, the top power switch (P-chan-
Short-Circuit Protection
The AME5252 uses a constant frequency, current mode
The output voltage is set by an external divider returned
When the output is shorted to ground, the frequency of
FB
pins. An error amplifier compares the divided
AME
FB
voltage is below the reference voltage.
FB
or V
OUT
rises above 0V.
FB
pin to the
FB
Dual Synchronous, 600mA, 1.5MHz
output voltage, the duty cycle increases to 100% which is
the dropout condition. In dropout, the P-channel MOSFET
switch is turned on continuously with the output voltage
being equal to the input voltage minus the voltage drops
across the internal P-channel MOSFET and the inductor.
the P-channel switch increases with decreasing input sup-
ply voltage (See Typical Performance Characteristics).
tion when the AME5252 is used at 100% duty cycle with
low input voltage.
n Application Information
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-resistance
inductor.
in the range of 1 H to 4.7 H. 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
rent as shown in equation 1. A reasonable starting point
for setting ripple current is IL = 240mA (40% of 600mA).
Dropout Operation
When the input supply voltage decreases toward the
An important design consideration is that the R
Therefore, the user should calculate the power dissipa-
The DC current rating of the inductor should be at least
Inductor Selection
For most applications, the value of the inductor will fall
Step-Down DC/DC Converter
I
L
f
1
IN
L
or V
V
OUT
OUT
c
also increases the ripple cur-
1 (
V
V
OUT
IN
)
Rev. A.01
DSON
of