AME5251 Datasheet, PDF (8/19 Pages) Analog Microelectronics – Dual 1A, 1.5MHz Synchronous Step-Down Converter

AME
AME5251
n Detailed Description
Main Control Loop
AME5251 uses a constant frequency, current mode
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.
Short-Circuit Protection
When the output is shorted to ground, the frequency of
the oscillator is reduced to about 180KHz. This frequency
foldback ensures that the inductor current hsa more time
do decay, thereby preventing runaway. The oscillator’ s
frequency will progressively increase to 1.5MHz when V_FB
or V_OUT rises abole 0V.
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.
8
Dual 1A, 1.5MHz Synchronous
Step-Down Converter
n Application Information
The basic AME5251 application circuit is shown in Typi-
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_IN and
C_OUT.
Inductor Selecton
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current DIL increases with higher V_IN and decreases
with higher inductance.
∆I _L
=
(
f
1
)(L
)
V_OUT
1− ^VOUT
V_IN
A reasonable starting point for setting ripple current is
∆IL=0.4(lmax). 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. For better
efficiency, choose a low DC-resistance inductor.
C_IN and C_OUT Selection
The input capacitance, C_IN is needed to filter the trap-
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:
I _RMS
=
I_{OUT (MAX )}
V_OUT
V_IN
V_{IN −1}
V_OUT
This formula has a maximum at V_IN=2V_OUT, where
IRMS=I_OUT/2. This simple worst-case condition is com-
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 temperature than required.
Rev. A.05