AME5250 Datasheet, PDF (8/21 Pages) Analog Microelectronics – 1A, 1.5MHz Synchronous Step-Down Converter

AME
AME5250
1A, 1.5MHz Synchronous
Step-Down Converter
n Detailed Description
Main Control Loop
AME5250 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 inter-
nal top power MOSFET is turned on each cycle when the
oscillator sets the RS latch, and turned off when the cur-
rent 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 above 0V.
Dropout Operation
As the input supply voltage decreases to a value ap-
proaching the output voltage, the duty cycle increases to-
ward the maximum on-time. Further reduction of the sup-
ply 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.
n Application Information
The basic AME5250 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 Selection
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 −
V_OUT
V_IN
)
A reasonable starting point for setting ripple current is
∆I_L=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 pre-
vent 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 )}
× ^VOUT
V_IN
×
V_{I N −1}
V_OUT
This formula has a maximum at V_IN=2V_OUT, where
I_RMS=I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations 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 re-
quired.
8
Rev. B.04