EV2108DQ/DK-00A Просмотр технического описания (PDF) - Monolithic Power Systems
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EV2108DQ/DK-00A Datasheet PDF : 12 Pages
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MP2108 – 2A, 6V, 740KHz SYNCHRONOUS BUCK CONVERTER
rate as the voltage at SS, forcing a linear
output voltage ramp from 0V to the desired
regulation voltage during soft-start.
The soft-start period is determined by the
equation:
t SS = 0.45 × C5
Where C5 (in nF) is the soft-start capacitor from
SS to GND, and tSS (in ms) is the soft-start
period. Determine the capacitor required for a
given soft-start period by the equation:
C5 = 2.22 × tSS
Use values for C5 between 10nF and 22nF to
set the soft-start period between 4ms and
10ms.
Setting the Output Voltage (see Figure 2)
Set the output voltage by selecting the resistive
voltage divider ratio. The voltage divider drops
the output voltage to the 0.9V feedback voltage.
Use 10kΩ for the low-side resistor of the
voltage divider. Determine the high-side resistor
by the equation:
R2
=
⎜⎜⎝⎛
VOUT
0.9V
− 1⎟⎟⎠⎞ × R1
Where R2 is the high-side resistor, R1 is the
low-side resistor and VOUT is the output voltage.
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous, so a capacitor is required to
supply the AC current to the step-down
converter while maintaining the DC input
voltage. A low ESR capacitor is required to
keep the noise at the IC to a minimum. Ceramic
capacitors are preferred, but tantalum or low
ESR electrolytic capacitors are also an option.
The capacitor can be electrolytic, tantalum or
ceramic. Because it absorbs the input switching
current, it must have an adequate ripple current
rating. Use a capacitor with RMS current rating
greater than 1/2 of the DC load current.
For stable operation, place the input capacitor
as close to the IC as possible. A smaller high
quality 0.1µF ceramic capacitor may be placed
closer to the IC with the larger capacitor placed
further away.
If using this technique, it is recommended that
the larger capacitor be a tantalum or electrolytic
type. All ceramic capacitors should be placed
close to the IC. For most applications, a 10µF
ceramic capacitor will work.
Selecting the Output Capacitor
The output capacitor (C2) is required to
maintain the DC output voltage. Low ESR
capacitors are preferred to keep the output
voltage ripple to a minimum. The characteristics
of the output capacitor also affect the stability of
the regulation control system. Ceramic,
tantalum, or low ESR electrolytic capacitors are
recommended.
The output voltage ripple is:
VRIPPLE =
VOUT
fSW × L
× ⎜⎜⎝⎛1 −
VOUT
VIN
⎟⎟⎠⎞ × ⎜⎜⎝⎛RESR
+
1
8 × fSW
× C2 ⎟⎟⎠⎞
Where VRIPPLE is the output voltage ripple, fSW is
the switching frequency, VIN is the input voltage
and RESR is the equivalent series resistance of
the output capacitors.
Choose an output capacitor to satisfy the output
ripple requirements of the design. A 22µF
ceramic capacitor is suitable for most
applications.
Selecting the Inductor
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor results in less ripple current that in turn
results in lower output ripple voltage. However,
the larger value inductor is likely to have a
larger physical size and higher series
resistance. Choose an inductor that does not
saturate under the worst-case load conditions.
A good rule for determining the inductance is to
allow peak-to-peak ripple current to be
approximately 30% to 40% of the maximum
load current. Make sure that the peak inductor
current (the load current plus half the peak-to-
peak inductor ripple current) is below 2.5A to
prevent loss of regulation due to the current
limit.
MP2108 Rev 1.1
www.MonolithicPower.com
8
10/2/2006
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© 2006 MPS. All Rights Reserved.
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