MP2106DQ MPS [Monolithic Power Systems], MP2106DQ Datasheet - Page 7

• Manufacturer Part Number: MP2106DQ
• Description: 1.5A, 15V, 800KHz Synchronous Buck Converter
• Manufacturer: MPS [Monolithic Power Systems]

voltage to ramp linearly from 0V to the desired
regulation voltage during soft-start.
The soft-start period is determined by the
equation:
Where C5 (in nF) is the soft-start capacitor from
SS to GND, and t
period. Determine the capacitor required for a
given soft-start period by the equation:
Use values between 10nF and 22nF for C5 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 0.895V feedback
voltage. Use 10kΩ for the low-side resistor of
the voltage divider. Determine the high-side
resistor by the equation:
Where R2 is the high-side resistor, V
output voltage and R1 is the low-side resistor.
Selecting the Input Capacitor
The input current to the step-down converter is
discontinuous, and 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 may also suffice.
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
MP2106 Rev. 1.6
2/22/2006
TM
R
2
=
t
C
SS
⎛
⎜ ⎜
⎝
5
. 0
SS
=
V
=
895
OUT
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
. 2
. 0
(in ms) is the soft-start
22
45
V
×
×
−
t
C
1
SS
⎞
⎟ ⎟
⎠
5
×
R
MP2106 – 1.5A, 15V, 800KHz SYNCHRONOUS BUCK CONVERTER
1
OUT
© 2006 MPS. All Rights Reserved.
www.MonolithicPower.com
is the
capacitors should be placed close to the
MP2106. 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
tantalum, or low ESR electrolytic capacitors are
recommended.
The output voltage ripple is:
Where V
the switching frequency, V
R
output capacitors and f
frequency.
Choose an output capacitor to satisfy the output
ripple requirements of the design. A 22µF
ceramic
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 will
result in lower output ripple voltage. However,
the larger value inductor is likely to have a
larger
resistance. Choose an inductor that does not
saturate under the worst-case load conditions.
A good rule for determining the inductance is to
allow the 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.
ESR
V
f
SW
V
RIPPLE
is the equivalent series resistance of the
regulation
OUT
×
physical
RIPPLE
L
×
capacitor
=
⎛
⎜ ⎜
⎝
1
−
is the output voltage ripple, f
V
V
OUT
IN
control
size
⎞
⎟ ⎟
⎠
×
is
⎛
⎜
⎜
⎝
R
SW
IN
and
ESR
suitable
system.
is the input voltage,
is the switching
+
8
higher
×
f
SW
1
for
Ceramic,
×
C
series
2
most
SW
⎞
⎟
⎟
⎠
is
7