MP3115 MPS, MP3115 Datasheet - Page 7

• Manufacturer Part Number: MP3115
• Description: High-efficiency, Single-cell Alkaline, 1.3mhz Synchronous Step-up Converter With Output Disconnect
• Manufacturer: MPS

APPLICATION INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. The voltage divider drops
the output voltage to the 1.2V feedback voltage.
Use 20kΩ for the low-side resistor (R2) of the
voltage divider. Determine the high-side resistor
(R1) by the equation:
Where V
1.2V feedback voltage and R2=20kΩ.
Selecting the Input Capacitor
An input capacitor is required to supply the AC
ripple current to the inductor while limiting noise
at
capacitors are recommended as they have
extremely low ESR and are available in small
footprints. Use an input capacitor of 4.7µF or
greater, and place it physically close to the
device.
Selecting the Output Capacitor
A single 4.7µF to 10µF ceramic capacitor
normally provides sufficient output capacitance
for most applications. Larger values (up to 22µF)
may be used to obtain extremely low output
voltage ripple and improve transient response.
The impedance of the ceramic capacitor at the
switching
capacitance, so the output voltage ripple is
mostly independent of ESR. The output voltage
ripple V
Where V
current, C2 is the capacitance of the output
capacitor and f
frequency.
MP3115 Rev. 0.9
1/25/2008
the
RIPPLE
IN
OUT
input
is the input voltage, I
frequency
V
RIPPLE
is calculated as:
is the output voltage, V
R
SW
1
source.
=
=
I
MP3115 – HIGH-EFFICIENCY 1.3MHz SYNCHRONOUS STEP-UP CONVERTER
V
is the 1.3MHz switching
LOAD
V
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
OUT
O
⎛
⎜ ⎜
⎝
is
UT
V
R
(
FB
V
−
2
×
O
Multi-layer
dominated
V
C
⎞
⎟ ⎟
⎠
UT
FB
2
×
−
f
LOAD
V
SW
IN
)
is the load
FB
© 2008 MPS. All Rights Reserved.
ceramic
by
www.MonolithicPower.com
is the
its
Selecting the Inductor
The inductor is required to force the output
voltage higher while being driven by the lower
input voltage. A good rule for determining the
inductance is to allow the peak-to-peak ripple
current to be approximately 30%-50% of the
maximum input current. Make sure that the
peak inductor current is below the minimum
current limit at the duty cycle used to prevent
loss of regulation due to current limit variation.
Calculate the required inductance value L using
the equations:
Where I
is the peak-to-peak inductor ripple current and η
is the efficiency. For the MP3115, 4.7µH is
recommended for most applications. Choose an
inductor that does not saturate at the peak
switch
additional margin to cover for heavy load
transients and extreme startup conditions.
Selecting the Feed-Forward Capacitor
A feed-forward capacitor in parallel with the
high-side resistor R1 can be added to improve
the
conduction modes and the load transient
response. A 47pF capacitor is recommended
for most applications.
LAYOUT CONSIDERATIONS
High frequency switching regulators require
very careful layout for stable operation and low
noise. All components must be placed as close
to the IC as possible. All feedback components
must be kept close to the FB pin to prevent
noise injection on the FB pin trace. The ground
return of C1 and C2 should be tied close to the
GND pin. See the MP3115 demo board layout
for reference.
output
LOAD(MAX)
current
I
IN
∆
(
MAX
I
ripple
L
=
is the maximum load current, ∆I
=
(
)
30
as
=
V
V
%
IN
OUT
V
(V
OUT
−
at
calculated
50
OUT
×
V
f
×
SW
%
IN
I
LOAD
both
-
)
×
I
V
IN
×
η
IN
(
∆
MAX
(
MAX
)
I
)
discontinuous
)
above
with
7