MP2354 – 2A, 23V, 380KHz STEP-DOWN CONVERTER
MP2354 Rev. 1.4 www.MonolithicPower.com 6
4/19/2011 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
APPLICATION INFORMATION
MP2354_TAC_F02
MP2354
LX
VIN BST
FB
SYNC
COMPGND
RUN
OUTPUT
2.5V / 2A
INPUT
4.75V to 23V
OPEN
IF NOT USED
OPEN
AUTOMATIC
STARTUP
C6
OPEN
C3
3.3nF
C5
10nF
D1
B230A
32
4
6
75
1
8
Figure 2—Typical Application Circuit
Sync Pin Operation
The SYNC pin driving waveform should be a
square wave with a rise time less than 20ns.
Minimum High voltage level is 2.7V. Low level
is less than 0.8V. The frequency of the external
sync signal needs to be greater than 445KHz.
A rising edge on the SYNC pin forces a reset of
the oscillator. The upper transistor M1 is
switched off immediately if it is not already off.
250ns later M1 turns on connecting LX to V
IN
.
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output to FB (see
Figure 2). The voltage divider divides the output
voltage down by the ratio:
2R1R
2RV
V
OUT
FB
+
×
=
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
Thus the output voltage is:
)
2R
2R1R23.1
V
OUT
+×
=
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using that value, R1 is determined by:
()()
Ω−×= k23.1V18.81R
OUT
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 17kΩ.
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 will result in less ripple current that will
result in lower output ripple voltage. However,
the larger value inductor will have a larger
physical size, higher series resistance, and/or
lower saturation current. A good rule for
determining the inductance to use is to allow
the peak-to-peak ripple current in the inductor
to be approximately 30% of the maximum
switch current limit. Also, make sure that the
peak inductor current is below the maximum
switch current limit. The inductance value can
be calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
×
=
IN
OUT
LS
OUT
V
V
1
∆If
V
L
Where V
IN
is the input voltage, f
S
is the 380KHz
switching frequency, and ∆I
L
is the peak-to-
peak inductor ripple current.
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×
××
+=
IN
OUT
S
OUT
LOADLP
V
V
1
Lf2
V
II
Where I
LOAD
is the load current.
