TM
MP1580 – 2A, 380KHz STEP-DOWN CONVERTER
APPLICATION INFORMATION
COMPONENT SELECTION
Sync Pin Operation
The SYNC pin driving waveform should be a
square wave with a rise time less than 20ns.
The Minimum High voltage level is 2.7V and the
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 SW to V
IN
.
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output to FB (see
Figure 3). The voltage divider divides the output
voltage down by the ratio:
V
FB
=
V
OUT
R2
R1
+
R2
A good rule for determining the inductance is to
allow the peak-to-peak ripple current in the
inductor to be approximately 30% of the
maximum load current. Also, make sure that the
peak inductor current (the load current plus half
the peak-to-peak inductor ripple current) is
below the 2.4A minimum current limit.
The inductance value can be calculated by the
equation:
L
=
V
OUT
×
( V
IN
−
V
OUT
)
V
IN
×
f
× ∆
I
Where V
IN
is the input voltage, f is the oscillator
frequency and
∆I
is the peak-to-peak inductor
ripple current. Table 1 lists a number of suitable
inductors from various manufacturers.
Table 1—Inductor Selection Guide
Package
Dimensions
(mm)
W
L
H
7.0
7.3
5.5
5.5
6.7
10.1
5.0
7.6
10.0
9.7
9.4
9.4
7.8
8.0
5.7
5.7
6.7
10.0
5.0
7.6
10.0
11.5
13.0
13.0
5.5
5.2
5.5
5.5
3.0
3.0
3.0
5.1
4.3
4.0
3.0
5.1
Where V
FB
is the feedback voltage and V
OUT
is
the output voltage.
Thus the output voltage is:
V
OUT
=
1.222
×
R1
+
R2
R2
Vendor/
Model
Sumida
CR75
CDH74
CDRH5D28
CDRH5D28
CDRH6D28
CDRH104R
Toko
D53LC
Type A
D75C
D104C
D10FL
Coilcraft
DO3308
DO3316
Core
Type
Open
Open
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Open
Open
Open
Core
Material
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
R2 can be as high as 100kΩ, but a typical value
is 10kΩ. Using this value, R1 is determined by:
R1
≅
8.18
×
( V
OUT
−
1.222)
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 results in less ripple current that in turn
results in lower output ripple voltage.
However, the larger value inductor has a larger
physical size, higher series resistance and/or
lower saturation current. Choose an inductor
that does not saturate under the worst-case
load conditions.
MP1580 Rev. 3.0
12/5/2005
www.MonolithicPower.com
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