TM
MP1580 – 2A, 380KHz STEP-DOWN CONVERTER
APPLICATION INFORMATION
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.
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.
The inductance value can be calculated by the
equation:
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 VIN.
(VIN − VOUT
VIN × f × ∆I
)
L = VOUT
×
Where VIN 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.
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:
Table 1—Inductor Selection Guide
R2
Package
Dimensions
VFB = VOUT
R1+ R2
(mm)
Vendor/
Model
Core
Type Material
Core
Where VFB is the feedback voltage and VOUT is
the output voltage.
W
L
H
Sumida
CR75
CDH74
CDRH5D28 Shielded Ferrite
CDRH5D28 Shielded Ferrite
CDRH6D28 Shielded Ferrite
Thus the output voltage is:
Open
Open
Ferrite
Ferrite
7.0
7.3
5.5
5.5
6.7
7.8 5.5
8.0 5.2
5.7 5.5
5.7 5.5
6.7 3.0
R1+ R2
VOUT = 1.222 ×
R2
R2 can be as high as 100kꢀ, but a typical value
is 10kꢀ. Using this value, R1 is determined by:
CDRH104R Shielded Ferrite 10.1 10.0 3.0
R1 ≅ 8.18 × (VOUT − 1.222)
Toko
D53LC
Type A
D75C
Shielded Ferrite
Shielded Ferrite
5.0
7.6
5.0 3.0
7.6 5.1
For example, for a 3.3V output voltage, R2 is
10kꢀ and R1 is 17kꢀ.
Inductor
D104C
Shielded Ferrite 10.0 10.0 4.3
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.
D10FL
Open
Ferrite
9.7 11.5 4.0
Coilcraft
DO3308
DO3316
Open
Open
Ferrite
Ferrite
9.4 13.0 3.0
9.4 13.0 5.1
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
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