MP2259 – 1A, 16V, 1.4MHz STEP-DOWN CONVERTER
APPLICATION INFORMATION
Setting the Output Voltage
The external resistor divider is used to set the
output voltage (see the schematic on front
page). The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor (see Figure 1). R2 can
be determined by:
R2
=
R1
V
OUT
−
1
0.81V
with X5R or X7R dielectrics are highly
recommended because of their low ESR and
small temperature coefficients. For most
applications, a 4.7µF capacitor is sufficient.
Selecting the Output Capacitor
The output capacitor (C2) keeps output voltage
ripple small and ensures loop stability. The
output capacitor impedance should be low at
the switching frequency. Ceramic capacitors
with X5R or X7R dielectrics are recommended
for their low ESR characteristics. A 10µF~ 22µF
capacitor is good for most applications.
PCB Layout Guide
PCB layout is very important to achieve stable
operation. Please follow these guidelines and
take Figure2 for references.
1)
Keep the path of switching current short
and minimize the loop area formed by Input
cap, high-side MOSFET and schottky diode.
Keep the connection of schottky diode
between SW pin and input power ground
as short and wide as possible.
Ensure all feedback connections are short
and direct. Place the feedback resistors
and compensation components as close to
the chip as possible.
Route SW away from sensitive analog
areas such as FB.
Connect IN, SW, and especially GND
respectively to a large copper area to cool
the chip to improve thermal performance
and long-term reliability. For single layer,
do not solder exposed pad of the IC.
Table 1—Resistor Selection for Common
Output Voltages
V
OUT
(V)
1.8
2.5
3.3
5
R1 (kΩ)
80.6 (1%)
49.9 (1%)
49.9 (1%)
49.9 (1%)
R2 (kΩ)
64.9 (1%)
23.7 (1%)
16.2 (1%)
9.53 (1%)
Selecting the Inductor
A 1µH to 10µH inductor is recommended for
most applications. For highest efficiency, the
inductor’s DC resistance should be less than
200mΩ. For most designs, the required
inductance value can be derived from the
following equation:
L
=
V
OUT
×
( V
IN
−
V
OUT
)
V
IN
× Δ
I
L
×
f
OSC
2)
3)
4)
5)
Where
ΔI
L
is the inductor ripple current.
Choose an inductor with a rating current higher
than the maximum load current. The maximum
inductor peak current can be calculated from:
I
L(MAX )
=
I
LOAD
+
Δ
I
L
2
Under light load conditions below 100mA, a
larger inductance is recommended for improved
efficiency.
Selecting the Input Capacitor
The input capacitor (C1) reduces the surge
current drawn from the input and the switching
noise from the device. The input capacitor
impedance at the switching frequency should
be less than the input source impedance to
prevent high frequency switching current from
passing through the input. Ceramic capacitors
MP2259 Rev. 0.92
9/15/2011
www.MonolithicPower.com
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6
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