MP2109 – DUAL 1.2MHz 800mA SYNCHRONOUS STEP-DOWN CONVERTER
MP2109 Rev.1.4 www.MonolithicPower.com 6
1/22/2010 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
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Dropout Operation
Each channel of the MP2109 allows the main
switch to remain on for more than one switching
cycle and increases the duty cycle while the
input voltage is dropping close to the output
voltage. When the duty cycle reaches 100%,
the main switch is held on continuously to
deliver current to the output up to the PFET
current limit. The output voltage then is the
input voltage minus the voltage drop across the
main switch and the inductor.
Short Circuit Protection
The MP2109 has short circuit protection. When
any output is shorted to ground, the oscillator
frequency is reduced to prevent the inductor
current from increasing beyond the PFET
current limit. The PFET current limit is also
reduced to lower the short circuit current. The
frequency and current limit will return to the
normal values once the short circuit condition is
removed and the feedback voltage reaches
0.6V.
Maximum Load Current
The MP2109 can operate down to 2.5V input
voltage, however the maximum load current
decreases at lower input due to large IR drop
on the main switch and synchronous rectifier.
The slope compensation signal reduces the
peak inductor current as a function of the duty
cycle to prevent sub-harmonic oscillations at
duty cycles greater than 50%. Conversely the
current limit increases as the duty cycle
decreases.
APPLICATION INFORMATION
Output Voltage Setting
The external resistor divider sets the output
voltage. The feedback resistor R1 also sets the
feedback loop bandwidth with the internal
compensation capacitor (see Figure 1).
Choose R1 around 300kΩ for optimal transient
response. R2 is then given by:
1
V6.0
V
1R
2R
OUT
−
=
Table 1—Resistor Selection vs. Output
Voltage Setting
V
OUT
R1 R2
1.2V
300k
Ω (1%) 300kΩ (1%)
1.5V
300kΩ (1%) 200kΩ (1%)
1.8V
300kΩ (1%) 150kΩ (1%)
2.5V
300kΩ (1%) 95.3kΩ (1%)
Inductor Selection
A 1µH to 10µH inductor with DC current rating
at least 25% higher than the maximum load
current is recommended for most applications.
For best efficiency, the inductor DC resistance
shall be <200mΩ. See Table 2 for
recommended inductors and manufacturers.
For most designs, the inductance value can be
derived from the following equation:
()
OSCLIN
OUTINOUT
fIV
VVV
L
×∆×
−×
=
Where ∆I
L
is inductor ripple current. Choose
inductor ripple current approximately 30% of the
maximum load current, 800mA.
The maximum inductor peak current is:
2
I
II
L
LOAD)MAX(L
∆
+=
Under light load conditions below 100mA, larger
inductance is recommended for improved
efficiency. Table 3 lists inductors recommended
for this purpose.
