MP2109 – DUAL 1.2MHz 800mA SYNCHRONOUS STEP-DOWN CONVERTER

MP2109 Rev.1.3

www.MonolithicPower.com

6

2/21/2006

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TM

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

V

6

.

0

V

1

R

2

R

=

Table 1—Resistor Selection vs. Output

Voltage Setting

R1

R2

1.2V

300k

300k

1.5V

300k

200k

1.8V

300k

150k

2.5V

300k

95.3k

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:

()

OSC

L

IN

OUT

IN

OUT

f

I

V

V

V

V

L

×

∆

×

−

×

=

inductor ripple current approximately 30% of the

maximum load current, 800mA.

The maximum inductor peak current is:

2

I

I

I

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.