MP2105 – 1MHz 800mA SYNCHRONOUS STEP-DOWN CONVERTER

MP2105 Rev. 1.3

www.MonolithicPower.com

6

5/11/2006

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TM

Current Mode PWM Control

Slope compensated current mode PWM control

provides stable switching and cycle-by-cycle

current limit for superior load and line response

and protection of the internal main switch and

synchronous rectifier. The MP2105 switches at a

constant frequency (1MHz) and regulates the

output voltage. During each cycle the PWM

comparator modulates the power transferred to the

load by changing the inductor peak current based

on the feedback error voltage. During normal

operation, the main switch is turned on for a certain

time to ramp the inductor current at each rising

edge of the internal oscillator, and switched off

when the peak inductor current is above the error

voltage. When the main switch is off, the

synchronous rectifier will be turned on immediately

and stay on until either the next cycle starts.

Dropout Operation

The MP2105 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 MP2105 has short circuit protection. When

the 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 MP2105 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 (see Figure 3). The feedback resistor R1

also sets the feedback loop bandwidth with the

internal compensation capacitor (see Figure 1).

Choose R1 around 500kΩ 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

499k

499k

1.5V

499k

332k

1.8V

499k

249k

2.5V

499k

158k

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

×

∆

×

−

×

=

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.