MP2235

– SYNCHRONOUS STEP-DOWN CONVERTER

MP2235 Rev. 1.0

www.MonolithicPower.com

13

5/16/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see Typical Application on page 1).

Choose R1 around 40kΩ for V

is then given by:

OUT

R1

R2

V

1

0.807V





The T-Type resistor R5 is used to control the

bandwidth

of

control

loop

which

will

be

introduced below.

Control Loop Compensation

MP2235 employs peak current mode control for

easy compensation and fast transient response.

To

simplify

the

compensation

design

and

minimize

external

components,

MP2235

integrates internal compensation. Figure 6 shows

an equivalent model for the device control loop.

VOUT

VOUT

VC

FB

300k

Ω 50pF

1pF

Figure 6: Equivalent Control Loop Model

The device power stage can be approximated to

a voltage controlled current source (duty cycle

modulator)

supplying

current

to

the

output

OUT

Z1

DC

C

P1

s

1

V

2f

A

s

V

1

2f











L

DC

i

R

A

R



Z1

ESR

OUT

1

f

2 R

C





P1

L

OUT

1

f

2 R C





output capacitance.

MP2235 uses voltage type amplifier for the

feedback

error

amplifier

and

integrates

compensation to ease the system design. The

output to control transfer function is given by:

C

Z2

Z3

EA

OUT

P2

P3

ss

(1

)(1

)

V

2 f

2 f

A

ss

V

s(1

)(1

)

2 f

2 f











2

EA

Z

P

1

T

1

2

2

T

R

A

(C

C )(R R

R R

R R )









Z2

ZZ

1

f

2 R C





Z3

1F

1

f

2 R C





P2

ZP

1

f

2 R C





P3

1

2

T

Z

1

f

2 (R // R // R )C





the T-type resistor between feedback resistor

divider

and

FB

pin.

is

the

type

III

capacitors.

The goal of compensation design is to shape the

converter transfer function to get a desired loop

gain. The system crossover frequency where the

feedback loop has the unity gain is important.

Lower crossover frequency results in slower line

and load transient responses, while higher

crossover

frequency

could

cause

system

instability. A good rule of thumb is to set the