MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER

MP2905 Rev. 0.91

www.monolithicpower.com

12

4/18/2011

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© 2011. All Rights Reserved.

Since the input capacitor (Cin) absorbs the

input switching current, it requires an adequate

ripple current rating. The RMS current in the

input capacitor can be estimated by:

OUT

OUT

Cin

LOAD

IN

IN

VV

II

(1

)

VV

=×

× −

where

LOAD

Cin

I

I

2

=

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic

capacitor, i.e. 0.1µF, should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure that they have enough

capacitance to provide sufficient charge to

prevent excessive voltage ripple at input

terminal. The input voltage ripple can be

estimated by capacitance:

LOAD

OUT

OUT

IN

Sin

IN

IN

IV

V

V(1

)

fC

V

V

D=

´

´

-

´

Setting the Output capacitor

The output capacitor (Cout) is required to

maintain the DC output voltage. Ceramic,

tantalum, or low ESR electrolytic capacitors are

recommended.

Low

ESR

capacitors

are

preferred to keep the output voltage ripple low.

The output voltage ripple can be estimated by:

OUT

OUT

OUT

ESR

SIN

S

OUT

VV

1

V(1

) (R

)

fL1

V

8 f

C

D=

´

-

´

+

´´

´

equivalent series resistance (ESR) value of the

output capacitor.

In the case of ceramic capacitors, the

impedance at the switching frequency is

dominated by the capacitance. The output

voltage ripple is mainly determined by the

capacitance. For simplification, the output

voltage ripple is:

OUT

OUT

OUT

2

IN

SOUT

VV

V(1

)

V

8f

L1 C

D=

´

-

´´

´

In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple is:

OUT

OUT

OUT

ESR

SIN

VV

V(1

) R

fL1

V

D=

´

-

´

´

The characteristics of the output capacitor also

affect the transient response and the stability of

the regulation system. Higher voltage ripple will

influence the voltage of the feed forward

capacitor to make the system be unstable. So,

at the condition which the tantalum or

electrolytic capacitors with higher ESR is used

or output current is higher, a RC filter is

necessary from Vout to GND. Connect the

resistor of filter between the Vout and feed

forward capacitor, and connect the capacitor of

filter from feed forward capacitor to GND.

Follow the R7 and C12 connection in MP2905

typical application. 10Ω/1uF or 2Ω/4.4uF is

recommended for good stability and better

transient response.