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

MP2905 Rev. 0.91

www.monolithicpower.com

11

4/18/2011

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it should be at least 20% higher than the input

supply rail at the high-side MOSFET’s drain.

4) Total gate charge Qg: the lower, the better it

will be. For high-side MOSFET, the main power

loss consists of conduction loss, switching loss,

and drive loss. The high-side MOSFET

conduction loss can be calculated by:

2

high side _ conduction

LOAD

highside _ DSON

PI

R

D

−

=×

×

Where D is the duct cycle, it’s defined by:

OUT

IN

V

D

V

=

High-side MOSFET switching loss is calculated

by:

high side _ switching

IN

LOAD

ON

OFF

S

1

PV

I

(t

t

) f

2

−

=×

×

+

×

switching frequency.

High-side MOSFET drive loss is calculated by:

high side _ drive

g _ high side

S

drive

PQ

f

V

−−

=× ×

Where Vdrive is the high-side MOSFET driving

voltage, typical value is 5V.

For low-side MOSFET, there isn’t switching loss,

conduction loss is the main loss, so we’d better

choice a MOSFET with lower Rds-on than high

side MOSFET. The recommended Rds-on of

low side MOSFET is one-third of high-side

MOSFET. The low-side MOSFET loss consists

of conduction loss, drive loss and body diode

conduction

loss.

The

Low-side

MOSFET

conduction loss is calculated by:

2

low side _ conduction

LOAD

low side _ DSON

PI

R

(1 D)

−−

=×

× −

Low-side MOS drive loss is calculated by:

low side _ drive

g _ low side

S

drive

PQ

f

V

−−

=× ×

Body diode conduction loss is calculated by:

bodydiode

F

LOAD

deadtime

S

P2 V

I

t

f

=×

×

×

×

MOFETS transition time.

Except the losses above, there still is output

cap loss in both high side MOSFET and low

side MOSFET. Output cap loss is defined by:

2

Cds

DS

DS

S

1

PC

V

f

2

= ××

×

For less switching noise, add drive resistors in

series with the gate of MOSFET to slow down

the transition between the high-side MOSFET

and low-side MOSFET switching.

Selecting the Feed Forward Capacitor

The feed forward capacitor (C8 in front page

typical application circuit) is a key factor to

affect the frequency. It can be calculated by:

3

O

FB

S

IN

FB

FB

IN

H

FB

V

V

1

f(1

)

VV

1

RC8

V

V

(90ns

20ns

)

C8

R

R

==

×

× −

×

+×

−

×

feedback reference voltage, typical is 590mV,

voltage-divided resistors. For example, in 2905

typical application:

FB

R1 R3

R

R1 R3

+

=

×

Select an X7R ceramic capacitor with the

closest Capacitance to the value calculated as

possible.

Increase

the

Capacitance,

the

switching frequency decrease, and vice versa,

decrease the Capacitance, the frequency

increase.

And output capacitor, inductor and inductor

DCR will affect the frequency, too, but those are

limited.

The frequency calculated by the formula has a

deviation within 30%.

Setting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down

converter while maintaining the DC input

voltage. Use low ESR capacitors for the best

performance. Ceramic capacitors with X5R or

X7R dielectrics are highly recommended

because

of

their

low

ESR

and

small

temperature coefficients.