MP3900 – HIGH EFFICIENCY BOOST CONTROLLER

MP3900 Rev. 1.0

www.MonolithicPower.com

7

12/17/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. If we use 10kΩ for the low-

side resistor (R2) of the voltage divider, we can

determine the high-side resistor (R1) by the

equation:

REF

REF

OUT

V

)

V

V

(

2

R

1

R

−

×

=

R1=301kΩ.

Selecting the Inductor and Current Sensing

Resistor

The inductor is required to transfer the energy

between the input source and the output

capacitors. A larger value inductor results in

less ripple current that results in lower peak

inductor current, and therefore reduces the

stress on the power MOSFET. However, the

larger value inductor has a larger physical size,

higher

series

resistance,

and/or

lower

saturation current.

A good rule of thumb is to allow the

peak-to-peak ripple current to be approximately

30-50% of the maximum input current. Make

sure that the peak inductor current is below

80% of the IC’s maximum current limit at the

operating duty cycle to prevent loss of

regulation. Make sure that the inductor does not

saturate under the worst-case load transient

and startup conditions. The required inductance

value can be calculated by :

I

f

V

)

V

-

(V

V

L

SW

OUT

IN(MIN)

OUT

)

MIN

(

IN

Δ

×

×

×

=

η

×

×

=

)

MIN

(

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

()

)

MAX

(

IN

I

%

50

%

30

I

−

=

Δ

ΔI is the peak-to-peak inductor ripple current

and η is the efficiency. For a typical design,

boost converter efficiency can reach 85%~95%.

ripple percentage being 30%,

η=95% and

8.8µH inductor (i.e. Sumida CDRH127/LDNP-

100MC).

The switch current is usually used for the peak

current mode control. In order to avoid hitting

the current limit, the voltage across the sensing

worst case current limit voltage, 200mV.

)

PEAK

(

L

SENSE

I

2

.

0

8

.

0

R

×

=

current.

MOSFET is used as the sensing resistor, be

calculated above, 30mΩ

Another factor to take into consideration is the

increases.. Device vendors will usually provide

temperature

coefficient

in

the

datasheet.

Generally, the MOSFET on resistance will

double from 25

°C to 125°C.

Selecting the Input Capacitor

An input capacitor (C1) is required to supply the

AC ripple current to the inductor, while limiting

noise at the input source. A low ESR capacitor

is required to keep the noise to the IC at a

minimum. Ceramic capacitors are preferred, but

tantalum or low-ESR electrolytic capacitors may

also suffice.

The capacitance can be calculated as:

SW

IN(RIPPLE)

1

f

V

8

I

C

×

Δ

×

Δ

≈

Where ΔI is the peak-to-peak inductor ripple

ripple. When using ceramic capacitors, take into

account the vendor specified voltage and

temperature coefficients for the particular

dielectric being used.