MP1517 – 3A, 25V, 1.1MHz STEP-UP CONVERTER

MP1517 Rev. 1.4

www.MonolithicPower.com

8

4/28/2006

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© 2006 MPS. All Rights Reserved.

The DC loop gain is:

2

OUT

FB

LOAD

IN

VEA

V

V

R

V

A

7

×

×

×

×

exists in the continuous conduction mode

(inductor current does not drop to zero on each

cycle) step-up converters. The frequency of the

right-half plane zero is:

2

2

OUT

LOAD

IN

V

L

2

R

V

×

×

π

×

×

To stabilize the regulation control loop, the

crossover frequency (The frequency where the

loop gain drops to 0dB or a gain of 1) should be

maximum output load current.

In some cases, an output capacitor with a high

capacitance

and

high

equivalent

series

resistance (ESR) is used, then a second

compensation capacitor (from COMP to SGND)

is required to compensate for the zero

introduced by the output capacitor ESR. The

extra capacitor is required if the ESR zero is

less than 4x the crossover frequency. The ESR

zero frequency is:

ESR

R

C2

2

1

×

×

π

×

If this is the case, calculate the second

compensation capacitor by the equation:

C6 =

3

R

R

C2

ESR

×

For most applications C6 is not required.

Typical

values

for

the

compensation

components are:

C3 = 10nF

R3 = 2.2kΩ

SEPIC CONVERTER COMPONENT

SELECTION

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor, while limiting

noise at the input source. The input capacitor

selection is the same as that in the General

Purpose Component Selection section above.

Selecting the Inductors

The SEPIC converter inductors (refer to Figure

4) are required to store energy, and generate

an output voltage that is less than or greater

than the input voltage. If a coupled inductor is

used in a SEPIC converter, then the mutual

inductance of each winding forces each

inductor

to

become

twice

the

original

inductance. Therefore smaller inductance can

be used with a coupled inductor. But the core

saturation of the coupled inductors is related to

the sum of both inductor currents.

There are two current paths to the internal

N-Channel MOSFET switch in a SEPIC

converter. One is from L1 and the other is from

L2.

Each inductor’s ripple current can be defined

as:

I

f

D

V

1

L

SW

IN

∆

×

×

=

I

f

D)

1

(

V

2

L

SW

UT

O

∆

×

−

×

=

IN

D

OUT

D

OUT

V

V

V

V

V

D

+

+

+

=

∆I is the peak to peak inductor ripple current.

Set ∆I to approximately 20% of the maximum

switch current. Each inductor’s peak current is:

IN

D

OUT

LOAD

)

PEAK

(

1

L

V

V

V

I

2

I

I

+

×

+

∆

=

IN

D

IN

LOAD

)

PEAK

(

2

L

V

V

V

I

2

I

I

+

×

+

∆

=

The total of these two currents is the total

switch current, and should be less than the

minimum device current limit of 3A.