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MP1517

3A, 25V, 1.1MHz Step-Up Converter

Monolithic Power Systems 

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

The DC loop gain is:

AVDC =

7 × A VEA

× VIN × RLOAD × VFB

V OUT 2

Where VFB is the feedback regulation threshold.

There is also a right-half-plane zero (fRHPZ) that

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:

fRHPZ =

VIN2 × RLOAD

2 × π ×L × VOUT 2

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

less than half of fRHPZ and should be at most

75KHz. fRHPZ is at its lowest frequency at

maximum output load current.

In some cases, an output capacitor with a high

capacitance and high equivalent series

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.

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:

fZESR =

1

2 × π × C2 × RESR

If this is the case, calculate the second

compensation capacitor by the equation:

C6 = C2×RESR

R3

For most applications C6 is not required.

Typical values for the compensation

components are:

C3 = 10nF

R3 = 2.2kΩ

Each inductor’s ripple current can be defined

as:

L1 = VIN × D

fSW × ∆I

L2 = VOUT × (1 − D)

fSW × ∆I

D=

VOUT + VD

VOUT + VD + VIN

Where VD is the voltage drop on diode D1, and

∆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:

IL1(PEAK )

=

∆I

2

+

ILOAD

×

VOUT + VD

VIN

IL2(PEAK )

=

∆I

2

+ ILOAD

×

VIN + VD

VIN

The total of these two currents is the total

switch current, and should be less than the

minimum device current limit of 3A.

MP1517 Rev. 1.4

www.MonolithicPower.com

8

4/28/2006

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© 2006 MPS. All Rights Reserved.

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