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

MP1517 Rev. 1.4

www.MonolithicPower.com

9

4/28/2006

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© 2006 MPS. All Rights Reserved.

Selecting the Output Diode

The output diode is typically a Schottky diode.

The Schottky diode is selected based upon

voltage requirement, current rating, and thermal

capability. The diode breakdown and power

switch breakdown voltage are set higher than

both of these devices. The current rating is set

based on the average load current in the diode.

The average diode current is equal to the load

current, but the peak current and power

consumption on D1 is:

IN

D

IN

LOAD

)

PEAK

(

1

D

V

V

V

I

2

I

I

+

×

+

∆

=

D

LOAD

1

D

V

I

P

×

=

Selecting the Coupling Capacitor

The steady state voltage across L1 and L2 is

equal to zero. Therefore the coupling capacitor

capacitor will need to be rated for an input

voltage plus some guard band. Also this

IN

D

OUT

LOAD

)

RMS

(

8

C

V

V

V

I

I

+

×

=

Application Examples

Figure 2 shows a typical application circuit

driving multiple strings of LEDs with the

MP1517. The 3 strings of 6 white LEDs can be

driven from a voltage supply range of 2.6V to

6V at an output current of 20mA. A 1µF output

capacitor is usually sufficient for this kind of

application. A 4.7µH inductor with low DCR

(inductor

resistance)

is

recommended

to

improve efficiency. A 10µF ceramic capacitor is

recommended

for

the

input

capacitance.

Schottky diodes have fast recovery and a low

forward voltage and are recommended. The

MP1517 soft-start helps to limit the amount of

the amount of overshoot on the output.

Figure 3 shows a typical application running the

MP1517 in flash mode. During preview mode,

resistor R2 sets the current through the white

LEDs to 20mA. When a flash is required the

N-Channel MOSFET Q1 is turned on and

150mA

flows

through

the

LEDs.

The

compensation capacitor has been increased to

47nF. This forces the compensation node to

slowly rise when Q1 is turned on which allows

the current through the inductor to slowly

increase without overshoot. By doing this the

inrush current on the input is minimized.

Figure 4 shows a SEPIC circuit using the

MP1517 to generate 3.3V output from a 3V to

4.2V input. A peak voltage detect circuit (D2

and C7) is added to the IN pin from the highest

voltage potential (SW node) in the entire circuit.

The average voltage on C7 is roughly the sum

of the internal MOSFET switch. A transformer

with two windings can be used to replace the

two separate inductors in Figure 4. In this case,

the effective inductance value is doubled due to

the mutual inductance of each winding. This

reduces the inductor ripple current and

improves efficiency. Figures 5 and 6 show

some other SEPIC application circuits.

Layout Consideration

High frequency switching regulators require

very careful layout for stable operation and low

noise. All components must be placed as close

to the IC as possible. Keep the path between

L1, D1, and C2 (also L2, C8 if applicable)

extremely short for minimal noise and ringing.

C1 must be placed close to the IN pin for best

decoupling. All feedback components must be

kept close to the FB pin to prevent noise

injection on the FB pin trace. The ground return

of C1 and C2 should be tied close to the PGND

pins. See the MP1517 demo board layout for

reference.