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

MP1517 Rev. 1.4

www.MonolithicPower.com

6

4/28/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

Internal Low-Dropout Regulator

The internal power to the MP1517 is supplied

from the IN pin through an internal 2.4V low-

dropout linear regulator, whose output is BP.

Bypass BP to SGND with a 10nF or greater

capacitor to insure the MP1517 operates

properly. The internal regulator can not supply

any more current than is required to operate the

MP1517, therefore do not apply any external

load to BP.

Soft-Start

The MP1517 includes a soft-start timer that limits

the voltage at COMP during startup to prevent

excessive current at the input. This prevents

premature termination of the source voltage at

startup due to input current overshoot. When

power is applied to the MP1517, and enable is

asserted, a 2µA internal current source charges

the external capacitor at SS. As the capacitor

charges, the voltage at SS rises. The MP1517

internally clamps the voltage at COMP to 700mV

above the voltage at SS. This limits the inductor

current at startup, forcing the input current to rise

slowly to the current required to regulate the

output voltage.

Open Load Shutdown

The MP1517 includes an open load detect that

will stop the output from switching. In a fault

condition where the connection to the LED’s is

27V, the MP1517 will stop switching and the

output will stop rising. When the output falls

below 27V the MP1517 will restart in soft-start

mode and switches until the OLS threshold is

exceeded again. This will continue until the part

is disabled. To disable the open load shutdown

feature, connect the OLS pin to GND.

APPLICATION INFORMATION

GENERAL PURPOSE COMPONENT

SELECTION

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. Use 10kΩ to 50kΩ for the

low-side resistor R2 of the voltage divider.

Determine the high-side resistor R1 by the

equation:

()

FB

FB

OUT

V

V

-

V

2

R

1

R

×

=

Selecting the Inductor

The inductor is required to force the higher

output voltage while being driven by the input

voltage. A larger value inductor results in less

ripple current, resulting in lower peak inductor

current and reducing stress on the internal

larger value inductor has a larger physical size,

higher

series

resistance,

and/or

lower

saturation current.

Choose an inductor that does not saturate

under the worst-case load transient and startup

conditions. A good rule for determining the

inductance is to allow the peak-to-peak ripple

current to be approximately 30% to 50% of the

maximum input current. Make sure that the

peak inductor current is below 3A to prevent

loss of regulation due to the current limit.

Calculate the required inductance value by the

equation:

I

f

V

)

V

-

(V

V

L

SW

OUT

IN

OUT

IN

∆

×

×

×

=

η

×

×

=

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

(

)

)

MAX

(

IN

I

%

50

%

30

I

×

−

=

∆

SW

f

is the

load current, ∆I is the peak-to-peak inductor ripple

current and η is the efficiency.

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. A low ESR capacitor

is required to keep the noise at the IC to a

minimum. Ceramic capacitors are preferred, but

tantalum or low-ESR electrolytic capacitors may

also suffice.