Data Sheet

ADD5207

Rev. A | Page 13 of 16

determine the on time with the following equation:

SW

ON

f

D

t

=

switch duty cycle (D) with the following equation:

OUT

IN

OUT

V

V

V

D

−

=

Choose the output capacitor based on the following equation:

(

)

OUT

OUT

SW

IN

OUT

L

OUT

V

V

f

V

V

I

C

∆

×

×

−

×

≥

Capacitor manufacturers include: Murata Manufacturing Co.,

Ltd., AVX, Sanyo, and Taiyo Yuden Co., Ltd.

Diode Selection

The output diode conducts the inductor current to the output

capacitor and loads while the switch is off. For high efficiency,

minimize the forward voltage drop of the diode. Schottky diodes

are recommended. However, for high voltage, high temperature

applications, where the Schottky diode reverse leakage current

becomes significant and degrades efficiency, use an ultrafast

junction diode. The output diode for a boost regulator must be

chosen depending on the output voltage and the output current.

The diode must be rated for a reverse voltage equal to or greater

than the output voltage used. The average current rating must

be greater than the maximum load current expected, and the peak

current rating must be greater than the peak inductor current.

Using Schottky diodes with lower forward voltage drop decreases

power dissipation and increases efficiency. The diode must be

rated to handle the average output load current. Many diode

manufacturers derate the current capability of the diode as a

function of the duty cycle. Verify that the output diode is rated

to handle the average output load current with the minimum

duty cycle.

The minimum duty cycle of the ADD5207 is:

OUT

IN_MAX

OUT

MIN

V

V

V

D

−

=

Schottky diode manufacturers include ON Semiconductor,

Diodes Incorporated, Central Semiconductor Corp., and Sanyo.

Loop Compensation

The external inductor, output capacitor, and the compensation

resistor and capacitor determine the loop stability. The induc-

tor and output capacitor are chosen based on performance, size,

stability. For typical LED application of the ADD5207, a 6.8 kΩ

compensation resistor in series with a 2.2 nF compensation

capacitor at COMP is adequate.

C2

VOUT_FB

HEADROOM CONTROL

Figure 15. Compensation Components

A step-up converter produces an undesirable right-half plane

zero in the regulation feedback loop. Capacitor C2 is chosen

to cancel the zero introduced by output capacitance ESR.

Solving for C2,

C

OUT

R

C

ESR

C2

×

=

For low ESR output capacitance, such as with a ceramic

capacitor, C2 is optional.

LAYOUT GUIDELINES

When designing a high frequency, switching, regulated power

supply, layout is very important. Using a good layout can solve

many problems associated with these types of supplies. The

main problems are loss of regulation at high output current

and/or large input-to-output voltage differentials, excessive

noise on the output and switch waveforms, and instability.

Using the following guidelines helps minimize these problems.

Make all power (high current) traces as short, direct, and thick

as possible. It is good practice on a standard printed circuit

board (PCB) to make the traces an absolute minimum of 15 mil

(0.381 mm) per ampere. The inductor, output capacitors, and

output diode should be as close to each other as possible. This

helps reduce EMI radiated by the power traces that carry high

switching currents. Close proximity of the components also

reduces lead inductance and resistance, which in turn reduce noise

spikes, ringing, and resistive losses that produce voltage errors.