SSL3250A_3

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 03 — 30 June 2009

18 of 26

NXP Semiconductors

SSL3250A

Photo flash dual LED driver

voltage of the output capacitor should be at least 16 V. For solution size reasons lower

value ceramic capacitors could be placed in parallel.

9.3 Inductor

The device has been designed to operate well with inductance values between

1.5

µH and 3.3 µH, in order to optimize for solution size. In a typical high current dual flash

LED application a 2.2

µH inductance is recommended. The inductors saturation current

should be greater than or equal to the inductor peak current limiter current, which is typical

2.2 A. During normal operation, it is recommended to keep the inductor peak current

below this value.

The copper losses and magnetic hysteresis losses in the inductor also contribute to the

total system losses.

9.4 Rectifier diode

Selecting a Schottky diode with low forward voltage drop improves efficiency. Although the

average current through the diode is equal to the load current and independent on duty

cycle for a boost converter topology, it is recommended to select a diode with an average

current rating that is significantly higher. The peak current rating of the diode should be

greater than the peak current through the inductor.

9.5 PCB layout

It is essential to have a good circuit layout to maximize efficiency and minimize EMI

disturbance. Because the circuit topology uses an inductor, it is often appointed as a main

source for EMI disturbance. But any loop of wire carrying a current is essentially an

electromagnet with a field strength that is proportional to the current. Therefore careful

circuit layout is important, keeping loop areas small and minimizing magnetic flux. Due to

the way an asynchronous boost converter operates, there are two power states. One state

is when the internal NMOS switch is on and one when the NMOS switch is off. During

each state there will be a current loop made by the power components that are

conducting. Arrange the input capacitor, rectifier diode and output capacitor in such a way

around the SSL3250A that during each of the two states the current loop is conducting in

the same direction. This prevents phase reversal of the magnetic field and reduces

radiated EMI. The current loop area should be kept small by placing the power

components as close as possible to the SSL3250A. Use ground planes to keep loop

areas to a minimum.

Priority should be given to positioning the output capacitor and the rectifier diode as close

as possible to the LX and PGND nodes of the SSL3250A. Since large currents will flow

from the input capacitor to the inductor and not into the VIN pin of the SSL3250A, it is wise

to locate the input capacitor near the inductor. The VIN pin should be star connected to

the positive pad of the input capacitor. It is recommended to place an extra 100 nF

capacitor from VIN to GND directly next to the SSL3250A.

PGND and GND of the SSL3250A should be directly connected to each other preferably

by using the die pad area under the SSL3250A. Place the ground connection of the output

capacitor as close as possible to the PGND pin of the SSL3250A.