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NXP Semiconductors
SSL3250A
Photo flash dual LED driver
maintaining loop stability. The SSL3250A overvoltage limit on V O is 10.3 V (typ). The rated
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 μ H and 3.3 μ H, in order to optimize for solution size. In a typical high current dual flash
LED application a 2.2 μ H 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.
SSL3250A_5
? NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 05 — 16 December 2009
18 of 26
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