NXP Semiconductors

UM10406

SSL1523 5 W LED driver

6. Board system optimization

To meet specific customer application requirements, the modifications described in the

following sections are possible.

6.1 Changing the output current and LED current

One of the major advantages of a flyback converter over other topologies, is its suitability

for driving LED configurations with a broad range of voltages. Essentially, changing the

winding ratio whilst maintaining the value of the primary inductance, will shift the output

working voltage accordingly. Part of the efficiency of the driver is linked to the output

voltage. A lower output voltage will require increased transformation ratio, and will cause

higher secondary losses. In practice, a mains operated flyback converter will have an

efficiency > 80 % for high output voltages (like 40 V) down to 50 % for very low output

voltages < 3 V. At low voltages, synchronous rectification becomes advisable to reduce

rectification losses.

The NXP TEA 1761/TEA1762 can be used for this purpose, see Ref. 1. For exact

calculations of transformer properties and peak current, refer to Ref. 2 application note

AN10754, “How to design an LED driver using the SSL2101”, see Ref. 2.

6.2 Changing the output ripple current

The output ripple current is mostly determined by the LED voltage, the LED dynamic

resistance and the output capacitor. The present value of C10 has been chosen to

optimize the capacitor size under typical load. The resulting ripple of ± 30 % will result in

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an expected deterioration of light output < 1 %.

The size for the buffer capacitor (C10) can be estimated from Equation 1:

C10 = Δ---I--I ⋅ 2----π---f--n---e--t---⋅--1-R----d---y--n---a--m----i-c-

(1)

Using a series of LEDs, the dynamic resistance of each LED can be multiplied by the

number of LEDs. The current sense resistor (R10) should also be included in this

calculation.

Example: For a ripple current of ± 30 %, and a mains frequency of 50 Hz, and a total

dynamic resistance of 7 Ω, the resulting capacitance value will be 3.3333 / (314*7) =

1500 μF. The capacitor must be specified for the HF switching related ripple current of

about 0.35 times the average effective LED current (ILED(AV)). For high lifetime

applications, the ripple current specification of the electrolytic capacitor must be

increased. For details, please contact the capacitor supplier.

6.3 Changing the load curve

The current load curve can be divided into the following two regions:

• Where the current control loop regulates the output current, the constant current

output

• Where the IC limits the peak input current of the converter, the constant power output

UM10406

User manual

All information provided in this document is subject to legal disclaimers.

Rev. 01 — 3 August 2010

© NXP B.V. 2010. All rights reserved.

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