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Driving LEDs: To Cap or Not to Cap

Introduction

High-brightness LEDs are available today with forward currents more than 100 times

greater than their predecessors. These new devices are not just high brightness, but

are high power as well. Single die with dissipations of 5W and multi-die modules with

power in excess of 25W are now available. The requirements of high efficiency and

low dissipation dictate a switching power supply for this new generation of High-

Brightness (HB), High-Power (HP) LEDs, as a voltage regulator and a current limiting

resistor are no longer appropriate. High-brightness, high-power LEDs require a

constant-current source to take full advantage of their ever-increasing luminous

efficiency and vibrant, pure color. The topology of choice for this new breed of

switching constant current sources is the basic buck converter. The most convincing

argument for using a buck converter is the ease with which this simple DC-DC

converter can be turned into a constant-current source. This article will explain the

selection of, or possible exclusion of, an output capacitor when designing a buck

regulator for constant-current drive of HB LEDs.

Controlled Current

The buck regulator is uniquely suited to be a constant current driver because the

output inductor is in series with the load. Regardless of whether a buck regulator is

used as a voltage source or a current source, selection of the inductor forms the

cornerstone of the system design. With an inductor in series with the output, the

average inductor current is always equal to the average output current, and the buck

converter naturally maintains control of the AC-current ripple. By definition, the LED

drive is a constant load system; hence a large amount of output capacitance is not

necessary to maintain VOduring load transients.

No Output Cap Yields High Output Impedance

In theory, a perfect current source has infinite output impedance, allowing the

voltage to slew infinitely fast in order to maintain a constant current. For switching

regulator designers who have concentrated on voltage regulators, this concept may

take a moment to sink in. Completely removing the output capacitor from a buck

regulator forces the output impedance to depend on the inductor. Without any

capacitance to oppose changes in VO, the output current (referred to as forward

current, or IF) slew rate depends entirely upon the inductance, the input voltage,

and the output voltage. (VO is equal to the combined forward voltage, VF, of each

series-connected LED)

LED manufacturers generally recommend a ripple

current, ΔIF, of ±5% to ±20% of

the DC forward current. Over the typical switching regulator frequency range of 50

kHz to 2 MHz the ripple itself is not visible to the human eye. These limits come from

increasing thermal losses at higher ripple current (a property of the LED

semiconductor PN junction itself) and a practical limit to the inductance used. The

percentages are similar to the recommended current ripple ratio in buck voltage

regulators. Inductor selection for a fixed-frequency current regulator is therefore

governed by the same equations as a voltage regulator:

One difference is that the inductance used for current regulators without output

capacitors tends to be higher because the drive currents for the emerging standards

of 1W, 3W, and 5W HB LEDs are 350 mA, 700 mA, and 1A respectively. Modern buck

voltage regulators tend to use inductors in the range of 0.1 ?H to 10 ?H with

saturation currents from 5A to 50A. Current drivers at similar switching frequencies

tend to require inductors ranging from 10 ?H to 1000 ?H and saturation currents

ranging from 0.5A to 5A.

The main goal of high output impedance is to create a system capable of responding

to PWM dimming signals, the preferred method of controlling the light output of

LEDs. The dimming signal might be applied to the enable pin of the regulator, in

which case the output current can slew from zero to the target and back to zero

without the delay of CO being charged and discharged. For even faster, higher

resolution dimming, a shunt switch, usually a MOSFET, can be placed in parallel with

the LED array, allowing the continuous flow of current at all times. Again, with no

output capacitor to slow the

slew rate, dimming frequencies into the 10’s of kHz

are

possible. This is a critical requirement in applications such as backlighting of flat-

panel displays, and the creation of white light using an RGB array.

Using an Output Capacitor Reduces Size and Cost

Some amount of output capacitance can be useful as an AC current filter.

Applications such as retrofitting of incandescent and halogen lights often require that

the LED and driver be placed in a small space formerly occupied by a light bulb.

Invariably the inductor is the largest, most expensive component after the LEDs

themselves. For the sake of efficiency (especially important in cramped quarters), the

designer generally chooses the lowest switching frequency that allows the solution

(mostly the inductor) to fit. Allowing a large ripple current in the inductor and

filtering the LED current results in a smaller, less expensive example, to

drive a single white LED (VF

≈

3.5V) at 1A with a ripple current

Δ

iFof ±

5% from an

input of 12V at 500 kHz would require a 50 ?H inductor with a current rating of 1.1A.

A typical ferrite core device that fits this application might be 10 mm square and 4.5

mm in height. In contrast, if the inductor ripple current is allowed to increase to ±

30%

(typical for a low-current voltage regulator) then the inductance required is less than

10 ?H, and an inductor measuring 6.0 mm square and only 2.8 mm in height size

can be used. The output capacitance required is calculated based on the dynamic

resistance, rD, of the LED, the sense resistance, RSNS , and the impedance of the

capacitor at the switching frequency, using the following expressions: