Design Hints

The schematic of the demonstration board is shown in page

2. Some hints on selection of the key parameters and com-

ponents will be described in below. For full details of the

design equations and theories, please refer to the LM3430

and LM3432 datasheets.

BOOST CONVERTER DESIGN WITH LM3430

The function of the boost converter is to step up the input

voltage to a higher LED rail voltage to drive strings of twelve

LEDs connected in series.

Determination of the Worst Case LED Rail Voltage, V

LED

The diode forward voltage, V

to 4.0V that varies from vendor to vendor. Considering only

the worst case situation, i.e. V

nected in series, a minimum of 48V is required to put the LED

strings in proper operation. On the top of this voltage, addi-

tional control headroom for the constant current regulators is

needed. The suggested start up LED rail voltage is 50V and

this is also the ceiling of the boost converter output voltage,

i.e. in case the load is disconnected, the boost converter out-

put will stay at this voltage, no risk of over-voltage need to be

considered. Once the demonstration board is in operation, the

LM3432 will communicate with the LM3430 to lower the LED

rail voltage to an appropriate level. This feature makes the

LM3430 with LM3432 a robust and efficient LED driver solu-

tion.

Selecting the Switching Frequency, F

SW

The selection of switching frequency is a trade off between

size, cost and efficiency. In general, a lower switching fre-

quency requires larger and more expansive external compo-

nents. For some of the applications, space is one of the key

considerations. A higher switching frequency can fulfill the

space requirement, however the switching losses will go up

and the overall system efficiency will be lowered. In this

demonstration board, a switching frequency of 1MHz was se-

lected.

Selection of the Power MOSFET

The power MOSFET plays a key role in system efficiency. An

ideal power MOSFET should be low gate capacitance, C

low gate charge, Q

Drain-Source breakdown voltage, V

the required V

drop, typically its about 0.5~0.7V. The average load current

with all LED strings fully ON is 120 mA, by quick estimation;

the worst case peak switch current is about 0.9A. The tiny

MOSFET selected can operate up to 60V and 2A with R

DS

Selecting the Boost Schottky Diode

The boost Schottky diode current equals to the average load

current. The forward voltage drop and reverse recovery time

determines the power loss with this component. The lower the

forward voltage drop and faster reverse recovery time always

results in better performance. The Schottky diode must also

be rated to handle the maximum output voltage plus any ring-

ing at the switching node caused by the diode parasitic ca-

pacitance and lead inductance.

Selection of Power Inductor

In fixed switching frequency boost converter applications, the

inductance is determined by the allowable peak-to-peak in-

ductor ripple current,

Δi

the switching frequency. The duty cycle, D is evaluated first

at both V

ductor current is calculated at both voltages respectively. With

the maximum average inductor current, the allowable induc-

tor ripple current is determined. Finally, the inductance value

can be calculated and an off-the-shelf inductance value clos-

est to the calculated value must be selected. The calculation

only gives the inductance required but not the size of the in-

ductor, the inductor selected must be capable to handle the

maximum peak inductor current without saturating the induc-

tor even at high temperature. The maximum peak inductor

current is equal to the maximum average inductor current plus

one half of the maximum allowable inductor ripple current,

Δi

Selection of Output Capacitor

The output capacitor in a boost converter supplies current to

the load during the MOSFET on-time and filters the AC com-

ponents of the load current during the off-time. The selection

of this capacitor determines the steady state output voltage

ripple,

ΔV

prised of three components. The first part of the ripple voltage

is the surge current created during the boost diode turns on.

The second part is due to the charging and discharging of the

output capacitor through the boost diode and the final part is

caused by the flow of inductor current through the output

capacitor's Equivalent Series Resistance, ESR. Both part 1

and part 3 are related to ESR, in case low ESR ceramic ca-

pacitors are used, the contribution of these two parts be-

comes insignificant. The output voltage ripple can be

estimated by the equation in below.

Where I

the converter switching frequency.

From the equation in above, the relationships between differ-

ent parameters are obvious. A higher output capacitance can

reduce the output voltage ripple; however this can slow down

the power-up time and the system transient respond. The

choice of the output capacitor depends mainly on the appli-

cation specifications. For most of the cases, multiple itera-

tions are required to come up with an appropriate value.

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