The buck power conversion topology can be used

when the LED string voltage is needed to be lower

then the input supply voltage. The design procedure

for a buck LED driver outlined in the previous

chapters can be applied to the low voltage LED

drivers as well. However, the designer must keep in

mind that the input voltage must be maintained

higher than 2 times the forward voltage drop across

the LEDs.

This limitation is rated to the output

current instability that may develop when the FC9910B

buck converter operates at a duty cycle greater than

0.5. This instability reveals itself as an oscillation of

the output current at a sub-harmonic of the switching

frequency.

The power conversion topology can be used when

the forward voltage drop of the LED string is higher,

equal or lower than the input supply voltage. For

example, the buck-booster topology can be appropriate

battery(12V) and output string consists of three to six

HB LEDs, as the case ma be for fail and break

signal lights.

In the buck-booster converter, the energy from the

input source is first stored in the inductor or a flyback

transformer when the switching transistor is ON. The

energy is then delivered to the output during the OFF

time of the transistor. When the energy stored in the

flyback inductor is not fully depleted by the next

switching cycle (continuous conduction mode ) the

DC conversion between input and output voltage is

given by:

Vout = -Vin * D / (1-D)

The output voltage can be either higher or lower than

the input voltage, depending on duty ratio.

Let us discuss the above exanple of an auto-motive

LED driver that needs to drive three HB LEDs at

350mA. Knowing the nominal input voltage VIN=12V,

the nominal duty ratio can be determined, as

3. Output Capacitor

Then, given the switching frequency, in this example

f

= 50kHz, the required on-time of the MOSFET

transistor can be calculated:

Unlike the buck topology, the buck-booster converter

requires an output filter capacitor to deliver power to

the LED string during the ON time of switching the

transistor, when the flyback inductor current is

diverted from the output of the converter.

In the order to average the current in the LED, this

capacitor must present impeadance to the switching

output AC ripple current that is much lower than the

dynamic impeadance Rout of the LED string.

If we assume Rout=3 Ohm in our example, in order to

attenuate the switching ripple by a factor of 10, a

capacitor with equivarent series resistance (ESR) of

0.3 Ohm is needed. A chip SMT tantalium capacitor

can be selected for this purpose.

FC9910B

2013. 08. 29

9/10

Revision No : 1

Application Information (Continued)

DC DC Low Voltage Application

1. Buck Converter Operation

2. Flyback(Buck-Booster) Operation

D = V

/ (Vin + V

) = 9 / (12+9) = 0.43

LEDs

LEDs

OSC

L = Vin * T

/ (0.3* I

) = 0.98mH, use 1mH

ON

LED