SP6121 datasheet(25/42 Pages) SIPEX

25 / 42 page

Nov 28 06 AppNote: SP6121 as a Synchronous Buck-Boost LED Driver

Page 5

500000

−

= 6.33µH

As we are using a dual winding coil we choose a 4.7µH inductor. Using a coupled

coil allows for this because the AC current is reduced considerably due to

internal coupling between the 2 inductors.

Transistor T1 is a P-Channel FET. It is directly controlled by the SP6121 PWM

controller. It must be able to saturate its VGS with the minimum input voltage of

3.4V. A typical 2.5V trench MOSFET is well suited for this environment. T2 is the

same but is an N Channel FET -- the VGS requirement is the same. Max VDS for

both is 20V; ID max is approximately equal to

ILED max + 50% = 2.3A. As the

Duty Cycle can get higher when the battery is low, the current in the primary cell

will increase a bit and then T1 will need to handle more current. Doubling the

output current is a good estimate for calibrating the PMOS transistor.

C1 is the energy transfer capacitor. This component does not transfer any DC

energy but only AC. So in order to minimize the loss we are choosing a 10µF

ceramic capacitor with low ESR.

COUT is very important as it stores all the energy during TOFF, and so maintains

the output voltage. This is a 100µF Tantalum capacitor. A lower value may not be

suitable to start the converter at full load with a low voltage battery.

The current regulation is done by sensing the LED current via R4. As it is a

100mΩ resistor, VR4 = 0.1 x

ILED. The converter is made to regulate 1.5A into the

LED. The voltage across R4 will then be 150mV. As the Feedback voltage of the

PWM controller is 1.25V, we will need an amplifier made out of an LMV321. This

amplifier is a simple non-inverting circuit with gain of 1.25 / 0.15 = 8.33.

In this case Gain is:

In order to have a safe design, we need to include overvoltage protection to

clamp the output voltage in case of a burned LED. This is due to the fact that

under a current-regulation architecture, an open circuit load is viewed by the

controller as 0 volts VOUT, and so it will infinitely try to increase its value.

The OVP circuit is made out of a simple Zener diode. We choose its voltage so it

is more than the maximum VF of the K2 LED and less than the maximum voltage

that transistors can support. In our case we took a Zener diode of 5.1V. In order

to prevent short-circuiting of the low impedance output of the LMV321, a 10kΩ

resistor (R5) is inserted between its output and the FB input of the SP6121.

SP6121 Datasheet(PDF) 25 Page - Sipex Corporation