SP6121CN-L Datasheet, PDF (25/42 Pages) Sipex Corporation – Low Voltage, Synchronous Step Down PWM Controller

L2
=
7.6 − 3.8
0.4×1.5× 500000×
0.5^−1
= 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: 1 + ^R2
R3
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.
Nov 28 06 AppNote: SP6121 as a Synchronous Buck-Boost LED Driver
Page 5
© 2006 Sipex Corporation