SP6683EB Datasheet, PDF (2/5 Pages) Sipex Corporation – Evaluation Board Manual

USING THE EVALUATION BOARD
1) Powering Up the SP6683 Circuit
The SP6683 Evaluation Board can be
powered from inputs from a +2.7V to +5.5V
battery or a power supply. Connect with
short leads directly to the “VIN” and “GND”
posts.
2) Using the evaluation board
The Evaluation Board was set up for a 4
15mA LED application even though there
are 6 LED spots on the board. The output of
SP6683 was open as default. If the
customer has discrete white LEDs, then the
discrete white LEDs should be soldered to
the D1-D6 positions and bias resistors
should be soldered to the R4-R9 positions.
Selection of the bias resistors please refers
to 3). If the customer has white LED
module, they can plug the two terminals of
the module to the “VOUT” and “Vfb” posts,
and use R4 as the bias current to regulate
the total current. The value of the bias
resistor could be calculated by equation (1).
3) Selecting the Bias Resistor
The bias resistor could be estimated by (1)
R_Total = V_FB I _{LED _ Total} = 0.3V 60mA = 5Ω (1)
Where I_{LED_Total} is the total output current.
R_4−9 = V_FB I _{LED _ D1−6} = 0.3V 15mA = 20Ω (2)
Where I_{LED_D1-D6} is the operating current of
D1-D6.
4) Selecting of V_mode and Divider Resistor
SP6683 can automatically change from X1
mode to X1.5 mode for highest efficiency.
To use this feature, divider resistors should
be chosen according to the specific
application. The guideline for divider resistor
selections is as follows. For high input
voltage, the SP6683 will work in X1 mode,
when the input voltage drops to V_th threshold
voltage, it will switch to X1.5 mode
automatically. The V_th threshold voltage for
mode change can be calculated by (3)
( ) V_th = V_F + 0.3 + m ⋅ I _LED ⋅ R_out
(3)
Where V_F and m are the forward voltage
and number of the white LEDs, R_out is the
output resistance of the SP6683.
The equation for the voltage divider R₁ and
R₂ with V_mode=1.25V is:
V_th = 1.25V ⋅ (1 + R₁ R ₂ )
(4)
which can be expressed as R1:
( R₁ = V_th 1.25 −1)⋅ R₂
(5)
For the typical Sp6683 Evaluation Board,
using V_F=3V, m=4, I_LED=15mA, R_out=6ohms,
the V_th will be 3.7V. Select R₂=100kohm,
then R₁=192kohm.
5) Selecting of Capacitors
Ceramic capacitors are used on the
evaluation board due to their inherently low
ESR, which will help produce low peak to
peak output ripple, and reduce high
frequency spikes.
Selection of the flying capacitor is a trade-off
between the output voltage ripple and the
output current capability. Decreasing the
flying capacitor will reduce the output
voltage ripple because less charge will be
delivered to the output capacitor. However,
smaller flying capacitor leads to larger
output resistance, thus decrease the output
current capability and the circuit efficiency.
In the evaluation board, the input, output
and flying capacitors are selected as 1uF
ceramic capacitors. Input and output ripple
could be further reduced by using larger low
ESR input and output capacitor.
6) Brightness Control
Obvious dimming control could be achieved
by applying a PWM control signal to the
ENABLE/PWM pin. The brightness of the
white LEDs is controlled by increasing and
decreasing the duty cycle of The PWM
signal. The recommend frequency range of
the PWM signal is from 60Hz to 700Hz. A
repetition rate of at least 60Hz is required to
prevent flicker.
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