SP6648LED Datasheet, PDF (3/5 Pages) Sipex Corporation – Evaluation Board Manual

5) Using the R_lim Function
The peak inductor current, I_PEAK, is programmed externally by the R_LIM resistor connected
between the RLIM pin and GND. The peak inductor current is defined by:
I_PEAK = 1400/R_LIM
The SP6648 datasheet specifications for R_LIM give a range of 1.87K to 4K ohms. Using the I_PEAK
equation above gives an I_PEAK range of 350 to 750mA.
The saturation current specified for the inductor needs to be greater then the peak current to
avoid saturating the inductor, which would result in a loss in efficiency and could damage the
inductor. The SP6648 evaluation board uses a R_lim value of 1.87K for an I_peak = 750mA to allow
the circuit to deliver up to 180mA for 1.3V input and 400mA for 2.6V input. Other values could
be selected using the above relationships.
6) Using the LBON - Low Battery Output Function
The SP6648 will regulate the output current until the input battery is completely discharged. To
provide an early low battery warning, the Low Battery Output function of the SP6648 switching
regulator can be used. LBON is programmed externally by the R₁ and R₂ resistor divider
connected between VIN, the LBI input pin and GND. When the LBI comparator falling threshold
of 0.61V is reached, D1 switches on, indicating low battery condition. Low battery detect trip
point can be calculated by the following formula:
V_LOWBAT
=
0.61⋅
R₁ + R₂
R₂
The SP6648 evaluation board R₁ & R₂ resistors have been set to trip for a falling battery
threshold of about 2.0V. Using this relationship, other low battery threshold values can be set
by the user.
7) Brightness Control
One approach to control LED brightness is to apply a PWM signal to the SHDN input of the
SP6648. In this case, the output current will be equal to the product of 350mA and the average
duty cycle at the SHDN pin. An additional 10K potentiometer (R₅) may also be used for dimming
LED. LED current is varied by the potentiometer between almost zero and full 350mA.
POWER SUPPLY DATA
For the standard evaluation board, The efficiency curve is shown in the figure below.
100
95
90
85
80
75
70
65
60
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Battery Voltage (V)
Fig.3 Efficiency curve of the SP6648LED evaluation board