SP6648 Datasheet, PDF (12/16 Pages) Sipex Corporation – Ultra-low Quiescent Current, High Efficiency Boost Regulator

SP6648LEDEB Evaluation Board with
LumiLED High Brightness White LED
For the high brightness LumiLED white LED
application, the SP6648 is generally programmed
as a current source. The bias resistor R_b is used
to set the operating current of the white LED as
equation:
R_b = V_REF/I_F
APPLICATIONS INFORMATION
where V_REF is around 0.61V, I_F is the operating
current of the LumiLED. To set the operating
current to be about 350mA, R_b is selected as 1.8 Ω
as shown in the following schematic. The effi-
ciency of the SP6648 LumiLED circuit is im-
proved by the use of a silicon diode D1 and
resistor R₁ to set the voltage at the current sense
resistor R₂ to 0.61V instead of the higher 1.25V
at the FB pin. An efficiency curve follows
showing the SP6648 efficiency driving 350mA
output current into the high brightness LumiLED.
V_{BATT 1.8-3.2V}
C1
10µF
R_LIM
1.87K
L1 10µH
Important:
1µF Ceramic Cap at V_OUT Pin
needed for stable regulation
1
V_BATT
10
V_OUT
2 _{LBI SP6648 LX} 9
3
LBON
4 _RLIM
5 _SHDN
8
P_GND
GND ⁷
FB ⁶
C4
1µF
R₁
16.2k
C2
10µF
D2
1W LED
350mA
R₅ 10K
D1 1N4148
V_REF = 0.61V
(R₅ optional)
I_OUT=0.61V/R_b
Rb
1.8Ω
SP6648LEDEB Evaluation Board Schematic
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)
SP6648LEDEB Efficiency Curve
As shown in following scope photos, if the
SP6648 is powered up before the LumiLED is
plugged in, the circuit will bring the Feedback
pin to 0V and the SP6648 has a feature to set the
output voltage to be 3.3V. Once the LumiLED is
plugged in, the Feedback pin will go up to 1.25V
and begin to regulate. The output voltage will go
from 3.3V to 3.68V (=V_F+0.61V), where V_F is
the forward voltage of the LumiLED. When the
LumiLED is open, the Feedback pin voltage
will go to 0V and the output voltage will go to
3.3V which will protect the part.
Date: 7/19/04
SP6648 Ultra-low Quiescent Current, High Efficiency Boost Regulator
12
© Copyright 2004 Sipex Corporation