SP6690 Datasheet, PDF (11/16 Pages) Sipex Corporation – Micro Power Boost Regulator Series White LED Driver

APPLICATION INFORMAMTION: Continued
Layout Consideration
Both the input capacitor and the output capacitor
should be placed as close as possible to the IC.
This can reduce the copper trace resistance
which directly effects the input and output
ripples. The feedback resistor network should
be kept close to the FB pin to minimize copper
trace connections that can inject noise into the
system. The ground connection for the feedback
resistor network should connect directly to the
GND pin or to an analog ground plane that is tied
directly to the GND pin. The inductor and the
schottky diode should be placed as close as
possible to the switch pin to minimize the noise
coupling to the other circuits, especially the
feedback network.
Power Efficiency
For the typical application circuit, the output
efficiency of the circuit is expressed by
η = ^{VOUT • IOUT}
V_IN • I_IN
Where V_IN , I_IN, V_OUT, I_OUT are the input and
output voltage and current respectively.
While the white LED efficiency is expressed by
η = ^{(VOUT - 1.22) • IOUT}
V_{IN •} I_IN
VIN _2.7-4.2V
Murata LQH32CN100K11
L1 10uH 0.45A
DS
MBR0530
C1
4.7uF
C2
2.2uF
U1
5
1
V
SW
IN
SP6690
4
SHDN
3
FB
GND
1.22V
2
R1
150Kohm
D1
DIODE
WLED MODULE
0.7V
Rb
34.8ohm
method.
Figure 23. Improve Efficiency with Diode in Feedback
Loop
To further improve the efficiency and reduce the
effects of the ambient temperature on the diode
D1 used in method 1, an op amp circuit can be
used as shown in Figure 24. The gain of the op
amp circuit can be calculated by:
Av = ^{R1 + R2}
R₁
If the voltage across the bias resistor is set to be
0.1V the current through R₁ and R₂ to be around
100µA, R₁ and R₂ can be selected as 1K and
11.2K respectively. LMV341 can be used be-
cause of its small supply current, offset voltage
and minimum supply voltage. By using this
method, the efficiency can be increased around
This equation indicates that the white LED
efficiency will be much smaller than the output
efficiency of the circuit when V_OUT is not very
large, compared to the feedback voltage (1.22V).
The other power is consumed by the bias resis-
tor. To reduce this power loss, two circuits can
be used, as shown in Figure 23 and Figure 24. In
Figure 23, a general-purpose diode (for ex-
ample, 1N4148) is used to bring the voltage
across the bias resistor to be around 0.7V. R₁ is
used to create a loop that provides around 100µA
operating current for the diode. 3% efficiency
improvement can be achieved by using this
Vbattery _2.7-4.2V
Murata LQH32CN100K11
L1 10uH 0.45A
DS
MBR0530
C1
4.7uF
5 U1 1
V
SW
IN
SP6690
4 _SHDN
FB ³
GND
1.22V
C2
2.2uF
2
Vbattery
6
5
1
+
4 _{OUT LMV341}
2
-3
R2
11.2K
WLED MODULE
0.1V
Rb
R1
1K
5.1Ω
7%.
Figure 24. Improve Efficiency with Op Amp in Feedback
Loop
Date: 05/25/04
SP6690 Micro Power Boost Regualtor, Series White LED Driver
11
© Copyright 2004 Sipex Corporation