SP6691EKSLASHTR datasheet(10/15 Pages) SIPEX

10 / 15 page

Jun26-07 Rev D

Micro Power Boost Regulator Series White LED Driver

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

VOUT • IOUT

VIN • IIN

Where VIN , IIN, VOUT, IOUT are the input and

output voltage and current respectively.

While the white LED efficiency is expressed by

(VOUT - 1.22) • IOUT

VIN • IIN

This equation indicates that the white LED

efficiency will be much smaller than the output

efficiency of the circuit when VOUT 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

method.

APPLICATION INFORMATION

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

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

7%.

VIN

WLED MODULE

4.7uF

Murata LQH32CN100K11

MBR0530

DIODE

34.8ohm

2.2uF

150Kohm

L1 10uH 0.45A

1.22V

2.7-4.2V

0.7V

SP6691

GND

SHDN

LMV341

OUT

0.1V

2.2uF

2.7-4.2V

L1 10uH 0.45A

1.22V

Vbattery

SP6691

S W

GND

SHDN

MBR0530

WLED MODULE

4.7uF

11.2K

Vbattery

Murata LQH32CN100K11

5.1Ω

Figure 24. Improve Efficiency with Op Amp in Feedback

Loop

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