6

Feb27-07 Rev K

Integrated White LED Driver With Organic LED Display Power Supply

© 2007 Sipex Corporation

The S

P645 is designed for white LED & OLED back-

lighting in mobile phone applications. It has a main

display loop which can drive up to 4 white LEDS in series

and a SUB display loop which is designed to drive OLED

up to 20V/0mA or 3 white LEDs.

The S

P645 operates at a wide input voltage range

suitable for Li-ion battery and has a constant off time

for smaller inductor and capacitors. The block diagram

refers the operation circuit. By SEL pin, it can choose the

different channel to turn on CH or CH2.

Current Limit Protection

The S

P645 has current limiting protection to prevent

excessive stress on itself and external components dur-

ing overload conditions. The internal current limit com-

parator will disable the NMOS power device at a typical

switch peak current limit of 300mA.

The output capacitor needs to be a minimum of uF

with 25V rating. For applications with 5V input, an

output capacitor of 2.2uF or 4.7uF may be needed to

lower output ripple and improve regulation. For better

voltage filtering, ceramic capacitors with low ESR are

recommended. X5R and X7R types are suitable be-

cause of their wider voltage and temperature ranges.

Output Over-Voltage Protection

The S

P645 contains dedicated circuitry for

monitoring the output voltage. In the event that the

primary LED network is disconnected the output will

increase and be limited to 22V(typ.). There is a ~V

hysteresis associated with this circuitry, which will turn

the NMOS off when the output voltage is at 23V(max.)

until the output voltage reach 2V(typ.) or lower. The

22V limit allows the use of 25V µF ceramic output

capacitors creating an overall small solution for white

LED applications.

Under Voltage Protection

The S

P645 has a UVP comparator to turn the NMOS

powerdeviceoffincasetheinputvoltageorbatteryvoltage

is too low

,preventing an on state of the power device con-

ductinglargeamountsofcurrent.Thereisa~Vhysteresis

associated

with this circuitry, which will turn the NMOS off

when the output voltage is at 23V(max.) until the output

voltage reaches 2V(typ.) or lower. The 22V limit allows

the use of 25V mF ceramic output capacitors creating an

overall small solution for white LED applications.

Capacitor Selection

A 4.7µF input capacitor can reduce input ripple. For

better voltage stability, to increase the input capacitor

value or using an LC filter is feasible, especially in Li-

Ion battery applications. The input capacitor acts as a

bypass to the IC and needs to be connected directly to

the VIN pin and the GND pin for proper low ripple reg-

ulation.

Diode Selection

A Schottky diode is a good choice for SP645

because of its lower forward voltage drop and faster

reverse recovery. Using

Schottky diodes can help ob-

tain better efficiency. High speed rectification is also a

good characteristic of

Schottky diodes for high switch-

ing frequency. Current rating of the diode must meet

the root mean square of the peak current and output

average current multiplication.

Inductor Selection

The recommended value of inductor for S

P645ap-

plications is 0uH. Small size and better efficiency are

the major concerns for portable device, such as S

P645

used for dual panel mobile phone. The inductor should

have low DCR for better efficiency. To avoid inductor

saturation, current rating should be at least 600mA. The

input range is 2.7V to 5.5V.

Detailed Description

Application Information