SC652

12

General Description

This design is optimized for handheld applications sup-

plied from a single Li-Ion cell and includes the following

key features:

A high efficiency fractional charge pump that

supplies power to all LEDs

Five matched current sinks that control LED

backlighting current, providing 500μA to 25mA

per LED

EN/PWM pin functions as an enable and pro-

vides PWM control of the LED brightness

High Current Fractional Charge Pump

The backlight outputs are supported by a high efficiency,

high current fractional charge pump output. The charge

pump multiplies the input voltage by 1, 1.5, or 2 times.

The charge pump switches at a fixed frequency of 250kHz

in 1.5x and 2x modes and is disabled in 1x mode to save

power and improve efficiency.

The mode selection circuit automatically selects the

mode as 1x, 1.5x, or 2x based on circuit conditions such

as LED voltage, input voltage, and load current. The 1x

mode is the most efficient of the three modes, followed

by 1.5x and 2x modes. Circuit conditions such as low

input voltage, high output current, or high LED voltage

place a higher demand on the charge pump output. A

higher numerical mode (1.5x or 2x) may be needed

momentarily to maintain regulation at the OUT pin

during intervals of high demand. The charge pump

responds to momentary high demands, setting the

charge pump to the optimum mode to deliver the output

voltage and load current while optimizing efficiency.

Hysteresis is provided to prevent mode toggling.

The charge pump requires two bucket capacitors for

proper operation. One capacitor must be connected

between the C1+ and C1- pins and the other must be con-

nected between the C2+ and C2- pins as shown in the

Typical Application Circuit diagram. These capacitors

should be equal in value, with a minimum capacitance of

1μF to support the charge pump current requirements.

The device also requires at least 1μF capacitance on the IN

pin and at least 1μF capacitance on the OUT pin to mini-

•

•

•

mize noise and support the output drive requirements of

I

OUT

up to 90mA. For output currents higher than 90mA, a

nominal value of 4.7μF is recommended for C

OUT

and C

IN

.

Capacitors with X7R or X5R ceramic dielectric are

strongly recommended for their low ESR and superior

temperature and voltage characteristics. Y5V capacitors

should not be used as their temperature coefficients

make them unsuitable for this application.

It is important that the minimum value of the capacitors

used is no lower than 1μF. This may require the use of

2.2μF capacitors to be sure that the degradation of

capacitance due to DC voltage does not cause the

capacitance to go below 1μF.

LED Backlight Current Sinks

The full scale backlight current (I

FS_BL

) is set via the

current through the ISET pin (I

ISET

). I

FS_BL

is regulated to

the value of I

ISET

multiplied by an internal gain of 100A/A.

R

ISET

is used to control the current through the ISET pin.

The relationship between R

ISET

and the full scale back-

light current is:

R

ISET

= 100/I

FS_BL

All backlight current sinks have matched currents, even

when there is a variation in the forward voltages (ΔV

F

)

of the LEDs. A ΔV

F

of 1.0V is supported when the input

voltage is at 2.9V. Higher ΔV

F

LED mis-match is sup-

ported when V

IN

is higher than 2.9V. All current sink

outputs are compared and the lowest output is used for

setting the voltage regulation at the OUT pin. This is

done to ensure that sufficient bias exists for all LEDs.

Any unused outputs must be left open and unused LED

drivers will remain disabled.

PWM Operation

A PWM signal can be used to adjust the DC current

through the LEDs. When the duty cycle is 100%, the

backlight current through each LED (I

BL

) equals the full

scale current set by R

ISET

. As the duty cycle decreases, the

EN/PWM input samples the control signal and converts

the duty cycle to a DC current level. In conventional

PWM controlled systems, the output current pulses on

and off with the PWM input to achieve an effective

Applications Information