SC614 datasheet(12/25 Pages) SEMTECH

12 / 25 page

POWER MANAGEMENT

SC614

United States Patent No. 6,504,422

www.semtech.com

General Operation

The SC614 contains a fractional charge pump, mode

selection circuitry, serial I/O logic, serial data registers

and current regulation circuitry for 7 LED outputs. All

are depicted in the Block Diagram on page 5.

The fractional charge pump multiplies the input voltage

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

switches at a fixed frequency that is bit selectable to

1.33MHz or 250kHz. The default frequency is 250kHz.

The charge pump does not switch during 1x mode, saving

power and improving efficiency.

The mode selection circuitry automatically selects the

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

as LED voltage, input voltage and load current. 1x is the

most efficient mode, followed by 1.5x and 2x modes. At

lower input voltages a stronger mode may be needed to

maintain regulation. If so, the mode will change first to

1.5x and then later to 2x. 2x mode usually operates for

a much shorter run time compared to 1x mode, and 2x

mode maintains the output until the battery is discharged

to 3V or less. The LED requiring the highest voltage drop

will determine the output voltage needed to drive all

outputs with sufficient anode voltage. Comparing all

cathodes and regulating VOUT for the LED with the lowest

cathode voltage ensures sufficient bias for all LEDs.

The LED outputs are controlled through the serial data

registers, found in the Register Map on page 8. LED on/

off functions are independently controlled, so that any

combination of LEDs may be switched on.

Seven (7) current regulating circuits sink currents from

the LEDs as set by the Current Control registers. For

LCD backlighting applications current matching is crucial,

and LEDs with matched forward voltage will produce the

best possible matched currents. The SC614 is capable

of dealing with V

F mis-matches up to 0.5V. For best

matching performance, however, it is recommended that

the LED to LED difference,

ΔV

F, be under 250mV.

Designing for Lowest Possible Battery Current

The battery current and efficiency of the SC614 are

mostly dependent on the charge pump mode of operation.

To get the best performance from the SC614 it is better

to use LEDs with consistantly lower V

F. Lower VF will keep

the charge pump in 1x mode longer and will use less

battery current, extending the run time of the battery.

Mode Transition Threshold Voltage and Hysteresis

Mode transition threshold voltage refers to the input

voltage at the point when the charge pump changes from

a weaker mode (lower numerically) to a stronger mode

(higher numerically). V

TRANS1X is the transition from 1x to

1.5x mode, and V

TRANS1.5X is the transition from 1.5x to 2x

mode.

Mode transition voltages V

TRANS1X and VTRANS1.5X can be

estimated by the following equations:

TRANS1X = VF + VILED + IOUT

1.0

TRANS1.5X = (VF + VILED + IOUT 5.5) / 1.5

where V

F is the highest forward voltage of the operating

LEDs, V

ILED is the current sink voltage for that LED (typically

ILED=150mV) and IOUT is the sum of all operating LED

currents.

The mode transition circuitry has hysteresis built in to

prevent the device from toggling between modes when

the input voltage is right at the threshold of mode change.

There is 100mV of hysteresis between 1.5x and 1x modes

and 200mV of hysteresis between 2x and 1.5x modes.

Efficiency

Power efficiency can be estimated for any particular

battery voltage as follows:

η = [V

OUT IOUT / VIN (IOUT Mode+IQ)] 100 %

where:

OUT = VF+ VILED (defined in the above section).

and,

Q = 1mA in 1x mode, 1.4mA in 1.5x mode and 1.8mA

in 2x mode (250kHz).

LED Current Accuracy

The LED current is set by the Current Control registers

0x01 through 0x07. The guaranteed accuracy of any

current sink is +/-8% at a current setting of 20mA, with

the typical accuracy much higher at +/-1.5%. For

example, if the Current Control registers are configured

such that each LED current will be 20mA (0x32 written

to register 0x01 through 0x07), the actual LED currents

Applications Information

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