SC604A datasheet(7/18 Pages) SEMTECH

7 / 18 page

 2005 Semtech Corp.

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SC604A

POWER MANAGEMENT

Applications Information

Designing for Lowest Possible Battery Current

The SC604A efficiency and battery current are shown in

the plots that follow on page 8. For this example, 4 LEDs

are matched at 15mA each. The battery current remains

low at 63mA well into the Li-Ion battery range as indicated

in the plot by a boundary box. The SC604A uses 1x mode

IN=IOUT+IQ) for part of the input voltage range, conserving

significant energy from the battery. A similar four (4)

output device uses only 1.5x mode (I

IN= IOUT

1.5+I

over the input voltage range. This means that the

SC604A will have about 25% higher efficiency than a

1.5x only charge pump. Where the competition drops

off at 3V, the SC604A uses 2x mode to extend the

operating range down to a battery voltage of only 2.85V.

The input voltages at which the mode transitions occur

are dependent on the forward voltage V

F of the LED used

and the LED current I

LED. To keep the battery current low

and in the 1x mode for as long as possible, it is best to

choose an LED with a lower V

The mode transition voltages V

TRANS1X and VTRANS1.5X can

be estimated by the following equations:

TRANS1X = VF + VILED + [(# of LEDs used)

LED

1.2]

TRANS1.5X = VF + VILED + [(# of LEDs used)

LED

16]

1.5

where, V

F is the forward LED voltage measured from

anode to cathode, V

ILED is the voltage at the ILED pin,

typically V

ILED = 100mV, ILED is the LED current.

Power efficiency can now be estimated for comparison

with the intended battery voltage range.

Efficiency [%] =

Detailed Description

The SC604A contains a fractional charge pump, mode

selection circuit, output selection logic, current setting

detection circuit, and four current sense circuits. All are

depicted in the block diagram on page 6.

The fractional charge pump multiplies the input voltage

a multiple of 1, 1.5 or 2 times the input voltage. The

charge pump switches at a fixed 250kHz whenever the

mode is 1.5x or 2x. The charge pump does not switch

during 1x mode, saving power and improving 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. 1x is the most

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

voltages a stronger mode may be needed to maintain

regulation, if so, the mode will change first to 1.5x and

then 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 2.85V or

less. The LED requiring the highest voltage drop will

determine the output voltage needed to drive all outputs

with adequate bias. Comparing all cathodes and

regulating VOUT for the LED with the lowest cathode

voltage ensures sufficient bias for all LEDs.

Output selection logic enables control over the LED

outputs for on and off functions with eight (8) different

output states. The states are defined in Table 1 on page

The current set and detection circuit uses an external

resistor and a 1.22V reference to program the LED

current.

Four (4) current regulating circuits sink matched currents

from the LEDs. LEDs with matched forward voltage will

produce the best possible matched currents. For best

matching performance it is recommended that the

∆Vf

between LEDs be under 250mV. (For more information

∆Vf considerations refer to Semtech application

notes).

VOUT IOUT

100%

VIN (IOUT Mode + IQ

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