SC604A
POWER MANAGEMENT
Applications Information
Detailed Description
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
(IIN=IOUT+IQ) for part of the input voltage range, conserving
significant energy from the battery. A similar four (4)
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.
output device uses only 1.5x mode (IIN= IOUT
ꢁ
1.5+IQ)
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 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.
The input voltages at which the mode transitions occur
are dependent on the forward voltage VF of the LED used
and the LED current ILED. 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 VF.
The mode transition voltages VTRANS1X and VTRANS1.5X can
be estimated by the following equations:
ꢁ
ꢁ
VTRANS1X = VF + VILED + [(# of LEDs used) ILED 1.2]
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
6.
ꢁ
ꢁ
VTRANS1.5X = VF + VILED + [(# of LEDs used) ILED 16]
1.5
where, VF is the forward LED voltage measured from
anode to cathode, VILED is the voltage at the ILED pin,
typically VILED = 100mV, ILED is the LED current.
The current set and detection circuit uses an external
resistor and a 1.22V reference to program the LED
current.
Power efficiency can now be estimated for comparison
with the intended battery voltage range.
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
on ∆Vf considerations refer to Semtech application
notes).
VOUT IOUT
Efficiency [%] =
100%
(
VIN (IOUT Mode + IQ
2005 Semtech Corp.
www.semtech.com
7
SEMTECH [ SEMTECH CORPORATION ]
