SC604A
POWER MANAGEMENT
Definitions
Input Current
ILED Accuracy
The total input current of the SC604A is a function of
the sum of the LED currents, the charge pump mode
and the quiescent current. The quiescent current trend
is charted on page 12 and used to calculate IIN in the
following examples.
The LED current is determined by the RSET resistor (ILED
vs. RSET data is found on pages 9 and 10). This term
does not include the tolerance of the resistor RSET. If
maximum accuracy is required, a precision resistor is
needed. To calculate the error ILED-ERR[%], use the formula
ꢁ
IIN = IOUT Mode + IQ =
ILED-ERR [%] = ±
I
- I
LED
(LED)
MEASURED
100
%
ꢁ
I
(ILED1+ILED2+ILED3+ILED4
Example 1:
Mode = 1x, IQ = 2.4mA,
)
Mode + IQ
LED
Current Matching
Current Matching refers to the difference in current from
one LED to the next. The ∆I between any two LEDs will
meet this requirement. To calculate the error ILED-LED-ERR
,
ꢁ
I
LED1+ILED2+ILED3+ILED4 = 4 15mA = 60mA
first identify the highest and lowest value of the 4 LED
currents, and use the formula:
Answer 1:
IMAX
IMAX + IMIN
2
ILED-LED-ERR [%] =
- 1
100%
100%
ꢁ
ꢁ
IIN = IOUT Mode + IQ = 60mA 1 + 2.4mA =
62.4mA
Example 2:
or
I
MIN
- 1
IMAX + IMIN
2
Mode = 1.5x, IQ = 2.4mA,
ꢁ
ILED1+ILED2+ILED3+ILED4 = 4 15mA = 60mA
Answer 2:
I
MAX
I
MIN
which reduces to ±
100%
+
I
I
MAX
MIN
ꢁ
ꢁ
IIN = IOUT Mode + IQ = 60mA 1.5 + 2.4mA =
92.4mA
1x Mode, 1.5x Mode and 2x Mode
1x Mode, 1.5x Mode and 2x Mode all refer to the charge
pump configuration. These modes boost the battery input
voltage and ensure there is enough voltage at VOUT so
that the regulated current will flow through the LEDs and
return via the ILED pins.
Mode Transition Voltage
Mode transition voltage refers to the input voltage at
the point just before the charge pump changes from a
weaker mode to a stronger mode. VTRANS1X is the
transition from 1x to 1.5x mode, and VTRANS1.5X is the
transition from 1.5x to 2x mode. Equations for VTRANS1X
and VTRANS1.5X are given on page 7.
2005 Semtech Corp.
www.semtech.com
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