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參數資料
| 型號： | SC614EVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Low Noise Backlight and Flash Driver With Serial Interface |
| 中文描述： | 低噪聲背光和閃光串行接口驅動程序 |
| 文件頁數： | 12/25頁 |
| 文件大小： | 395K |
| 代理商： | SC614EVB |
2006 Semtech Corp.
POWER MANAGEMENT
Applications Information
SC614
United States Patent No. 6,504,422
www.semtech.com
12
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
mis-matches up to 0.5V. For best
matching performance, however, it is recommended that
the LED to LED difference,
Δ
V
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
. Lower V
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
mode.
Mode transition voltages V
and V
TRANS1.5X
can be
estimated by the following equations:
V
TRANS1X
= V
F
+ V
ILED
+ I
OUT
1.0
V
TRANS1.5X
= (V
F
+ V
ILED
+ I
OUT
5.5) / 1.5
where V
F
is the highest forward voltage of the operating
LEDs, V
is the current sink voltage for that LED (typically
V
=150mV) and I
OUT
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
I
OUT
/ V
IN
(I
OUT
Mode+I
Q
)] 100 %
where:
V
OUT
= V
F
+ V
ILED
(defined in the above section).
and,
I
= 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
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