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參數(shù)資料
| 型號(hào)： | SC606AEVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Low Noise Smart LED Driver with Serial Interface |
| 中文描述： | 低噪聲智能串行接口的LED驅(qū)動(dòng)器 |
| 文件頁(yè)數(shù)： | 7/18頁(yè) |
| 文件大?。?/td> | 385K |
| 代理商： | SC606AEVB |
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7
2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
Applications Information
SC606
Detailed Description
The SC606 contains a fractional charge pump, mode selection
circuit, serial I/O logic, serial data registers and current regulation
circuitry for 6 LED outputs. All are depicted in the Block Diagram
on page 6.
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 1.33MHz. 250kHz may require additional input
capacitance of up to 10
μ
F. 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 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 2.85V 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 on page 10 in Table 1. LED on/off functions are
independently controlled, so that any combination of LEDs may be
switched on. LED current is set per LED pair. For example, when
ILEDA1 and ILEDA2 are both on, ILEDA1 = ILEDA2. But ILEDA1 and
ILEDA2 do not have to be on at the same time.
Six (6) current regulating circuits sink matched currents from the
LEDs on a per pair basis. LEDs with matched forward voltage will
produce the best possible matched currents. For best matching
performance 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 SC606 are mostly
dependent on the charge pump mode of operation. To get the
best performance from the SC606 it is better to use LEDs with
consistantly lower V
voltage. 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 voltages V
TRANS1X
and V
TRANS1.5X
can be estimated
by the following equations:
V
TRANS1X
= V
F
+ V
ILEDn
+(# of LEDs used) I
LED
0.83
V
TRANS1.5X
= [V
F
+ V
ILEDn
+(# of LEDs used) I
LED
10.1]/1.5
where, V
is the anode to cathode voltage and V
is the voltage
at the ILED pin. Typically V
ILED
=120mV and I
LED
is the LED current.
Power efficiency can be estimated for the intended battery voltage
range.
η
= [V
OUT
I
OUT
/ V
IN
(I
OUT
Mode+I
Q
)] 100 %
where, VOUT = V
+ V
and
I
Q
= 1.5mA, in 1x mode
I
Q
= 3mA, in 1.5x or 2x mode
Quiescent Current
I
at no load will vary with the device state. A sequence of steps is
now described which will demonstrate the effect that the device
state has on I
Q
when VIN = 3.8V.
(1) After power up (Enable High) and before any serial
communication, the I
is approximately 2.2mA. This is because
the output defaults to 5V and the charge pump is active to support
the 5V. The charge pump also defaults to 1.33MHz.
(2) After power up if the clock is changed to 250kHz, the I
Q
will
decrease to 1mA.
(3) If one LED is set on at 0.5mA, the input current will be 1.5mA.
For a typical white LED, the 5V output is much higher than
necessary, so the charge pump will switch off and the output will
adjust to a normal value for the LED, typically around 3.5V.
(4) Turn off the LED and I
= I
= 80
μ
A with Enable = High. This is
the lowest power state while Enable = High.
I
LED
Accuracy
I
is determined by the status of registers at 00h, 01h and 02h.
The tolerance of the I
current is +/-6.5% at the 10mA setting.
For example, if the data registers are configured so that each LED
current should be 10mA, the actual LED currents would be between
9.35mA and 10.65mA (+/-6.5%). All 6 outputs meet this
requirement over the industrial temperature range. To calculate
the error I
LED-ERR
[%], use the formula,
I
LED-ERR
[%] = ±
I
LED(measured) -
I
LED
I
LED
100%
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