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参数资料
| 型号： | LC41059FN |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHED CAPACITOR CONVERTER, QCC28 |
| 封装： | QFN-28 |
| 文件页数： | 4/4页 |
| 文件大小： | 59K |
| 代理商： | LC41059FN |
4
///// Operation /////
1. Circuit Description
The LC41059 is a white LED supply which accepts the input voltage from the Li-Ion battery.
While the
Li-Ion battery has the output voltage range from 2.7 to 5.5V, the forward voltage (VF) of the white LED is
about 3.6V typically.
So when the battery degrades the output voltage to about 3.6V, the higher voltage
must be generated internally.
The LC41059 employs a charge pump to step up the output voltage to 1.5
times or 2 times the input voltage.
This charge pump selects the optimum boost mode, and changes the
boost mode automatically.
LED current (ILED) is determined by the BRGT input voltage and the resistance of the external resistor.
Same voltage of BRGT (VBRGT) is forced on the external resistor with high accuracy by an internal Op-Amp,
so the current passing through the external resistor (IRSET) will be IRSET = VBRGT / RSET.
LC41059 includes
the current regulator to deliver the regulated current to the LEDs, which are composed of current mirrors
with a 40 to 1 ratio.
For example, when VBRGT= 1.2V,
RSET= 2000ohms, then IRSET =0.6mA, and ILED=
IRSET x 40= 24mA .
If four LEDs are attached, the device drive up a total of 24mA x 4= 96mA through the
LEDs.
The LED brightness can be controlled by both analog and/or digital methods. The digital technique uses a PWM
(Pulse Width Modulation) signal applied to the SD pin (see 4.Brightness Control Using PWM). The analog
technique applies an analog voltage to the BRGT pin (see 5.BRGT Pin).
2. Soft Start
LC41059 includes a soft start function to reduce the inrush current.
Inrush current will be generated when
the charge pump operates with flying capacitors discharged.
When the capacitors are being charged with
low impedance, excess current may surge into the capacitors from the battery.
Soft start is done to reduce
stress on the battery and external components.
During soft start, the switch resistances limit the inrush
current used to charge the flying and hold capacitors.
3. Shutdown Mode
A shutdown pin is available to disable the LC41059 and reduce the quiescent current to 1A maximum.
During normal operation mode, applying a high level input to SD pin will enable the device.
Pulling a low
level input to SD pin will disable the device.
4. Brightness Control Using PWM
Brightness control can be implemented by pulsing a signal at the SD pin.
The recommended frequency is
between 100Hz to 2kHz.
If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs.
Likewise, if frequency is much higher, brightness in the LEDs will not be linear.
Increasing and decreasing
the duty cycle of the PWM signal control the brightness.
Zero duty cycle will turn off the LED, and a 50%
duty cycle waveform produces an average current of 10mA if RSET is set to produce a maximum LED
current of 20mA.
So the LED current varies linearly with the duty cycle.
5. BRGT Pin
The BRGT pin can be used to smoothly vary the brightness of the White LEDs.
In the LC41059, this
voltage is fed to the Op-Amp that controls the current through the mirror resistor RSET.
The nominal range
on BRGT is 0V to 1.25V.
Care must be taken to prevent voltages on BRGT that cause LED current to
exceed a total of 100 mA.
Although this will not cause damage to the IC, it will not meet the guaranteed
specifications listed in the “Electrical Characteristics” chapter.
TENTATIVE
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