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參數(shù)資料
| 型號(hào)： | LC410591FN |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHED CAPACITOR CONVERTER, 600 kHz SWITCHING FREQ-MAX, QCC16 |
| 封裝： | 4 X 4 MM, VQFN-16 |
| 文件頁數(shù)： | 10/11頁 |
| 文件大小： | 84K |
| 代理商： | LC410591FN |
LC410591FN
No.A0089-8/11
Functional Description
Circuit description
The LC410591FN is a white LED driver that uses a Li-ion battery as its power supply. The output voltage range of the
Li-ion battery is approximately 2.7V to 5.5V, and a typical white LED forward voltage (VF) is approximately 3.6V.
Therefore, when the supply voltage is 3.6V or less, the voltage must be stepped up internally. The LC410591FN
incorporates a charge pump that steps up the supply voltage 1.5 times. This charge pump automatically switches to the
optimum step-up mode.
The 100% LED current is determined by the resistance values of the external resistors. In other words, the internal
reference voltage (VREF≠1.2) is applied precisely to the external resistors by the internal Op amplifier, and the current
flowing to the external resistors (IRSET) becomes IRSET=VREF/RSET. The LED current (ILED) is regulated by a
current mirror circuit so that 40 times the IRSET current flows. For example, if VREF=1.2V and RSET=2k, then
IRSET=0.6mA, and ILED=IRSET×40=24mA. In this case, for 4-light control, the total current flowing to the LED is
24mA
×4=96mA. However, this formula does not include the variation in the resistance values of the external resistors.
If accurate LED current is required, high-precision resistors (with a variation of 1% or less, for instance) may be
required.
The LED brightness is adjusted by a serial signal to the BRGT pin.
Shutdown mode
The LC410591FN enters a shutdown mode when the low level is applied to the BRGT pin for 22ms or longer. In the
shutdown mode, the LEDs turn off, and the supply current is reduced to 1
A or less.
Soft start
The LC410591FN performs a soft start to prevent rush current, and then 100% current flows to the LEDs. A rush
current is generated if the charge pump is activated when the flying capacitor is discharged. If the impedance of the
capacitor is low, an excessive current will flow in from the battery. Soft start is performed to reduce the stress on the
battery and external components. During the soft start, the resistance of the switch connected to the flying capacitor and
output smoothing capacitor is adjusted to prevent a rush current.
Brightness adjustment
LED brightness can be adjusted by controlling the current to the LED with the PWM signal supplied to the BRGT pin.
The average current flowing to the LEDs depends on the PWM signal.
[LED average current=ILED×Duty ratio] …(Fig. 2)
The current flowing to the LED (average value) changes in a nearly linear fashion with respect to the duty ratio of the
PWM signal input to the BRGT pin. …(Fig. 3)
The upper limit of the PWM signal frequency is 50kHz.
Fix the BRGT signal to the low level for at least 22ms from power supply (VIN) rising.…(Fig. 4)
LED current
t
T
Duty ratio=t/T
Frequency=1/T
(Fig. 2)
ILED
22ms or longer
VIN
BRGT
(Fig. 4)
(Fig. 3)
ILED - BRGT Duty
ILED
-m
A
BRGT Duty - %
0
20
40
60
100
80
70
60
50
40
30
20
10
0
ILC01771
VIN=3.6V,Rset=3.3k
BRGT=1kHz,4LEDs
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