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參數(shù)資料
| 型號： | LM27953 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | LED驅(qū)動器 |
| 英文描述： | White LED Driver with Four LED Current Sinks and 3/2x Switched Capacitor Boost |
| 中文描述： | 白光LED 4個LED電流吸收和3/2x開關(guān)電容升壓驅(qū)動 |
| 文件頁數(shù)： | 9/11頁 |
| 文件大小： | 681K |
| 代理商： | LM27953 |
Circuit Description
(Continued)
When the voltage on the EN pin is low (
<
0.5V), the part is in
shutdown mode. All internal circuitry is OFF and the part
consumes very little supply current when the LM27953 is
shutdown. When the voltage on the EN pin is high (
>
1.1V),
the part is active. The charge pump is ON, and the corre-
sponding output current drivers are active.
EN is also used to turn the output currents ON and OFF.
SETTING LED CURRENTS
The output currents of the LM27953 can be set to a desired
value simply by connecting an appropriately sized resistor
(R
) between the I
pin of the LM27953 and GND. The
output currents (LED currents) are proportional to the current
that flows out of the I
pin. The output currents are a factor
of 100 greater than the I
current. The feedback loop of an
internal amplifier sets the voltage of the I
pin to 1.25V
(typ.). Placing a resistor between I
and GND programs
the I
current, and thus the LED currents. The statements
above are simplified in the equations below:
I
Dx
= 100 x (V
SET
/ R
SET
)
R
SET
= 100 x (1.25V / I
Dx
)
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM27953 can drive 4 LEDs at 15mA each from an input
voltage as low as 2.7V, so long as the LEDs have a forward
voltage of 3.5V or less (room temperature).
The statement above is a simple example of the LED drive
capabilities of the LM27953. The statement contains the key
application parameters that are required to validate an LED-
drive design using the LM27953: LED current (I
LEDx
), num-
ber of active LEDs (N), LED forward voltage (V
LED
), and
minimum input voltage (V
IN-MIN
).
The equation below can be used to estimate the total output
current capability of the LM27953:
I
LED_MAX
= ((1.5 x V
IN
) - V
LED
) / ((N x R
OUT
) + k
HR
) (eq. 1)
I
LED_MAX
= ((1.5 x V
IN
) - V
LED
) / ((N x 2.7
) + 22mV/mA)
R
– Output resistance. This parameter models the inter-
nal losses of the charge pump that result in voltage droop at
the pump output P
. Since the magnitude of the voltage
droop is proportional to the total output current of the charge
pump, the loss parameter is modeled as a resistance. The
output resistance of the LM27953 is typically 2.7
(V
IN
=
3.0V, T
A
= 25C). In equation form:
V
POUT
= 1.5xV
IN
– NxI
LED
xR
OUT
k
HR
– Headroom constant. This parameter models the mini-
mum voltage required to be present across the current
sources for them to regulate properly. This minimum voltage
is proportional to the programmed LED current, so the con-
stant has units of mV/mA. The typical k
HR
of the LM27953 is
22mV/mA. In equation form:
(V
POUT
– V
LED
)
>
k
HR
xI
LED
The "I
LED-MAX
" equation (eq. 1) is obtained from combining
the R
OUT
equation (eq. 2) with the k
HR
equation (eq. 3) and
solving for I
LED
. Maximum LED current is highly dependent
on minimum input voltage and LED forward voltage. Output
current capability can be increased by raising the minimum
input voltage of the application, or by selecting an LED with
a lower forward voltage. Excessive power dissipation may
also limit output current capability of an application.
(eq. 2)
(eq. 3)
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT
CAPABILITY
Outputs D1 through D4 may be connected together in any
combination to drive higher currents through fewer LEDs.
For example in
Figure 1
, outputs D1A and D2A are con-
nected together to drive one LED. D3A and D4A are con-
nected to drive a second LED.
With this configuration, two parallel current sources of equal
value provide current to one of the LEDs. R
should
therefore be chosen so that the current through each output
is programmed to 50% of the desired current through the
parallel connected LED. For example, if 40mA is the desired
drive current for the parallel connected LED, R
should be
selected so that the current through each of the outputs is
20mA. Other combinations of parallel outputs may be imple-
mented in similar fashions, such as in
Figure 2
.
Connecting outputs in parallel does not affect internal opera-
tion of the LM27953 and has no impact on the Electrical
Characteristics and limits previously presented. The avail-
able diode output current, maximum diode voltage, and all
other specifications provided in the Electrical Characteristics
table apply to parallel output configurations, just as they do
to the standard application circuit on pg1 of the datasheet.
SOFT START
The LM27953 contains internal soft-start circuitry to limit
input inrush currents when the part is enabled. Soft start is
implemented with a controlled turn-on of the internal voltage
reference. During soft start, the current through the LED
20128019
FIGURE 1. Two Parallel Connected LEDs
20128020
FIGURE 2. One Parallel Connected LED
L
www.national.com
9
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM27953TL | 制造商:Rochester Electronics LLC 功能描述: 制造商:Texas Instruments 功能描述:IC LED DRIVER WHITE SMD 27953 |
| LM27953TL/NOPB | 功能描述:IC LED DRVR WHT BCKLT 18USMD RoHS:是 類別:集成電路 (IC) >> PMIC - LED 驅(qū)動器 系列:- 標(biāo)準(zhǔn)包裝:1 系列:- 恒定電流:- 恒定電壓:- 拓?fù)?PWM，切換式電容器（充電泵） 輸出數(shù):1 內(nèi)部驅(qū)動器:是 類型 - 主要:背光 類型 - 次要:白色 LED 頻率:642kHz 電源電壓:2.7 V ~ 5.5 V 輸出電壓:5V 安裝類型:表面貼裝 封裝/外殼:10-VFDFN 裸露焊盤 供應(yīng)商設(shè)備封裝:10-VSON 包裝:剪切帶 (CT) 工作溫度:-30°C ~ 85°C 產(chǎn)品目錄頁面:1371 (CN2011-ZH PDF) 其它名稱:BD1603NUV-E2CT |
| LM27953TLX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:White LED Driver with Four LED Current Sinks and 3/2x Switched Capacitor Boost |
| LM27953TLX/NOPB | 功能描述:IC LED DRVR WHT BCKLT 18USMD RoHS:是 類別:集成電路 (IC) >> PMIC - LED 驅(qū)動器 系列:- 標(biāo)準(zhǔn)包裝:1 系列:- 恒定電流:- 恒定電壓:- 拓?fù)?PWM，切換式電容器（充電泵） 輸出數(shù):1 內(nèi)部驅(qū)動器:是 類型 - 主要:背光 類型 - 次要:白色 LED 頻率:642kHz 電源電壓:2.7 V ~ 5.5 V 輸出電壓:5V 安裝類型:表面貼裝 封裝/外殼:10-VFDFN 裸露焊盤 供應(yīng)商設(shè)備封裝:10-VSON 包裝:剪切帶 (CT) 工作溫度:-30°C ~ 85°C 產(chǎn)品目錄頁面:1371 (CN2011-ZH PDF) 其它名稱:BD1603NUV-E2CT |
| LM2795BL | 制造商:Texas Instruments 功能描述:IC WLED DRIVER QUAD POWERWISE |
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