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參數(shù)資料
| 型號： | LM3570SD |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | Low Noise White LED Driver System |
| 中文描述： | LED DISPLAY DRIVER, DSO14 |
| 封裝： | 4 X 3 MM, 0.80 MM HEIGHT, LLP-14 |
| 文件頁數(shù)： | 11/13頁 |
| 文件大小： | 352K |
| 代理商： | LM3570SD |
Application Information
(Continued)
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 3-LED application circuit.
LED HEADROOM VOLTAGE (V
HR
)
Three current sources are connected internally between
V
OUT
and D
-D
. The voltage across each current source,
(V
V
), is referred to as headroom voltage (V
). The
current sources require a sufficient amount of headroom
voltage to be present across them in order to regulate prop-
erly. Minimum required headroom voltage is proportional to
the current flowing through the current source, as dictated by
the equation:
V
HR-MIN
= k
HR
x I
DX
The parameter k
, typically 25mV/mA in the LM3570, is a
proportionality constant that represents the ON-resistance of
the internal current mirror transistors. For worst-case design
calculations, using a k
of 30mV/mA is recommended.
(Worst-case recommendation accounts for parameter shifts
from part-to-part variation and applies over the full operating
temperature range).
Figure 4
shows how output current of
the LM3570 varies with respect to headroom voltage.
On the flat part of the graph, the currents regulate properly
as there is sufficient headroom voltage for regulation. On the
sloping part of the graph the headroom voltage is too small,
the current sources are squeezed, and their current drive
capability is limited. Changes in headroom voltage from one
output to the next, possible with LED forward voltage mis-
match, will result in different output currents and LED bright-
ness mismatch. Thus, operating the LM3570 with insufficient
headroom voltage across the current sources should be
avoided.
TABLE 1. I
, R
and V
HR-MIN
k
HR
= 30 mV/mA (worst-case), V
OUT
=4.3V
I
OUT
10mA
R
SET
12.4k
V
HEADROOM
300mV
I
OUT
15mA
25mA
R
SET
8.35k
6.25k
V
HEADROOM
450mV
750mV
SOFT START
Soft start is implemented internally by ramping the reference
voltage more slowly than the applied voltage. During soft
start, the current through the LED outputs and the V
voltage will ramp up in proportion to the rate that the refer-
ence voltage is being ramped up.
ENABLE / SHUTDOWN
When the voltage on the active-high-logic enable pin (EN) is
low, the LM3570 will be in shutdown. While disabled, the
LM3570 typically draws 0.1μA. When the EN pin is uncon-
nected, the part automatically goes into shutdown due to an
internal 300k
pull-down resistor that is tied between EN
and GND. When the part is in shutdown, it is important to
have the PWM pin also set to ground to avoid a leakage
current resulting from an internal 300k
pull-down resistor
tied between the PWM pin and ground
PWM Pin
The PWM pin on the LM3570 is responsible for turning the
three constant current sources (D1-D3) on or off without
disabling the charge pump. This pin allows for PWM bright-
ness control on the diode outputs without affecting whatever
load is tied to the V
pin. The PWM pin has an internal
300k
pull-down resistor that by default turns off the diode
outputs when no control signal is active.
I
CURRENT SELECTION PROCEDURES USING THE
PWM PIN
The following procedures illustrate how to set and adjust
output current levels using the PWM pin.
Brightness Control Using PWM
1.
Determine the maximum desired I
LED
current. Use the
I
Dx
equation to calculate R
SET
2.
Brightness control can be implemented by pulsing a
signal at the PWM pin. LED brightness is proportional to
the duty cycle (D) of the PWM signal. For linear bright-
ness control over the full duty cycle adjustment range,
the PWM frequency (f) should be limited to accommo-
date the turn-on time (T
ON
= 100μs) of the current
sources.
D x (1/f)
>
T
ON
f
MAX
= D
MIN
÷ T
ON
If the PWM frequency is much less than 100Hz, flicker
may be seen in the LEDs. For the LM3570, zero duty
cycle will turn off the LEDs and a 50% duty cycle will
result in an average I
LED
being half of the programmed
LED current. For example, if R
SET
is set to program
15mA, a 50% duty cycle will result in an average I
LED
of
7.5mA.
CAPACITOR SELECTION
The LM3570 requires 4 external capacitors for proper opera-
tion (C
=C
=1μF, C
= 2.2μF, C
=3.3μF). Surface-mount
multi-layer ceramic capacitors are recommended. These ca-
pacitors are small, inexpensive and have very low equivalent
series resistance (
≤
10m
typ.). Tantalum capacitors, OS-
CON capacitors, and aluminum electrolytic capacitors gen-
erally are not recommended for use with the LM3570 due to
20085008
FIGURE 4. I
LED
vs V
HR
3 LEDs, V
IN
= 3.6V
L
www.national.com
11
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3570SD/NOPB | 功能描述:IC LED DRIVR WHITE BCKLGT 14-LLP RoHS:是 類別:集成電路 (IC) >> PMIC - LED 驅(qū)動器 系列:PowerWise® 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 |
| LM3570SDX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Low Noise White LED Driver System |
| LM3578 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Switching Regulator |
| LM3578A | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Switching Regulator |
| LM3578AM | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
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