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| 型号： | LM2795TLX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 14/23页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR WHT BCKLT 14USMD |
| 标准包装： | 3,000 |
| 恒定电流： | 是 |
| 拓扑： | PWM，切换式电容器（充电泵） |
| 输出数： | 4 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 背光 |
| 类型 - 次要： | 白色 LED |
| 频率： | 325kHz ~ 515kHz |
| 电源电压： | 2.7 V ~ 5.5 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-WFBGA |
| 供应商设备封装： | 14-TuSMD |
| 包装： | 带卷 (TR) |
| 工作温度： | -30°C ~ 85°C |
| 配用： | LM2795EVAL-ND - BOARD EVALUATION LM2795 |
| 其它名称： | LM2795TLX |
SNVS168L – JANUARY 2002 – REVISED MAY 2013
18
16
14
12
10
8
6
4
2
0
R SET = 124Ω
R SET = 475Ω
R SET = 2.67kΩ
0.05
0.20
0.35
0.50
0.65
0.80
V HR (V)
Figure 24. I LED vs V HR
4 LEDs, V IN = 3.0V
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 mismatch, will result in different output currents and LED brightness mismatch. Thus, operating
the LM2794/5 with insufficient headroom voltage across the current sources should be avoided.
OUTPUT CURRENT CAPABILITY
The primary constraint on the total current capability is the headroom voltage requirement of the internal current
sources. Combining the V POUT and V HR equations from the previous two sections yields the basic inequality for
determining the validity of an LM2794/5 LED-drive application:
V POUT = 1.5 × V IN ? I TOTAL × R OUT
V HR-MIN = k HR × I DX
V POUT ? V DX ≥ V HR-MIN
1.5 × V IN ? I TOTAL × R OUT ? V DX ≥ (k HR × I DX )
Rearranging this inequality shows the estimated total output current capability of an application:
I TOTAL ≤ [(1.5 × V IN-MIN ) ? V DX-MAX ? (k HR × I DX )] ÷ R OUT
(4)
(5)
(6)
(7)
(8)
Examining the equation above, the primary limiting factors on total output current capability are input and LED
forward voltage. A low input voltage combined with a high LED voltage may result in insufficient headroom
voltage across the current sources, causing them to fall out of regulation. When the current sources are not
regulated, LED currents will be below desired levels and brightness matching will be highly dependent on LED
forward voltage matching.
Typical LM2794/5 output resistance is 3.0 ? . For worst-case design calculations, using an output resistance of
3.5 ? is recommended. LM2794/5 has a typical k HR constant of 20mV/mA. For worst-case design calculations,
use k HR = 25mV/mA. (Worst-case recommendations account for parameter shifts from part-to-part variation and
apply over the full operating temperature range). R OUT and k HR increase slightly with temperature, but losses are
typically offset by the negative temperature coefficient properties of LED forward voltages. Power dissipation and
internal self-heating may also limit output current capability but is discussed in a later section.
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT DRIVE
Outputs D 1 through D 4 may be connected together in any combination to drive higher currents through fewer
LEDs. For example in Figure 25 , outputs D 1 and D 2 are connected together to drive one LED while D 3 and D 4
are connected together to drive a second LED.
14
Product Folder Links: LM2794 LM2795
Copyright ? 2002–2013, Texas Instruments Incorporated
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| LM27961TL/NOPB | 功能描述:IC LED DRVR WHT BCKLT 18USMD RoHS:是 类别:集成电路 (IC) >> PMIC - LED 驱动器 系列:- 标准包装:1 系列:- 恒定电流:- 恒定电压:- 拓扑:PWM，切换式电容器（充电泵） 输出数:1 内部驱动器:是 类型 - 主要:背光 类型 - 次要:白色 LED 频率:642kHz 电源电压:2.7 V ~ 5.5 V 输出电压:5V 安装类型:表面贴装 封装/外壳:10-VFDFN 裸露焊盘 供应商设备封装:10-VSON 包装:剪切带 (CT) 工作温度:-30°C ~ 85°C 产品目录页面:1371 (CN2011-ZH PDF) 其它名称:BD1603NUV-E2CT |
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