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| 型号： | SC443ULTRT |
| 厂商： | Semtech |
| 文件页数： | 14/19页 |
| 文件大小： | 0K |
| 描述： | IC LED DRIVR CONST CURRENT 16MLP |
| 标准包装： | 1 |
| 恒定电流： | 是 |
| 拓扑： | PWM，升压（升压） |
| 输出数： | 3 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 背光 |
| 类型 - 次要： | 白色 LED |
| 频率： | 640kHz ~ 960kHz |
| 电源电压： | 4.5 V ~ 21 V |
| 输出电压： | 42V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-UFQFN 裸露焊盘 |
| 供应商设备封装： | 16-MLP（3x3） |
| 包装： | 标准包装 |
| 工作温度： | -40°C ~ 85°C |
| 其它名称： | SC443ULTRTDKR |
SC443
Applications Information (continued)
Output Capacitor Selection
When the boost converter runs in DCM ( L < L boundary ), it
takes advantage of small inductance and quick transient
response while avoiding the bandwidth-limiting instabil-
ity of the RHP zero found in CCM boost converters.
The inductor peak current is,
The converter will work in CCM if L > L boundary . Generally
the converter has higher efficiency under CCM and the
inductor peak current is,
The next task in design is targeting the proper amount
of ripple voltage due to the constant-current LED loads.
The two error amplifiers that control the PWM converter
sense the delta between requested current and actual
current in each output current regulator. On a cycle-
by-cycle basis, a small amount of output ripple ensures
good sensing and tight regulation, while the output cur-
rent regulators keep each LED current at a fixed value.
Overall, this allows usage of small output caps while
ensuring precision LED current regulation. Although the
mechanics of regulation and frequency dependence may
be complex, actual selection of the output capacitor can
be simplified because this capacitor is mainly selected
for the output ripple of the converter. Assume a ceramic
capacitor is used. The minimum capacitance needed for
a given ripple can be estimated by,
For many applications an inductor with a value of 4.7 μ H
to 22 μ H should be fine. The inductor peak current must
be less than its saturation rating. When the inductor cur-
C OUT
(V OUT
V OUT
V I N ) I OUT
F S V RIPPLE
rent is close to the saturation level, its inductance can
decrease 20% to 35% from the 0A value depending on
the vendor specifications. Using a small value inductor
forces the converter into DCM, in which case the induc-
tor current ramps down to zero before the end of each
switching cycle. It also reduces the boost converter ’s
maximum output current and produces large input volt-
age ripple. An inductor with larger inductance will reduce
the bandwidth of the feedback loop and possibly higher
DC resistance (DCR). Inductor ’s DCR plays a significant
role for the total efficiency since the power transistor is
integrated inside the SC443. Of course, there is a trade-off
between the DCR and inductor size. Table 2 lists recom-
mended inductors and their vendors.
Table 2. Recommended Inductors
Inductor Website
DR74, 4.7μH ~ 15μH www.cooperet.com
IHLP-2525CZ-01, 4.7μ ~ 10μH www.vishay.com
V RIPPLE – Peak-to-peak output ripple;
I OUT – Output current;
V OUT – Boost output voltage;
V IN – Input voltage;
F S – Switching frequency.
During load transient, the output capacitor supplies or
absorbs additional current before the inductor current
reaches its steady state value. Larger capacitance helps
with the overshoot and undershoots during load tran-
sient, and loop stability. Recommended ceramic capacitor
manufacturers are listed in Table 3.
Table 3. Recommended Ceramic Capacitor
Manufacturers
Vendor Phone Website
Kemet 408-986-0424 www.kemet.com
DS85LC, 6.8μH ~ 10μH
www.tokoam.com
Murata
Taiyo Yuden
814-237-1431
408-573-4150
www.murata.com
www.t-yuden.com
? 2009 Semtech Corp.
www.semtech.com 14
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