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S P 7 6 4 8
L o w R e f e r e n c e H i g h E f f i c i e n c y B o o s t R e g u l a t o r
For the example, a 10μH inductor would have
an inductor current ripple of 350mA, while a
4.7μH inductor would have an in ductor current
ripple value of 740mA. Do not allow tantalum
capacitors to exceed their ripple-current
ratings. An input filter capacitor can reduce
peak currents drawn from the battery and
improve efficiency. For most applications, use
the same capacitor for the input and output.
Low-ESR tantalum capacitors are acceptable
provided they meet the ESR requirement of
0.2 ? to 0.3 ? . In selecting an inductor, the
saturation current specified for the inductor
needs to be greater than the SP7648 peak
current to avoid saturating the inductor, which
would result in a loss of efficiency and could
damage the inductor. The SP7648 evaluation
board uses a Wurth 4. 7μH inductor with an
I SAT value of 1.7A and a DCR of 0.065 ? , which
handles the I PEAK of 1.6A of the SP7648 and
will deliver high efficiencies. Other inductors
could be selected provided their I SAT is greater
than the I PEAK of the SP7648.
V OUT P ROGRAMMING
The SP7648 can be programmed as either a
voltage source or a current source. To
program the SP7648 as voltage source, the
SP7648 requires 2 feedback resistors R1 & R2
to control the output voltage. To set V OUT in
the voltage mode, use the equation:
R1 = [(V OUT /0.8)-1]xR2 where flash < 0.4V,
R1 = [(V OUT /0.288)-1]xR2 where flash > 1.0V.
U SING THE R LIM F UNCTION
The peak inductor current, I PEAK , is
programmed externally by the R LIM resistor
connected between the R LIM pin and GND. The
peak inductor current is defined by:
I PEAK = 1600/R LIM
The saturation current specified for the
inductor needs to be greater than the peak
current to avoid saturating the inductor, which
would result in a loss in efficiency and could
damage the inductor. The SP7648 evaluation
board uses a R LIM value of 1K ? for an I PEAK =
1.6A to allow the circuit to deliver up to
700mA for V IN = 3.3V and V OUT = 4.2V. Other
values could be selected using the above
relationships.
U SING THE FLASH C ONTROL P IN
The SP7648 will regulate the output by the
equations above depending on the state of the
FLASH pin. When the FLASH pin is low
(<0.4V), the internal reference voltage is
defined as 0.8V. When the FLASH pin is high
(>0.4V), the internal reference voltage is
defined as 0.288V. This allows the use of
smaller values for the sense resistor for
current regulation mode. This improves
efficiency and reduces the physical size of the
sense resistor. An external MOSFET switch can
be used to change the sense resistor when
changing to the Flash Mode.
H IGH B RIGHTNESS W HITE LED
For the high brightness LumiLED white LED
application, the SP7648 is generally
programmed as a current source. The bias
resistors R1 and R2 are used to set the
operating current of the white LED with the
equation:
R = V FB /I F
where V FB is 0.8V in torch mode and 0.288V in
flash mode, I F is the operating current of the
LED. To set the operating current to be about
200mA in torch mode, the flash pin is forced
low, R2 is selected as 0.8V/ 0.2 = 4 ? , as
shown in the typical application circuit. To set
the operating current to 700mA in flash mode,
the flash pin is forced high, R is selected as
0.288V/0.41 ? = 700mA. In real ity R in Flash
includes the series MOSFET R DSON and the
parallel combination of R 2 = 4 ? shown by the
formula:
R in Flash = (R 1 x (R 2 + Q 1 R DSON )) / (R 1 + R 2 +
Q 1 R DSON )
If the SP7648 is powered up before the LED is
plugged in, the circuit will bring the feed-back
pin to ZeroV and the SP7648 has a feature to
set the output voltage to be 5V. Once the LED
is plugged in, the feedback pin will go up to
0.8V in torch mode or 0.288V in flash mode
and begin to regulate. The output voltage will
go from 5V to V F +V FB , where V F is the forward
voltage of the LED. When the LED is open, the
? 2012 Exar Corporation
9/13
Rev. 2.0.0
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相关代理商/技术参数
SP7648ER
制造商:SIPEX 制造商全称:Sipex Corporation 功能描述:Low Reference High Efficiency Boost Regulator
SP7648ER/TR
制造商:SIPEX 制造商全称:Sipex Corporation 功能描述:PackageLow Reference High efficiency Boost Regulator
SP7648ER-L
功能描述:LED照明驱动器 Efficiency Boost Regulator RoHS:否 制造商:STMicroelectronics 输入电压:11.5 V to 23 V 工作频率: 最大电源电流:1.7 mA 输出电流: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:SO-16N
SP7648ER-L/TR
功能描述:DC/DC 开关控制器 RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK
SP7648EU
制造商:SIPEX 制造商全称:Sipex Corporation 功能描述:Evaluation Board Manual
SP7648EU/TR
制造商:SIPEX 制造商全称:Sipex Corporation 功能描述:Low Reference High Efficiency Boost Regulator
SP7648EU-L
功能描述:DC/DC 开关控制器 RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK
SP7648EU-L/TR
功能描述:DC/DC 开关控制器 RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK