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8-String White LED Driver with
SMBus for LCD Panel Applications
24 . 7 mV × ( 0 . 75 ? D )
I OUT _ CCM ( MAX ) = ? I LIM ?
The output current capability of the step-up converter is
a function of current limit, input voltage, operating fre-
quency, and inductor value. Because the slope com-
pensation is used to stabilize the feedback loop, the
inductor current limit depends on the duty cycle, and is
determined with the following equation:
I LIM = 1 . 9 A +
R S
where 24.7mV is the scale factor from the slope com-
pensation, 1.9A is a typical current limit at 75% duty
cycle, and D is the duty cycle.
The output current capability depends on the current-
limit value and operating mode. The maximum output
current in CCM is governed by the following equation:
? 0.5 × D × V IN ?
?
? f OSC × L ?
where I LIM is the current limit calculated above, η is the
nominal regulator efficiency (85%), and D is the duty
cycle. The corresponding duty cycle for this current is:
Finding the best inductor involves the compromises
among circuit efficiency, inductor size, and cost.
In choosing an inductor, the first step is to determine
the operating mode: continuous conduction mode
(CCM) or discontinuous conduction mode (DCM). The
MAX17061A has a fixed internal slope compensation
that requires minimum inductor value. When CCM
mode is chosen, the ripple current and the peak cur-
rent of the inductor can be minimized. If a small-size
inductor is required, DCM mode can be chosen. In
DCM mode, the inductor value and size can be mini-
mized, but the inductor ripple current and peak current
are higher than those in CCM. The controller can be
stable, independent of the internal slope compensation
mode, but there is a maximum inductor value require-
ment to ensure the DCM operating mode.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current
to the average DC inductor current at the full load cur-
rent. The controller operates in DCM mode when LIR is
higher than 2.0, and it works in CCM mode when LIR is
lower than 2.0. The best trade-off between inductor size
D =
V OUT ? V IN + V DIODE
V OUT ? I LIM × R ON + V DIODE
and converter efficiency for step-up regulators generally
has an LIR between 0.3 and 0.5. However, depending
on the AC characteristics of the inductor core material
where V DIODE is the forward voltage of the rectifier
diode and R ON is the internal MOSFET’s on-resistance
(0.15 Ω typ).
The maximum output current in DCM is governed by
the following equation:
and ratio of inductor resistance to other power-path
resistances, the best LIR can shift up or down. If the
inductor resistance is relatively high, more ripples can
be accepted to reduce the number of required turns and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
L × I LIM OSC × η
× f
I OUT _ DCM ( MAX ) =
2
2 × ( V OUT + V DIODE ? V IN )
current can reduce losses throughout the power path. If
extremely thin high-resistance inductors are used, as is
common for LCD panel applications, LIR higher than 2.0
can be chosen for DCM operating mode.
? V IN _ MIN ? ? V OUT ? V IN _ MIN ? ? η TYP ?
L = ? ? ? ? ? ?
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when selecting
an inductor. These factors affect the converter’s operating
mode, efficiency, maximum output load capability, tran-
sient response time, output voltage ripple, and cost.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance minimizes the current rip-
ple, and therefore reduces the peak current, which
decreases core losses in the inductor and I 2 R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increases physical size and I 2 R copper
losses. Low inductor values decrease the physical size,
but increase the current ripple and peak current.
Once a physical inductor is chosen, higher and lower val-
ues of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detail
design procedure for CCM can be described as follows:
Inductor Selection in CCM Operation
1) Calculate the approximate inductor value using the
typical input voltage (V IN ), the maximum output cur-
rent (I OUT(MAX) ), the expected efficiency ( η TYP )
taken from an appropriate curve in the Typical
Operating Characteristics, and an estimate of LIR
based on the above discussion:
2
? V OUT ? ? I OUT ( MAX ) × f OSC ? ? LIR ?
20
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MAX17061AEVCMAXQU+ 功能描述:LED 照明开发工具 Evaluation Kit/Evaluation system for MAX17061A RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装:
MAX17061AEVKIT+ 功能描述:LED 照明开发工具 MAX17061A Eval Kit RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装:
MAX17061ETI+ 功能描述:LED照明驱动器 8-String White LED Driver w/SMBus 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
MAX17061ETI+T 功能描述:LED照明驱动器 8-String White LED Driver w/SMBus 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
MAX17061EVCMAXQU+ 功能描述:LED 照明开发工具 Evaluation Kit/Evaluation System for the MAX17061 RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装:
MAX17061EVKIT+ 功能描述:LED 照明开发工具 MAX17061 Eval Kit RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装:
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MAX17062ETB+T 功能描述:直流/直流开关转换器 TFT-LCD Step-Up DC/DC Converter RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT