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| 型号： | MAX8727ETB+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 9/13页 |
| 文件大小： | 0K |
| 描述： | IC DC/DC CONV TFT-LCD 10-TDFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 应用： | 转换器，TFT，LCD |
| 输入电压： | 2.6 V ~ 5.5 V |
| 输出数： | 1 |
| 输出电压： | 2.6 V ~ 24 V |
| 工作温度： | 0°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 10-WFDFN 裸露焊盘 |
| 供应商设备封装： | 10-TDFN-EP（3x3） |
| 包装： | 带卷 (TR) |
TFT-LCD Step-Up DC-DC Converter
? 5 V ? ?
? ? 0 . 85 ?
L = ?
? ≈ 3 . 2 μ H
? 15 V ? ? 0 . 6 A × 1 . 2 MHz ? ? 0 . 35 ?
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 best trade-off between inductor size and cir-
cuit 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 and the
ratio of inductor resistance to other power path resis-
tances, the best LIR can shift up or down. If the induc-
tor resistance is relatively high, more ripple can be
accepted to reduce the number of turns required and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can decrease losses throughout the power
path. If extremely thin high-resistance inductors are
used, as is common for LCD panel applications, the
The inductor ’s saturation current rating and the
MAX8727’s LX current limit (I LIM ) should exceed I PEAK ,
and the inductor’s DC current rating should exceed
I IN(DC,MAX) . For good efficiency, choose an inductor
with less than 0.1 Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I MAIN(MAX) ) is 600mA with a 15V output and
a typical input voltage of 5V. Choosing an LIR of 0.35 and
estimating efficiency of 85% at this operating point:
2
15 V ? 5 V
? ? ? ?
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 85% at that operating point:
best LIR can increase to between 0.5 and 1.0.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficien-
I IN ( DC , MAX ) =
0.6 A × 15 V
4 . 5 V × 0 . 85
≈ 2 . 35 A
cy improvements in typical operating regions.
Calculate the approximate inductor value using the typ-
The ripple current and the peak current are:
ical input voltage (V IN ), the maximum output current
(I MAIN(MAX) ), the expected efficiency ( η TYP ) taken from
an appropriate curve in the Typical Operating
I RIPPLE =
4 . 5 V × ( 15 V ? 4 . 5 V )
3 . 6 μ H × 15 V × 1 . 2 MHz
≈ 0 . 73 A
Characteristics , and an estimate of LIR based on the
? ?
? ? η
? V
?
? ? TYP ?
L = ?
? ??
I PEAK = 2 . 35 A +
≈ 2 . 70 A
above discussion:
2
IN V MAIN ? V IN
?
? V MAIN ? ? I MAIN ( MAX ) × f OSC ? ? LIR ?
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage V IN(MIN) using con-
servation of energy and the expected efficiency at that
operating point ( η MIN ) taken from an appropriate curve
in the Typical Operating Characteristics :
0 . 73 A
2
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohmic ripple due
to the capacitor’s equivalent series resistance (ESR):
V RIPPLE = V RIPPLE ( C ) + V RIPPLE ( ESR )
I IN ( DC , MAX ) =
I MAIN(MAX) × V MAIN
V IN ( MIN ) × η MIN
V RIPPLE ( C ) ≈
I MAIN ? V MAIN ? V IN ?
? ? and
C OUT ? V MAIN f OSC ?
Calculate the ripple current at that operating point and
the peak current required for the inductor:
V RIPPLE ( ESR ) ≈ I PEAK R ESR ( COUT )
I RIPPLE =
V IN ( MIN ) × ( V MAIN ? V IN ( MIN ) )
L × V MAIN × f OSC
where I PEAK is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors,
the output voltage ripple is typically dominated by
I PEAK IN ( DC , MAX ) + RIPPLE
= I
I
2
V RIPPLE(C) . The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
_______________________________________________________________________________________
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