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参数资料
| 型号： | MAX8784ETL+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 19/24页 |
| 文件大小： | 0K |
| 描述： | IC REG STP UP W/AMP 40-TQFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 应用： | LCD 电视机/监控器 |
| 电源电压： | 4 V ~ 5.5 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 40-WFQFN 裸露焊盘 |
| 供应商设备封装： | 40-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
Step-Up Regulator, Internal Charge Pumps, Switch
Control, and Operational Amplifier for TFT LCDs
Design Procedure
Step-Up Regulator
Inductor Selection
The inductance value, peak-current rating, and series
resistance are factors to consider when selecting the
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:
inductor. These factors influence the converter’s effi-
ciency, maximum output-load capability, transient
response time, and output voltage ripple. Physical size
and cost are also important factors to be considered.
,
I IN ( DCMAX ) =
I AVDD(MAX ) × V AVDD
V IN ( MIN ) × η MIN
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
Calculate the ripple current at that operating point and
the peak current required for the inductor:
value. Very high-inductance values minimize the cur-
rent ripple, and therefore, reduce the peak current,
which decreases core losses in the inductor and I 2 R
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
I LI _ RIPPLE =
V IN(MIN) × ( V AVDD ? V IN(MIN) )
L I × V AVDD × f SW
turns of wire, which increase physical size and can
increase I 2 R losses in the inductor. Low-inductance val-
ues decrease the physical size, but increase the cur-
,
I AVDD _ PEAK = I IN ( DCMAX ) +
I LI _ RIPPLE
2
14 V ? 5 V
? 5 V ? ?
? ? 0 . 85 ?
L I = ? ? ? ? ? ? ≈ 3 . 0 μ H
rent ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size, and cost.
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 at the full-load current. The
best trade-off between inductor size and circuit effi-
ciency for step-up regulators generally has an LIR
between 0.3 to 0.5. However, depending on the AC
characteristics of the inductor core material and ratio of
inductor resistance to the 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
The inductor ’s saturation current rating and the
MAX8784’s LX current limit should exceed I LI _ 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 ? series resistance.
Considering the typical operating circuit in Figure 1, the
maximum load current (I AVDD(MAX) ), with charge-pump
loads, is 820mA with a 14V output and a typical input
voltage of 5V. Choosing an LIR of 0.35 and estimating
efficiency of 85% at this operating point:
2
? 14 V ? ? 0 . 82 A × 1 . 2 MHz ? ? 0 . 35 ?
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 85% at that operating point:
path. If extremely thin high-resistance inductors are
used, as is common for LCD applications, the best LIR
can increase to between 0.5 and 1.0.
,
I IN ( DCMAX ) =
0.82A × 14V
4 . 5 V × 0 . 85
≈ 3 . 00 A
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficien-
The ripple current and the peak current are:
cy improvements in typical operating regions.
Calculate the approximate inductor value using the typ-
ical input voltage (VIN), the maximum output current
(I AVDD(MAX) ), the expected efficiency ( η TYP ) taken from
I LI _ RIPPLE =
4 . 5 V × ( 14 V ? 4 . 5 V )
3 . 0 μ H × 14 V × 1 . 2 MHz
≈ 0 . 69 A
an appropriate curve in the Typical Operating Char-
acteristics, and an estimate of LIR based on the above
discussion:
I LI _ PEAK = 3 . 0 A +
0.69A
2
≈ 3 . 35 A
V IN ? ?
? ? η TYP ?
?
V AVDD ? V IN
? V AVDD ? ? I AVDD ( MAX ) × f SW ? ? ? LIR ?
2
L I = ? ? ? ? ?
______________________________________________________________________________________
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