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| 型号： | MAX8739ETP+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 16/19页 |
| 文件大小： | 0K |
| 描述： | IC DC/DC CONV W/OPAMP 20TQFN-EP |
| 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： | 1.8 V ~ 5.5 V |
| 输出数： | 1 |
| 输出电压： | 1.8 V ~ 13 V |
| 工作温度： | 0°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 20-WQFN 裸露焊盘 |
| 供应商设备封装： | 20-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
TFT, LCD, DC-DC Converter with
Operational Amplifiers
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
ratio of inductor resistance to other power-path resis-
I IN(DC,MAX) . For good efficiency, choose an inductor
with less than 0.1 ? series resistance.
Considering the Typical Operating Circuit , the maxi-
mum load current (I MAIN(MAX) ) is 300mA, with an 8V
output and a typical input voltage of 2.5V. Choosing an
LIR of 0.4 and estimating efficiency of 85% at this oper-
ating point:
L = ?
? 2 . 5 V ?
? 8 V ?
? ≈ 3 . 0 μ H
× ?
? × ?
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
?
2
? 8 V ? 2 . 5 V ? ? 0 . 85 ?
? 0 . 3 A × 1 . 2 MHz ? ? 0 . 4 ?
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can decrease losses throughout the power
Using the circuit’s minimum input voltage (2.2V) and
estimating efficiency of 80% at that operating point:
path. If extremely thin, high-resistance inductors are
used, as is common for LCD panel applications, the
best LIR can increase to between 0.5 and 1.0.
I IN ( DC , MAX ) =
0.3 A × 8 V
2 . 2 V × 0 . 8
≈ 1 . 36 A
I RIPPLE =
≈ 0 . 44 A
I PEAK = 1 . 36 A + ≈ 1 . 58 A
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficien-
cy improvements in typical operating regions.
Calculate the approximate inductor value using the
typical 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
Characteristics , and an estimate of LIR based on the
above discussion:
The ripple current and the peak current are:
2 . 2 V × ( 8 V ? 2 . 2 V )
3 . 0 μ H × 8 V × 1 . 2 MHz
0 . 44 A
2
Output-Capacitor Selection
L = ?
? V MAIN ?
? η TYP ?
?
? × ?
?
? V IN ?
?
2
?
× ?
?
V MAIN ? V IN
I MAIN ( MAX ) × f OSC ? ? LIR ?
The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and dis-
charging of the output capacitance, and the ohmic rip-
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-
ple due to the capacitor’s equivalent series resistance
(ESR):
V RIPPLE = V RIPPLE(C) + V RIPPLE(ESR)
× ? MAIN IN ?
servation of energy and the expected efficiency at that
operating point ( η MIN ) taken from an appropriate curve
in the Typical Operating Characteristics :
V RIPPLE ( C ) ≈
I MAIN
C OUT
? V ? V ?
? V MAIN × f SW ?
I IN ( DC , MAX ) =
I MAIN(MAX) × V MAIN
V IN ( MIN ) × η MIN
and:
V RIPPLE(ESR) ≈ I PEAK x R ESR
Calculate the ripple current at that operating point and
the peak current required for the inductor:
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 RIPPLE =
V IN ( MIN ) × ( V MAIN ? V IN ( MIN ) )
L × V MAIN × f OSC
V RIPPLE(C) . The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
Input-Capacitor Selection
I PEAK IN ( DC , MAX ) + RIPPLE
= I
I
2
The inductor ’s saturation current rating and the
MAX8739’s LX current limit (I LIM ) should exceed I PEAK
and the inductor’s DC current rating should exceed
The input capacitor (C IN ) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion into the IC. A 10μF ceramic capacitor is used in the
Typical Application Circuit (Figure 1) because of the
high source impedance seen in typical lab setups.
Actual applications usually have much lower source
16
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