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| 型号： | MAX8795AGCJ+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 21/31页 |
| 文件大小： | 0K |
| 描述： | IC CONV DC-DC TFT-LCD 32LQFP |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,000 |
| 应用： | 转换器，TFT，LCD |
| 输入电压： | 2.5 V ~ 6 V |
| 输出数： | 8 |
| 输出电压： | 2.5 V ~ 18 V |
| 工作温度： | -40°C ~ 105°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-LQFP |
| 供应商设备封装： | 32-LQFP（7x7） |
| 包装： | 带卷 (TR) |
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TFT-LCD DC-DC Converter with
Operational Amplifiers
L = ?
? ≈ 3 . 3 μ H
? 5 V ? 2 ? 14 V ? 5 V ? ? 0 . 85 ?
? 14 V ? ? 0 . 5 A × 1 . 2 MHz ? ? 0 . 5 ?
a typical input voltage of 5V. Choosing an LIR of 0.5 and
estimating efficiency of 85% at this operating point:
? ? ? ?
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 80% at that operating point:
be tolerated on C IN if IN is decoupled from C IN using an
RC lowpass filter (see R10 and C13 in Figure 1).
Rectifier Diode
The MAX8795A’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. In general, a 2A Schottky
,
I IN ( DCMAX ) =
0.5A × 14V
4 . 5 V × 0 . 8
≈ 1 . 94 A
diode complements the internal MOSFET well.
Output-Voltage Selection
I RIPPLE =
≈ 0 . 77 A
I PEAK = 1 . 94 A + ≈ 2 . 33 A
R 1 = R 2 × ? MAIN ? 1 ?
The ripple current and the peak current are:
4 . 5 V × ( 14 V ? 4 . 5 V )
3 . 3 μ H × 14 V × 1 . 2 MHz
0 . 77 A
2
Output-Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
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 )
The output voltage of the main step-up regulator can be
adjusted by connecting a resistive voltage-divider from the
output (V MAIN ) to AGND with the center tap connected to
FB (see Figure 1). Select R2 in the 10k ? to 50k ? range.
Calculate R1 with the following equation:
? V ?
? V FB ?
where V FB , the step-up regulator’s feedback set point,
is 1.233V. Place R1 and R2 close to the IC.
Loop Compensation
Choose R COMP to set the high-frequency integrator
V RIPPLE ( C ) ≈ MAIN ? MAIN IN ?
and :
I ? V ? V ?
C OUT ? V MAIN f OSC ?
V RIPPLE ( ESR ) ≈ I PEAK R ESR ( COUT )
gain for fast transient response. Choose C COMP to set
the integrator zero to maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
where I RIPPLE is the RIPPLE inductor current (see the
Inductor Selection section). For ceramic capacitors, the
output voltage ripple is typically dominated by
V RIPPLE(C) . The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
R COMP ≈
C COMP ≈
253 × V IN × V OUT × C OUT
L × I MAIN ( MAX )
V OUT × C OUT
10 × I MAIN ( MAX ) × R COMP
Input-Capacitor Selection
The input capacitor (C IN ) reduces the current peaks
drawn from the input supply and reduces noise injection
into the IC. A 22μF ceramic capacitor is used in the typi-
cal applications circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source impedance
since the step-up regulator often runs directly from the
output of another regulated supply. Typically, C IN can
be reduced below the values used in the typical applica-
tions circuit. Ensure a low-noise supply at IN by using
adequate C IN . Alternately, greater voltage variation can
To further optimize transient response, vary R COMP in
20% steps and C COMP in 50% steps while observing
transient-response waveforms.
Charge Pumps
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output
requirement. Figures 8 and 9 show the positive and
negative charge-pump output voltages for a given
V MAIN for one-, two-, and three-stage charge pumps.
______________________________________________________________________________________
21
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