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参数资料
| 型号： | MAX8798EVKIT+ |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 23/31页 |
| 文件大小： | 0K |
| 描述： | KIT EVAL FOR MAX8798 |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 1 |
| 主要目的： | 特殊用途 DC/DC，LCD 电源 |
| 输出及类型： | 4，非隔离 |
| 功率 - 输出： | 3.56W |
| 输出电压： | 8V，3.2V，21V，-13V |
| 电流 - 输出： | 300mA，150mA，20mA，20mA |
| 输入电压： | 1.8 ~ 6 V |
| 稳压器拓扑结构： | 升压 |
| 频率 - 开关： | 1.2MHz |
| 板类型： | 完全填充 |
| 已供物品： | 板，CD |
| 已用 IC / 零件： | MAX8798 |
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Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
8 V ? 3 . 3 V
? 3 . 3 V ? ?
? ? 0 . 85 ?
L = ? ? ? ? ? ? ≈ 2 . 8 μ H
Choosing an LIR of 0.5 and estimating efficiency of
85% at this operating point:
2
? 8 V ? ? 0 . 4 A × 1 . 2 MHz ? ? 0 . 5 ?
A 2.6μH inductor is chosen. Then, using the circuit’s
minimum input voltage (3V) and estimating efficiency of
80% at that operating point:
Figure 2 because of the high source impedance seen
in typical lab setups. Actual applications usually have
much lower source impedance since the step-up regu-
lator often runs directly from the output of another regu-
lated supply. Typically, C IN can be reduced below the
values used in Figure 2. Ensure a low noise supply at
IN by using adequate C IN . Alternatively, greater voltage
variation can be tolerated on C IN if IN is decoupled
from C IN using an RC lowpass filter (seen in Figure 2).
,
I IN ( DCMAX ) =
0.4A × 8V
3 V × 0 . 8
≈ 1 . 33 A
Rectifier Diode
The MAX8798’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
The ripple current and the peak current at that input
voltage are:
time and low forward voltage. In general, a 2A Schottky
diode complements the internal MOSFET well.
I RIPPLE =
3 V × ( 8 V ? 3 V )
2 . 6 μ H × 8 V × 1 . 2 MHz
≈ 0 . 6 A
Output Voltage Selection
The output voltage of the main step-up regulator is
adjusted by connecting a resistive voltage-divider from
I PEAK = 1 . 33 A +
0.6A
2
= 1 . 53 A
the output (V MAIN ) to AGND with the center tap con-
nected to FB (see Figure 2). Select R2 in the 10k Ω to
50k Ω range. Calculate R1 with the following equation:
R 1 = R 2 × ? MAIN ? 1 ?
V RIPPLE ( C ) ≈ MAIN ? MAIN IN ?
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 ? V ? V ?
C OUT ? V MAIN f OSC ?
and:
? V ?
? V REF ?
where V REF , the step-up regulator’s feedback set point,
is 1.235V (typ). Place R1 and R2 close to the IC.
Loop Compensation
Choose R COMP to set the high-frequency integrator
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:
V RIPPLE ( ESR ) ≈ I PEAK R ESR ( COUT )
where I PEAK is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors, the
R COMP ≈
1000 × V IN × V OUT × C OUT
L × I MAIN ( MAX )
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.
C COMP ≈
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 injec-
tion into the IC. A 10μF ceramic capacitor is used in
To further optimize transient response, vary R COMP in
20% steps and C COMP in 50% steps while observing
transient response waveforms.
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