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| 型号： | MAX8798EVKIT+ |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 24/31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
Setting the VCOM Adjustment Range
The external resistive voltage-divider sets the maximum
value of the VCOM adjustment range. R SET sets the
full-scale sink current, I OUT , which determines the mini-
mum value of the VCOM adjustment range. Large R SET
values increase resolution, but decrease the VCOM
adjustment range. Calculate R3, R4, and R SET using
the following procedure:
1) Choose the maximum VCOM level (V MAX ), the mini-
mum VCOM level (V MIN ), and the V MAIN supply
voltage.
2) Select R3 between 10k Ω and 500k Ω based on the
acceptable power loss from the V MAIN supply rail
connected to BOOST.
3) Calculate R4:
ground plane layer, can dissipate about 2.18W into
+70°C still air. More PCB copper, cooler ambient air,
and more airflow increase the possible dissipation,
while less copper or warmer air decreases the IC’s dis-
sipation capability. The major components of power
dissipation are the power dissipated in the step-up reg-
ulator and the power dissipated by the operational
amplifiers.
The MAX8798 ’s largest on-chip power dissipation
occurs in the step-up switch, the VCOM amplifier, and
the high-voltage scan-driver outputs.
Visit www.maxim-ic.com/thermal-tutorial for more
information on the general topic of improving thermal
performance.
Step-Up Regulator
V BOOST MAX )
R 4 ?
(
V MAX
-V
× R 3
The largest portions of the power dissipated by the
step-up regulator are the internal MOSFET, the induc-
tor, and the output diode. If the step-up regulator with
4) Calculate R SET :
3.3V input and 300mA output has about 85% efficiency,
about 5% of the power is lost in the internal MOSFET,
R SET =
V MAX
20 × (V MAX ? V MIN )
× R 3
about 3% in the inductor, and about 5% in the output
diode. The remaining few percent are distributed
among the input and output capacitors and the PCB
5) Verify that I SET does not exceed 120μA:
traces. If the input power is about 3W, the power lost in
I SET =
V BOOST
20 × R SET
the internal MOSFET is about 150mW.
Operational Amplifier
The power dissipated in the operational amplifier
PD SCAN SCAN PANEL × ( V GON GOFF )
= 3 × f
× C
? V
6) If I SET exceeds 120μA, return to step 2 and choose
a larger value for R1.
7) The resulting resolution is:
(V MAX ? V MIN )
127
A complete design example is given below:
V MAX = 4V, V MIN = 2.4V, V BOOST = 8V
If R3 = 200k Ω , then R4 = 200k Ω and R SET =
24.9k Ω .
Resolution = 12.5mV
Applications Information
Power Dissipation
An IC’s maximum power dissipation depends on the
thermal resistance from the die to the ambient environ-
ment and the ambient temperature. The thermal resis-
tance depends on the IC package, PCB copper area,
other thermal mass, and airflow.
The MAX8798, with its exposed backside paddle sol-
dered to 1in 2 of PCB copper and a large internal
depends on the output current, the output voltage, and
the supply voltage:
PD SOURCE = I VCOM _ SOURCE × ( V BOOST ? V VCOM )
PD SINK = I VCOM _ SINK × V VCOM
where I VCOM_SOURCE is the output current sourced by
the operational amplifier, and I VCOM_SINK is the output
current that the operational amplifier sinks.
In a typical case where the supply voltage is 8V and
the output voltage is 4V with an output source current
of 30mA, the power dissipated is 120mW.
Scan-Driver Outputs
The power dissipated by the scan-driver outputs (CKV,
CKVB, and STVP) depends on the scan frequency, the
capacitive load, and the difference between the GON
and GOFF supply voltages:
2
If the scan frequency is 50kHz, the load of the three
outputs is 5nF, and the supply voltage difference is
30V, then the power dissipated is 675mW.
24
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