MAX8798AETX+T Maxim Integrated Products, MAX8798AETX+T Datasheet - Page 24

• Manufacturer Part Number: MAX8798AETX+T
• Description: Display Drivers Internal-Switch Boos t Regulator with Int
• Manufacturer: Maxim Integrated Products

Specifications of MAX8798AETX+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
The external resistive voltage-divider sets the maximum
value of the VCOM adjustment range. R
full-scale sink current, I
mum value of the VCOM adjustment range. Large R
values increase resolution, but decrease the VCOM
adjustment range. Calculate R3, R4, and R
the following procedure:
1) Choose the maximum VCOM level (V
2) Select R3 between 10kΩ and 500kΩ based on the
3) Calculate R4:
4) Calculate R
5) Verify that I
6) If I
7) The resulting resolution is:
A complete design example is given below:
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
24
mum VCOM level (V
voltage.
acceptable power loss from the V
connected to BOOST.
a larger value for R1.
V
If R3 = 200kΩ, then R4 = 200kΩ and R
24.9kΩ.
Resolution = 12.5mV
______________________________________________________________________________________
MAX
SET
Setting the VCOM Adjustment Range
= 4V, V
exceeds 120µA, return to step 2 and choose
R
SET
2
R
SET
SET
Applications Information
of PCB copper and a large internal
4
MIN
=
≅
:
does not exceed 120µA:
20 (V
(
I
SET
V
(V
= 2.4V, V
BOOST
×
MAX
OUT
=
V
MIN
MAX
V
127
MAX
20 R
MAX
V
, which determines the mini-
−
BOOST
- V
×
), and the V
V
−
MIN
BOOST
MAX
V
SET
Power Dissipation
MIN
)
)
×
)
= 8V
R
×
MAIN
3
R
MAX
3
MAIN
SET
), the mini-
supply rail
SET
sets the
supply
SET
using
SET
=
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
information on the general topic of improving thermal
performance.
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
3.3V input and 300mA output has about 85% efficiency,
about 5% of the power is lost in the internal MOSFET,
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
traces. If the input power is about 3W, the power lost in
the internal MOSFET is about 150mW.
The power dissipated in the operational amplifier
depends on the output current, the output voltage, and
the supply voltage:
where I
the operational amplifier, and I
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.
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:
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.
PD
PD
www.maxim-ic.com/thermal-tutorial
SCAN
SOURCE
VCOM_SOURCE
= ×
PD
3
=
SINK
I
VCOM SOURCE
f
SCAN
=
I
VCOM SINK
_
is the output current sourced by
×
C
PANEL
_
VCOM_SINK
Operational Amplifier
×
×
Scan-Driver Outputs
(
×
(
V
V
Step-Up Regulator
BOOST
V
GON
VCOM
−
is the output
−
V
GOFF
V
VCOM
for more
)
2
)