MAXIM MAX8798

19-0971; Rev 0; 9/07
KIT
ATION
EVALU
E
L
B
AVAILA
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
The MAX8798 includes a high-performance, step-up
regulator; a high-speed operational amplifier; a digitally
adjustable VCOM calibration device with nonvolatile
memory and an I2C interface; and a high-voltage, levelshifting scan driver. The device is optimized for thin-film
transistor (TFT) liquid-crystal display (LCD) applications.
The step-up DC-DC converter provides the regulated
supply voltage for panel source driver ICs. The high
switching frequency allows the use of ultra-small inductors and ceramic capacitors. The current-mode control
architecture provides fast transient response to pulsed
loads typical of source driver loads. The step-up regulator features soft-start and current limit.
The high-current operational amplifier is designed to
drive the LCD backplane (VCOM). The amplifier features high output current (±150mA), fast slew rate
(45V/µs), wide bandwidth (20MHz), and rail-to-rail
inputs and outputs.
The programmable VCOM calibrator is externally
attached to the VCOM amplifier’s resistive voltage-divider
and sinks a programmable current to adjust the VCOM
output-voltage level. An internal 7-bit digital-to-analog
converter (DAC) controls the sink current. The DAC is
ratiometric relative to BOOST and is guaranteed to be
monotonic over all operating conditions. The calibrator IC
includes an EEPROM to store the desired VCOM voltage
level. The 2-wire I2C interface between the LCD panel
and the programming circuit minimizes panel connector
pin count and simplifies production equipment.
The high-voltage, level-shifting scan driver is designed
to drive the TFT panel gate drivers. Its three outputs
swing from +35V (maximum) to -25V (minimum) and
can swiftly drive capacitive loads. To save power, the
two complementary outputs are designed to allow
charge sharing during state changes.
The MAX8798 is available in a 36-pin, thin QFN package with a maximum thickness of 0.8mm for ultra-thin
LCD panels.
Features
o 1.8V to 5.5V IN Supply Voltage Range
o 1.8V to 4.0V VDD Input Voltage Range
o 1.2MHz Current-Mode Step-Up Regulator
Fast Transient Response
Built-In 20V, 1.9A, 150mΩ MOSFET
o High-Speed (20MHz) Operational Amplifier
±150mA Output Current
o High-Voltage Drivers with Scan Logic
+35V to -25V Outputs
Output Charge Sharing
o Programmable VCOM Calibrator
7-Bit Adjustable Current-Sink Output
I2C Interface
EEPROM Setting Memory
o Thermal-Overload Protection
Simplified Operating Circuit
VP
VN
VIN
VMAIN
50kΩ
SHDN
IN
LX
DISH
1kΩ
CPV
SCAN DRIVER
LOGIC
AND
GATE DRIVERS
STV
OECON
VN
PGND
FB
COMP
OE
SYSTEM
1.9A
STEP-UP
REG
AGND
GON
GOFF
VP
STVP
BOOST
CKVCS
NEG
PANEL
CKVB
VCOM
CKVBCS
Applications
TO VCOM
BACKPLANE
CKV
Notebook Computer Displays
VL
3.3V
LINEAR
REG
VDD
LCD Monitor Panels
VIN
Ordering Information
VCOM CALIBRATOR
7
GND
POS
BGND
OUT
SDA
SCL
PART
TEMP RANGE
PINPACKAGE
MAX8798ETX+
-40°C to +85°C
36 Thin QFN
6mm x 6 mm
PKG
CODE
I2 C
BUS
SCLS
WPP
WPN
SET
T-3666M-1
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
MAX8798
General Description
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
ABSOLUTE MAXIMUM RATINGS
IN, VL, SHDN to AGND .........................................-0.3V to +7.5V
VDD, SDA, SCL, SCLS, WPN, WPP, SET to GND...-0.3V to +4.0V
OECON, CPV, OE, STV to AGND..........................-0.3V to +4.0V
COMP, FB to AGND ......................................-0.3V to (VL + 0.3V)
DISH to GND ............................................................-6V to +0.3V
LX to PGND ............................................................-0.3V to +20V
OUT, VCOM, NEG, POS to BGND........-0.3V to (BOOST + 0.3V)
PGND, BGND, AGND to GND...............................-0.3V to +0.3V
GON to AGND ........................................................-0.3V to +40V
GOFF to AGND .............................................-30V to (VIN + 0.3V)
BOOST to BGND ....................................................-0.3V to +20V
CKV, CKVB, STVP, CKVCS,
CKVBCS to AGND..................(GOFF - 0.3V) to (GON + 0.3V)
LX, PGND RMS Current Rating.............................................2.4A
Continuous Power Dissipation (TA = +70°C)
NiPd Lead Frame with Nonconductive Epoxy
36-Pin, 6mm x 6mm Thin QFN
(derate 27.2mW/°C above +70°C) .........................2179.8mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
VDD Input Voltage Range
MIN
TYP
1.8
MAX
UNITS
4.0
V
4
10
μA
1.3
1.75
V
VDD Quiescent Current
VDD = 3V
VDD Undervoltage Lockout
VDD rising; typical hysteresis 100mV
IN Input Voltage Range
(Note 1)
6.0
V
IN Quiescent Current
VIN = 3V, VFB = 1.5V, not switching
0.04
0.1
mA
IN Undervoltage Lockout
IN rising; typical hysteresis 100mV
1.4
1.75
Thermal Shutdown
1.8
o
Rising edge, hysteresis = 15 C
V
o
160
C
BOOTSTRAP LINEAR REGULATOR (VL)
VL Output Voltage
I VL = 100μA
3.15
3.3
3.45
VL Undervoltage Lockout
VL rising, typical hysteresis 200mV
2.4
2.7
3.0
VL Maximum Output Current
VFB = 1.1V
10
V
V
mA
MAIN DC-DC CONVERTER
BOOST Supply Current
LX not switching, no load on VL
LX switching, no load on VL
Operating Frequency
990
Oscillator Maximum Duty Cycle
FB Regulation Voltage
1170
1350
kHz
mA
92
96
%
1.235
1.254
V
-0.15
-0.08
+0.15
%/V
125
200
nA
160
280
0 < ILOAD < 200mA, transient only
VIN = 1.8V to 5.5V, FB to COMP
FB Input Bias Current
VFB = 1.25V
50
FB Transconductance
I = 5μA at COMP
70
FB Voltage Gain
FB to COMP
FB Fault Timer Trip Threshold
Falling edge
LX On-Resistance
LX Leakage Current
LX Current Limit
Duty cycle = 65%
2
4
88
FB Line Regulation
Soft-Start Period
2
3
1.216
FB Load Regulation
Current-Sense Transresistance
1.5
-1
%
2400
0.96
μS
V/V
1
1.04
V
ILX = 1.2A
150
300
m
VLX = 18V
0.01
20
μA
1.6
1.9
2.2
A
0.25
0.42
0.55
V/A
3
_______________________________________________________________________________________
ms
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
18
V
19
19.9
V
1.4
V
OPERATIONAL AMPLIFIER
BOOST Supply Range
5
BOOST Overvoltage
Fault Threshold
(Note 2)
BOOST Undervoltage
Fault Threshold
(Note 3)
1.0
1V < (VNEG, VPOS) < (VBOOST - 1V)
1V < (VNEG, VPOS) < (VBOOST - 1V)
120
Large-Signal Voltage Gain
Common-Mode Rejection Ratio
18.1
dB
75
dB
1V < (VNEG, VPOS) < (VBOOST - 1V)
-25
-5
+25
VBOOST/2
+15
-2.5
+12
Input Bias Current
1V < (VNEG, VPOS) < (VBOOST - 1V)
-50
+50
nA
Input Common-Mode
Voltage Range
1V < (VNEG, VPOS) < (VBOOST - 1V)
0
VBOOST
V
VCOM Output Voltage
Swing High
I VCOM = 5mA
VCOM Output Voltage Swing Low
I VCOM = -5mA
50
VCOM Output-Current High
VVCOM = VBOOST - 1V
-75
mA
VCOM Output-Current Low
VVCOM = 1V
+75
mA
Slew Rate
1V < (VNEG, VPOS) < (VBOOST - 1V)
40
V/μs
-3dB Bandwidth
1V < (VNEG, VPOS) < (VBOOST - 1V)
20
MHz
Input Offset Voltage
VCOM Short-Circuit Current
VBOOST VBOOST
- 100
- 50
Short to VBOOST/2, sourcing
50
150
Short to VBOOST/2, sinking
50
150
mV
mV
100
mV
mA
PROGRAMMABLE VCOM CALIBRATOR
GON Input Range
16.1
GON Threshold to Enable Program
Rising edge, 60mV hysteresis
SET Voltage Resolution
SET Differential Nonlinearity
35.0
15.6
16.0
7
Monotonic overtemperature
V
V
Bits
-2
+1
+2
LSB
SET Zero-Scale Error
-1
+2
LSB
SET Full-Scale Error
-3
+3
LSB
120
μA
To GND, VBOOST = 18V
8.5
170.0
To GND, VBOOST = 6V
2.5
50.0
SET Current
SET External Resistance
(Note 4)
k
VSET/VBOOST Voltage Ratio
DAC full scale
0.05
V/V
OUT Leakage Current
When OUT is off
1
nA
OUT Settling Time
To ±0.5 LSB error band
20
μs
VSET +
0.5V
OUT Voltage Range
EEPROM Write Cycles
(Note 5)
1000
18
V
Times
_______________________________________________________________________________________
3
MAX8798
ELECTRICAL CHARACTERISTICS (continued)
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.3 x
VDD
V
2-WIRE INTERFACE
Logic-Input Low Voltage (VIL)
SDA, SCL, WPN, VDD = 3V
Logic-Input High Voltage (VIH)
SDA, SCL, WPN, VDD = 3V
WPP Logic-Output Low Voltage
IWPP = 1mA
WPP Logic-Output High Voltage
IWPP = 1mA
0.7 x
VDD
V
+0.1
VDD 0.1
SDA Logic-Output Low Sink Current SDA forced to 3.3V
6
Logic Input Current
SDA, SCL, SCL_S,WPN to VDD or GND
-1
Input Capacitance
SDA, SCL, SCL_S
V
V
mA
+1
μA
500
kHz
5
pF
SCL Frequency (fCLK)
DC
SCL High Time (tCLH)
SCL Low Time (tCLL)
600
ns
1300
ns
SDA, SCL, SCLS Rise Time (tR)
CBUS = total capacitance of bus line in pF
20 + 10
x CBUS
SDA, SCL, SCLS Fall Time (tF)
CBUS = total capacitance of bus line in pF
20 + 10
x CBUS
START Condition Hold Time
(tHDSTT)
10% of SDA to 90% of SCL
START Condition Setup Time
(tSVSTT)
300
ns
300
ns
600
ns
600
ns
Data Input Hold Time (tHDDAT)
0
ns
Data Input Setup Time (tSUDAT)
150
ns
STOP Condition Setup Time
(tSVSTP)
600
ns
Bus Free Time (tUF)
Input Filter Spike Suppression (tSP)
SCL-SCLS Switch Resistance
1300
ns
SDA, SCL (Note 5)
WPN = GND
250
1
WPN = VDD
ns
M
20
50
35
V
HIGH-VOLTAGE SCAN DRIVER
GON Input Voltage Range
12
GOFF Input Voltage Range
-2
V
GON Supply Current
STV, CPV, OE, OECON = AGND
-25
250
350
μA
GOFF Supply Current
STV, CPV, OE, OECON = AGND
100
200
μA
Output-Voltage Low
CKV, CKVB, STVP,
-5mA output current
Output-Voltage High
CKV, CKVB, STVP,
5mA output current
Propagation Delay Between OE
CPV = 0, STV = 0, CLOAD = 4.7nF, 50
Rising Edge and CKV/CKVB Edge
4
VGOFF
+ 0.2
VGOFF
+ 0.05
V
VGON
- 0.05
VGON
- 0.2
V
250
450
ns
_______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
Output Slew Rate CKV, CKVB
Without charge sharing,
STV = VDD, CLOAD = 4.7nF, 50
Propagation Delay Between
STV and STVP
CLOAD = 4.7nF
STVP Output Slew Rate
CLOAD = 4.7nF, 50 Charge-Sharing Discharge
Path Resistance
CKV to CKVCS and
CKVB to CKVBCS
MIN
TYP
20
40
250
20
DISH Discharge Path Resistance GOFF to -3V, DISH = -3V
DISH Turn-On Threshold
450
ns
V/μs
250
400
250
500
STV, CPV, OE Input Low Voltage
-1.8
V
0.8
V
1.6
V
OECON Input Low Voltage
1.5
OECON Input High Voltage
UNITS
V/μs
40
Dish falling
STV, CPV, OE Input High Voltage
MAX
2.0
V
V
OECON Sink Current
OECON = 5W = VDD
0.4
STV, CPV, OE
Input Current
VSTV = VDD or GND,
VCPV = VDD or GND,
VOE = VDD or GND,
VOECON = VDD or GND
0.8
mA
-1
+1
μA
CKV, CKVB, STVP Output
Three-State Current
VCKV = GON or GOFF, three-state
VCKVB = GON or GOFF, three-state
VCKVCS = GON or GOFF, three-state
VCKVBCS = GON or GOFF, three-state
VSTVP = GON or GOFF, three-state
-1
+1
μA
0.6
V
CONTROL INPUTS
SHDN
Input Low Voltage
Input High Voltage
SHDN Input Current
SHDN, 1.8V < VIN < 3.0V
1.8
SHDN, 3.0V < VIN < 5.5V
SHDN = 0V or 3V
2.0
V
-1
+1
μA
ELECTRICAL CHARACTERISTICS
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 6)
PARAMETER
CONDITIONS
VDD Input Voltage Range
MIN
1.8
TYP
MAX
UNITS
4.0
V
10
μA
1.75
V
VDD Quiescent Current
VDD = 3V
VDD Undervoltage Lockout
VDD rising; typical hysteresis 100mV
IN Input Voltage Range
(Note 1)
6.0
V
IN Quiescent Current
VIN = 3V, VFB = 1.5V, not switching
0.1
mA
IN Undervoltage Lockout
IN rising; typical hysteresis 100mV
1.75
V
1.8
_______________________________________________________________________________________
5
MAX8798
ELECTRICAL CHARACTERISTICS (continued)
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 6)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
BOOTSTRAP LINEAR REGULATOR (VL)
VL Output Voltage
I VL = 100μA
3.15
3.45
V
VL Undervoltage Lockout
VL rising, typical hysteresis 100mV
2.4
3.0
V
MAIN DC-DC CONVERTER
BOOST Supply Current
LX not switching, no load on VL
2
LX switching, no load on VL
4
Operating Frequency
Oscillator Maximum Duty Cycle
FB Regulation Voltage
FB Line Regulation
VIN = 1.8V to 5.5V, FB to COMP
FB Transconductance
I = 5μA at COMP
FB Fault-Timer Trip Threshold
Falling edge
LX On-Resistance
ILX = 1.2A
LX Current Limit
Duty cycle = 65%
mA
990
1350
kHz
88
96
%
1.216
1.254
V
-0.15
+0.15
%/V
70
280
μS
0.96
1.04
V
300
m
2.2
A
1.6
OPERATIONAL AMPLIFIER
BOOST Supply Range
BOOST Overvoltage Fault Threshold (Note 2)
5
18
V
18.1
19.9
V
BOOST Undervoltage Fault Threshold (Note 3)
1.4
V
Input Offset Voltage
1V < (VNEG, VPOS) < (VBOOST - 1V)
-25
+25
mV
Input Common-Mode Voltage Range
1V < (VNEG, VPOS) < (VBOOST - 1V)
0
VBOOST
V
VCOM Output-Voltage Swing High
I VCOM = 5mA
VCOM Output-Voltage Swing Low
I VCOM = -5mA
VCOM Short-Circuit Current
VBOOST
- 100
mV
100
Short to VBOOST/2, sourcing
50
Short to VBOOST/2, sinking
50
mV
mA
PROGRAMMABLE VCOM CALIBRATOR
GON Input Range
GON Threshold to Enable Program
16.1
Rising edge, 60mV hysteresis
SET Voltage Resolution
SET Differential Nonlinearity
35.0
16.0
7
Monotonic overtemperature
V
V
Bits
-2
+2
LSB
SET Zero-Scale Error
-1
+2
LSB
SET Full-Scale Error
-3
+3
LSB
120
μA
SET Current
SET External Resistance
(Note 4)
To GND, VBOOST = 18V
8.5
170.0
To GND, VBOOST = 6V
2.5
50.0
VSET +
0.5V
18
OUT Voltage Range
EEPROM Write Cycles
6
(Note 5)
1000
_______________________________________________________________________________________
k
V
Times
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 6)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.3 x
VDD
V
2-WIRE INTERFACE
Logic-Input Low Voltage (VIL)
SDA, SCL, WPN, VDD = 3V
Logic-Input High Voltage (VIH)
SDA, SCL, WPN, VDD = 3V
WPP Logic-Output Low Voltage
IWPP = 1mA
WPP Logic-Output High Voltage
IWPP = -1mA
SDA Logic-Output Low Sink Current
SDA forced to 3.3V
SCL Frequency (fCLK)
0.7 x
VDD
V
+0.1
VDD 0.1
V
6
DC
V
mA
500
kHz
SCL High Time (tCLH)
600
ns
SCL Low Time (tCLL)
1300
ns
SDA, SCLS, and SCL Rise Time (tR)
CBUS = total capacitance of bus line in pF
20 + 10
x CBUS
SDA, SCLS, and SCL Fall Time (tF)
CBUS = total capacitance of bus line in pF
20 + 10
x CBUS
START Condition Hold Time
(tHDSTT)
10% of SDA to 90% of SCL
300
ns
300
ns
600
ns
START Condition Setup Time
(tSVSTT)
600
ns
Data Input Hold Time (tHDDAT)
0
ns
Data Input Setup Time (tSUDAT)
150
ns
STOP Condition Setup Time (tSVSTP)
600
ns
Bus Free Time (tUF)
Input Filter Spike Suppression (tSP)
1300
ns
SCL-SCLS Switch Resistance
SDA, SCL (Note 5)
WPN = GND
250
1
WPN = VDD
ns
M
50
HIGH-VOLTAGE SCAN DRIVER
GON Input Voltage Range
12
35
V
GOFF Input Voltage Range
-25
-2
V
GON Supply Current
STV, CPV, OE, OECON = AGND
350
μA
GOFF Supply Current
STV, CPV, OE, OECON = AGND
200
μA
Output-Voltage Low
CKV, CKVB, STVP,
-5mA output current
Output-Voltage High
CKV, CKVB, STVP,
5mA output current
VGOFF
+ 0.2
V
VGON
- 0.2
V
_______________________________________________________________________________________
7
MAX8798
ELECTRICAL CHARACTERISTICS (continued)
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VDD = VSHDN = +3V, circuit of Figure 2, BOOST = 8V, GON = 23V, GOFF = -12V, VPOS = 0V, VNEG = 1.5V, OE = CPV = STV =
OECON = 0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 6)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
450
ns
Propagation Delay Between OE
Rising Edge and CKV/CKVB Edge
CPV = 0, STV = 0, CLOAD = 4.7nF, 50
Output Slew Rate CKV, CKVB
Without charge sharing,
STV = VDD, CLOAD = 4.7nF, 50
Propagation Delay Between STV
and STVP
CLOAD = 4.7nF
STVP Output Slew Rate
CLOAD = 4.7nF, 50
Charge-Sharing Discharge Path
Resistance
CKV to CKVCS and
CKVB to CKVBCS
400
DISH Switch Resistance
GOFF to -3V, DISH = -3V
500
DISH Turn-On Threshold
Dish falling
-1.8
V
0.8
V
20
V/μs
450
20
V/μs
STV, CPV, OE Input Low Voltage
STV, CPV, OE Input High Voltage
1.6
V
OECON Input Low Voltage
1.5
OECON Input High Voltage
OECON Sink Current
OECON = STV = VDD
ns
V
2.0
V
0.4
mA
CONTROL INPUTS
Input Low Voltage
SHDN
Input High Voltage
SHDN, 1.8V < VIN < 3.0V
SHDN, 3.0V < VIN < 5.5V
0.6
1.8
V
V
2.0
Note 1: For 5.5V < VIN < 6.0V, use the MAX8798 for no longer than 1% of IC lifetime. For continuous operation, the input voltage
should not exceed 5.5V.
Note 2: Inhibits boost switching if VBOOST exceeds the threshold This fault is not latched.
Note 3: Step-up regulator switching is not enabled until BOOST is above undervoltage threshold.
Note 4: SET external resistor range is verified at DAC full scale.
Note 5: Guaranteed by design, not production tested.
Note 6: -40°C specs are guaranteed by design, not production tested.
tHDSTT
tR
tCLH
tCLL
SCL
tF
SDA
tHDDAT
tSUDAT
tSUSTP
VIH
VIL
Figure 1. Timing Definitions Used in the Electrical Characteristics
8
_______________________________________________________________________________________
tBF
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
VIN = 2.2V
60
70
SUPPLY CURRENT (μA)
OUTPUT ERROR (%)
70
-0.2
VIN = 3.0V
-0.4
VIN = 2.2V
-0.6
60
50
0.2A
40
30
20
50
-0.8
40
100
1000
0
1
10
LOAD CURRENT (mA)
100
2.7
2.5
40
1.20
35
1.19
30
2.0
25
20
1.5
IBOOST
15
1.0
10
0.5
5
0
10
35
60
1.18
200mA LOAD
1.17
1.16
1.15
1.14
1.13
1.12
1.10
1.6
85
TEMPERATURE (°C)
2.7
3.8
4.9
6.0
INPUT VOLTAGE (V)
STEP-UP REGULATOR LOAD TRANSIENT
RESPONSE (20mA TO 300mA)
STEP-UP REGULATOR
HEAVY-LOAD SOFT-START
MAX8798 toc07
MAX8798 toc06
LX
5V/div
VLX
10V/div
0V
0V
VMAIN
5V/div
IL
1A/div
0V
IL
500mA/div
0A
0A
LOAD CURRENT
200mA/div
0mV
VMAIN (AC-COUPLED)
200mV/div
SHDN CONTROL
5V/div
0V
2ms/div
6.0
1.11
0
-15
SWITCHING FREQUENCY (MHz)
IIN
4.9
STEP-UP CONVERTER SWITCHING
FREQUENCY vs. INPUT VOLTAGE
IN SUPPLY CURRENT (μA)
MAX8798 toc04
3.0
3.8
SUPPLY VOLTAGE (V)
LOAD CURRENT (mA)
STEP-UP REGULATOR SUPPLY CURRENT
vs. TEMPERATURE
-40
1.6
1000
MAX8798 toc05
10
NO LOAD
10
-1.0
1
BOOST SUPPLY CURRENT (mA)
EFFICIENCY (%)
0
VIN = 5.0V
VIN = 3.0V
80
MAX8798 toc02
VIN = 5.0V
80
0.2
MAX8798 toc01
100
90
IN SUPPLY QUIESCENT CURRENT
vs. IN SUPPLY VOLTAGE
STEP-UP REGULATOR OUTPUT
LOAD REGULATION vs. LOAD CURRENT
MAX8798 toc03
STEP-UP REGULATOR EFFICIENCY
vs. LOAD CURRENT
0mA
100μs/div
L = 3.6μH
RCOMP = 100kΩ
CCOMP1 = 220pF
_______________________________________________________________________________________
9
MAX8798
Typical Operating Characteristics
(Circuit of Figure 2, VIN = 3V, TA = +25°C, unless otherwise noted.)
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
MAX8798
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 3V, TA = +25°C, unless otherwise noted.)
STEP-UP REGULATOR PULSED LOAD
TRANSIENT RESPONSE (20mA TO 1A)
TIMER-DELAY LATCH
RESPONSE TO OVERLOAD
MAX8798 toc08
L = 3.6μH
RCOMP = 100kΩ
CCOMP1 = 220pF
MAX8798 toc09
VLX
10V/div
0V
VLX
10V/div
0V
IL
1A/div
0A
VMAIN
5V/div
0V
0mV
VMAIN
AC-COUPLED
200mV/div
IL
2A/div
0A
LOAD CURRENT
1A/div
IMAIN
1A/div
0A
0A
10μs/div
10ms/div
POWER-UP SEQUENCE
OF ALL SUPPLY OUTPUTS
BOOST SUPPLY CURRENT
vs. BOOST SUPPLY VOLTAGE
0V
VCOM
5V/div
VIN
5V/div
0V
0V
VIN = 3.3V
2.5
VGOFF
20V/div
SHDN
5V/div
SHDN
CONTROL
0V
2.0
VIN = 5V
1.5
1.0
0.5
9
12
VBOOST = 8V
1.5
1.0
15
-40
-15
10
35
85
TEMPERATURE (°C)
OPERATIONAL AMPLIFIER PSRR
vs. FREQUENCY
OPERATIONAL AMPLIFIER RAIL-TO-RAIL
INPUT/OUTPUT WAVEFORMS
MAX8798 toc15
MAX8798 toc14
VPOS
5V/div
-10
GAIN (dB)
-15
-5
NO LOAD
-10
-20
0V
-25
-30
-15
-35
-20
VVCOM
5V/div
-40
0V
-45
-25
100
1k
10k
FREQUENCY (Hz)
10
60
OUTPUT VOLTAGE (V)
-5
0
GAIN (dB)
6
0
MAX8798 toc13
33pF LOAD
2.0
0
0
3
100pF LOAD
VBOOST = 15V
0.5
OPERATIONAL AMPLIFIER
FREQUENCY RESPONSE
5
2.5
NO LOAD ON VMAIN
2ms/div
10
3.0
BOOST CURRENT (mA)
VGON
20V/div
3.5
MAX8798 toc11
0V
3.0
INPUT CURRENT (mA)
0V
VL
5V/div
VMAIN
5V/div
MAX8798 toc12
MAX8798 toc10
BOOST SUPPLY CURRENT
vs. TEMPERATURE
100k
100
1k
10k
100k
10μs/div
FREQUENCY (Hz)
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
OPERATIONAL AMPLIFIER
LOAD TRANSIENT RESPONSE
OPERATIONAL AMPLIFIER
SMALL-SIGNAL STEP RESPONSE
OPERATIONAL AMPLIFIER
LARGE-SIGNAL STEP RESPONSE
MAX8798 toc16
MAX8798 toc18
MAX8798 toc17
VVCOM
(AC-COUPLED)
200mV/div
0mV
VPOS
(AC-COUPLED)
100mV/div
VPOS
2V/div
0mV
0V
IVCOM
100mA/div
VCOM
(AC-COUPLED)
100mV/div
0mV
VCOM
2V/div
0mA
0V
200ns/div
2μs/div
200ns/div
STV/STVP INPUT/OUTPUT
WAVEFORMS WITH LOGIC INPUT
CPV AND OE/CKV AND CKVB INPUT/OUTPUT
WAVEFORMS WITH LOGIC INPUT
(STV = 0V, CLOAD = 5.0nF AND 50Ω, R1, R2 = 200Ω)
STV RISING EDGE
PROPAGATION DELAY
MAX8798 toc19_1
MAX8798 toc19
MAX8798 toc20
CPV
5V/div
0V
OE
5V/div
0V
STV
5V/div
0V
STV
5V/div
0V
CKV
20V/div
0V
STVP
10V/div
STVP
10V/div
0V
0V
CKVB
20V/div
0V
4μs/div
4μs/div
100ns/div
OE/CKV RISING EDGE
PROPAGATION DELAY
STV FALLING EDGE
PROPAGATION DELAY
OE/CKV FALLING EDGE
PROPAGATION DELAY
MAX8798 toc20_1
MAX8798 toc21_1
MAX8798 toc21
OE
5V/div
STV
5V/div
OE
5V/div
0V
0V
0V
CKV
10V/div
0V
CKV
10V/div
100ns/div
0V
STVP
10V/div
0V
100ns/div
100ns/div
______________________________________________________________________________________
11
MAX8798
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 3V, TA = +25°C, unless otherwise noted.)
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Typical Operating Characteristics (continued)
(Circuit of Figure 2, VIN = 3V, TA = +25°C, unless otherwise noted.)
CALIBRATOR SIGNAL LSB
DOWNWARD STEP RESPONSE
CALIBRATOR SIGNAL LSB
UPWARD STEP RESPONSE
MAX8798 toc22
MAX8798 toc23
SCL
5V/div
0V
SDA
5mV/div
0V
VCOM
(AC-COUPLED)
10mV/div
0mV
VSET
(AC-COUPLED)
10mV/div
0mV
SCL
5V/div
0V
SDA
5V/div
0V
VCOM
(AC-COUPLED)
10mV/div
0mV
VSET
(AC-COUPLED)
10mV/div
0mV
40μs/div
40μs/div
CALIBRATOR FULL-SCALE
UPWARD STEP RESPONSE
CALIBRATOR FULL-SCALE
DOWNWARD STEP RESPONSE
MAX8798 toc24
MAX8798 toc25
SCL
5V/div
SCL
5V/div
0V
0V
SDA
5V/div
0V
SDA
5V/div
0V
4.025V
VCOM
2V/div
2.422V
4.025V
VCOM
2V/div
2.422V
VSET
200mV/div
0mV
0mV
40μs/div
12
VSET
200mV/div
40μs/div
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
PIN
NAME
FUNCTION
1
CKV
High-Voltage, Gate-Pulse Output. When enabled, CKV toggles between its high state (connected to
GON) and its low state (connected to GOFF) on each falling edge of the CPV input. Further, CKV floats
whenever CPV and OE are both low or whenever CPV is low and OECON is high.
2
CKVCS
CKV Charge-Sharing Connection. CKVCS connects to CKV whenever CKV floats to allow connection to
CKVB, sharing charge between the capacitive loads on these two outputs.
3
CKVBCS
4
CKVB
High-Voltage, Gate-Pulse Output. CKVB is the inverse of CKV during active states and floats whenever
CKV floats.
5
STVP
High-Voltage, Start-Pulse Output. STVP is low (connected to GOFF) whenever STV is low and is high
(connected to GON) only when STV is high and CPV and OE are both low. When STV is high and either
CPV or OE is high, STVP floats.
6
STV
Vertical Sync Input. The rising edge of STV begins a frame of data. The STV input is used to generate the
high-voltage STVP output.
7
OECON
Active-Low, Output-Enable Timing Input. OECON is driven by an RC-filtered version of the OE input signal. If
OE remains high long enough for the resistor to charge the capacitor up to the OECON threshold, the OE
signal is masked until OE goes low and the capacitor is discharged below the threshold through the resistor.
8
OE
Active-High, Gate-Pulse Output Enable. CKV and CKVB leave the floating charge-sharing state on the
rising edge of OE.
9
CPV
Vertical Clock-Pulse Input. CPV controls the timing of the CKV and CKVB outputs, which change state
(by first sharing charge) on its falling edge.
10
GND
Logic Ground
11
DISH
GOFF Discharge Input. Pulling DISH below ground activates an internal connection between GOFF and
GND, rapidly discharging the GOFF supply. Typically, DISH is capacitively connected to IN, so that
when VIN falls GOFF is discharged.
12
VDD
Supply Input. Logic supply input for the VCOM calibrator. Bypass to GND through a minimum 0.1μF
capacitor.
13
WPN
Active-Low, Write-Protect Input. When WPN is low, I2C commands are ignored and the VCOM calibrator
settings cannot be modified.
14
SCLS
Alternate I2C-Compatible Clock Input. When WPN is high, SCLS connects to SCL to drive SCL from an
alternate clock source.
15
SCL
I2C-Compatible Clock Input and Output
16
SDA
I2C-Compatible Serial Bidirectional Data Line
17
WPP
Write-Protect Output. WPP is the inverse of WPN. It can be used to control active-high, write-protect
inputs on other devices.
18
SET
Full-Scale, Sink-Current Adjustment Input. Connect a resistor, R SET, from SET to GND to set the fullscale adjustable sink current, which is VBOOST / (20 x R SET). I OUT is equal to the current through RSET.
19
VL
3.3V On-Chip Regulator Output. This regulator powers internal analog circuitry for the step-up regulator,
op amp, and VCOM calibrator. External loads up to 10mA can be powered. Bypass VL to GND with a
0.22μF or greater ceramic capacitor.
CKVB Charge-Sharing Connection. CKVBCS connects to CKVB whenever CKVB floats to allow
connection to CKV, sharing charge between the capacitive loads on these two outputs.
______________________________________________________________________________________
13
MAX8798
Pin Description
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
MAX8798
Pin Description (continued)
14
PIN
NAME
20
BGND
Amplifier Ground
FUNCTION
21
BOOST
Operational Amplifier Supply Input. Connect to VMAIN and bypass to BGND with a 1μF or greater
ceramic capacitor.
22
OUT
Adjustable Sink-Current Output. OUT connects to the resistive voltage-divider at the op amp input POS
(between BOOST and GND) that determines the VCOM output voltage. I OUT lowers the divider voltage by
a programmable amount.
23
POS
Operational Amplifier Noninverting Input
24
NEG
25
VCOM
Operational Amplifier Output
26
SHDN
Shutdown Control Input. Pull SHDN low to disable the step-up regulator. The VCOM calibrator, op amp,
and scan driver functions remain enabled.
27
IN
Operational Amplifier Inverting Input
Step-Up Regulator Supply Pin. Bypass IN to AGND (pin 34) with a 1μF or greater ceramic capacitor.
28, 29
LX
30, 31
PGND
Switching Node. Connect inductor/catch diode here and minimize trace area for lowest EMI.
32
FB
33
COMP
Compensation Pin for Error Amplifier. Connect a series RC from this pin to AGND. Typical values are
180k and 470pF.
34
AGND
Ground
35
GOFF
Gate-Off Supply. GOFF is the negative supply voltage for the CKV, CKVB, and STVP high-voltage driver
outputs. Bypass to PGND with a minimum of 0.1μF ceramic capacitor.
36
GON
Gate-On Supply. GON is the positive supply voltage for the CKV, CKVB, and STVP high-voltage driver
outputs. Bypass to VMAIN or PGND with a minimum of 0.1μF ceramic capacitor.
—
EP
Power Ground. Source connection of the internal step-up regulator power switch.
Feedback Pin. Reference voltage is 1.24V nominal. Connect external resistor-divider
midpoint here and minimize trace area. Set V OUT according to: VOUT = 1.24V (1 + R1/R2).
Exposed Backside Paddle
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
MAX8798
0.1μF
0.1μF
VN
-12V, 20mA
VP
20V, 20mA
0.1μF
0.1μF
0.1μF
0.1μF
10Ω
0.1μF
0.1μF
1μF
D1
2.6μH
VIN
2.2V TO 3.6V
0.1μF
10μF
10μF
SHDN
VN
VP
IN
DISH
LX
GOFF
PGND
R1
200kΩ
GON
FB
CPV
OE
SYSTEM
20kΩ
VMAIN
8V, 300mA
RCOMP
100kΩ
COMP
STV
R2
34kΩ
CCOMP
220pF
OECON
AGND
3nF
STVP
200Ω
CKVCS
PANEL
MAX8798
BOOST
CKVB
NEG
200Ω
CKVBCS
VCOM
CKV
TO VCOM
BACKPLANE
R3
200kΩ
VL
VL
POS
0.22μF
VDD
BGND
0.1μF
20kΩ
GND
VIN
R4
200kΩ
OUT
20kΩ
SDA
I2C
BUS
SCL
SCLS
WPP
WPN
SET
RSET
25kΩ
Figure 2. MAX8798 Typical Operating Circuit
______________________________________________________________________________________
15
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
VN
VP
VIN
1.8V TO 5.5V
VMAIN
IN
SHDN
250kΩ
LX
DISH
1.6A
min
STEP-UP
500Ω
CPV
SCAN DRIVER
LOGIC
AND
GATE DRIVERS
STV
OECON
VN
FB
COMP
OE
SYSTEM
PGND
AGND
GON
GOFF
VP
STVP
BOOST
CKVCS
NEG
PANEL
CKVB
VCOM
CKVBCS
CKV
POS
VL
VL
VL
VDD
BGND
OUT
VIN
GND
7
2
IC
INTERFACE
DAC
SDA
I2C BUS
SCL
SCLS
WPP
WPN
SET
Figure 3. MAX8798 Functional Diagram
16
______________________________________________________________________________________
TO VCOM
BACKPLANE
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Detailed Description
The MAX8798 typical application circuit (Figure 2) generates a +8V source-driver supply and approximately +20V
and -12V gate-driver supplies for TFT displays. The inputvoltage range for the IC is from +1.8V to +5.5V, but the
Figure 2 circuit is designed to run from 2.2V to 3.6V.
Table 1 lists recommended components and Table 2 lists
contact information of component suppliers.
Table 1. Component List
DESIGNATION
DESCRIPTION
C1
10μF, 6.3V X5R ceramic capacitor (1206)
TDK C3216X5ROJ106M
C21, C22
4.7μF, 10V X5R ceramic capacitors (1206)
TDK C3216X5R1A475M
D1
D2–D5
L1
3A, 30V Schottky diode (M-flat)
Toshiba CMS02
200mA, 100V, dual, ultra-fast diodes (SOT23)
Fairchild MMBD4148SE
3.6μH, 1.8A inductor
Sumida CM0611BHPNP-3R6MC
The MAX8798 contains a high-performance step-up
switching regulator; one high-speed operational amplifier; one three-channel, high-voltage level-shifting scan
driver for active-matrix TFT LCDs; and an I2C-controlled
VCOM calibrator. Figure 3 shows the MAX8798 functional diagram.
Step-Up Regulator
The step-up regulator employs a current-mode, fixedfrequency PWM architecture to maximize loop bandwidth and provide fast transient response to pulsed
loads found in source drivers of TFT LCD panels. The
high switching frequency (1.2MHz) allows the use of
low-profile inductors and ceramic capacitors to minimize
the thickness of LCD panel designs. The integrated
high-efficiency MOSFET and the IC’s built-in digital
soft-start functions reduce the number of external components required while controlling inrush current. The
output voltage can be set from VIN to 18V with an external resistive voltage-divider.
The regulator controls the output voltage and the power
delivered to the output by modulating the duty cycle (D)
of the internal power MOSFET in each switching cycle.
The duty cycle of the MOSFET is approximated by:
V
−V
D ≈ MAIN IN
VMAIN
Table 2. Component Suppliers
SUPPLIER
PHONE
FAX
WEBSITE
Fairchild
408-822-2000
408-822-2102
www.fairchildsemi.com
Sumida
847-545-6700
847-545-6720
www.sumida.com
TDK
847-803-6100
847-390-4405
www.component.tdk.com
Toshiba
949-455-2000
949-859-3963
www.toshiba.com/taec
______________________________________________________________________________________
17
MAX8798
Typical Application Circuit
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Figure 4 shows the block diagram of the step-up regulator. An error amplifier compares the signal at FB to
1.24V and changes the COMP output. The voltage at
COMP determines the current trip point each time the
internal MOSFET turns on. As the load varies, the error
amplifier sources or sinks current to the COMP output
accordingly to produce the inductor peak current necessary to service the load. To maintain stability at high
duty cycles, a slope-compensation signal is summed
with the current-sense signal.
On the rising edge of the internal clock, the controller
sets a flip-flop, turning on the n-channel MOSFET and
applying the input voltage across the inductor. The current through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the current-feedback signal and the slope compensation
exceed the COMP voltage, the controller resets the flipflop and turns off the MOSFET. Since the inductor current is continuous, a transverse potential develops
across the inductor that turns on the diode (D1). The
voltage across the inductor then becomes the difference between the output voltage and the input voltage.
This discharge condition forces the current through the
inductor to ramp back down, transferring the energy
stored in the magnetic field to the output capacitor and
the load. The MOSFET remains off for the rest of the
clock cycle.
Undervoltage Lockout (UVLO)
The undervoltage lockout (UVLO) circuit compares the
input voltage at IN with the UVLO threshold (1.3V rising
and 1.2V falling) to ensure that the input voltage is high
enough for reliable operation. The 100mV (typ) hysteresis prevents supply transients from causing a restart.
Once the input voltage exceeds the UVLO rising
threshold, startup begins. When the input voltage falls
below the UVLO falling threshold, the controller turns
off the main step-up regulator and the linear regulator,
disables the switch-control block, and the operational
amplifier output becomes high impedance.
LX
CLOCK
LOGIC AND DRIVER
PGND
ILIM
COMPARATOR
SOFTSTART
ILIMIT
SLOPE COMP
PWM
COMPARATOR
CURRENT
SENSE
1.2MHz
OSCILLATOR
TO FAULT LOGIC
1.0V
FAULT
COMPARATOR
ERROR AMP
FB
1.24V
COMP
Figure 4. Step-Up Regulator Block Diagram
18
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Bootstrapping and Soft-Start
The MAX8798 features bootstrapping operation. In normal operation, the internal linear regulator supplies
power to the internal circuitry. The input of the linear
regulator (BOOST) should be directly connected to the
output of the step-up regulator. The MAX8798 is
enabled when the voltages at IN and BOOST are above
their UVLO thresholds and the fault latch is not set.
After being enabled, the regulator starts open-loop
switching to generate the supply voltage for the linear
regulator. The internal reference block turns on when
the VL voltage exceeds its 2.7V (typ). When the reference voltage reaches regulation, the PWM controller
and the current-limit circuit are enabled and the stepup regulator enters soft-start. During soft-start, the main
step-up regulator directly limits the peak inductor current, allowing from zero up to the full current-limit value
in 128 equal current steps. The maximum load current
is available after the output voltage reaches regulation
(which terminates soft-start), or after the soft-start timer
expires in approximately 3ms. The soft-start routine
minimizes the inrush current and voltage overshoot and
ensures a well-defined startup behavior.
Fault Protection
During steady-state operation, the MAX8798 monitors
the FB voltage. If the FB voltage does not exceed 1V
(typ), the MAX8798 activates an internal fault timer. If
there is a continuous fault for the fault-timer duration,
the MAX8798 sets the fault latch, turning off the main
step-up regulator and the linear regulator, disabling the
switch-control block and the operational amplifier.
Once the fault condition is removed, cycle the input
voltage to clear the fault latch and reactivate the
device. The fault-detection circuit is disabled during the
soft-start time.
The MAX8798 monitors BOOST for undervoltage and
overvoltage conditions. If the BOOST voltage is below
1.4V (typ) or above 19V (typ), the MAX8798 disables the
gate driver of the step-up regulator and prevents the
internal MOSFET from switching. The BOOST undervoltage and overvoltage conditions do not set the fault latch.
Operational Amplifier
The MAX8798 has an operational amplifier that is typically used to drive the LCD backplane (VCOM) or the
gamma-correction-divider string. The operational
amplifier features ±150mA output short-circuit current,
40V/µs slew rate, and 20MHz bandwidth. While the op
amp is a rail-to-rail input and output design, its accuracy is significantly degraded for input voltages within 1V
of its supply rails (BOOST and BGND).
Short-Circuit Current Limit
The operational amplifier limits short-circuit current to
approximately ±150mA if the output is directly shorted to
BOOST or to AGND. If the short-circuit condition persists, the junction temperature of the IC rises until it
reaches the thermal-shutdown threshold (+160°C typ).
Once the junction temperature reaches the thermal-shutdown threshold, an internal thermal sensor immediately
sets the thermal-fault latch, shutting off the main step-up
regulator, the linear regulator, the switch-control block,
and the operational amplifier. Those portions of the
device remain inactive until the input voltage is cycled.
Driving Pure Capacitive Loads
The operational amplifier is typically used to drive the
LCD backplane (VCOM) or the gamma-correctiondivider string. The LCD backplane consists of a distributed series capacitance and resistance, a load that can
be easily driven by the operational amplifier. However,
if the operational amplifier is used in an application with
a pure capacitive load, steps must be taken to ensure
stable operation.
As the operational amplifier’s capacitive load increases,
the amplifier’s bandwidth decreases and gain peaking
increases. A 5Ω to 50Ω small resistor placed between
VCOM and the capacitive load reduces peaking, but
also reduces the gain. An alternative method of reducing
peaking is to place a series RC network (snubber) in parallel with the capacitive load. The RC network does not
continuously load the output or reduce the gain. Typical
values of the resistor are between 100Ω and 200Ω and
the typical value of the capacitor is 10pF.
______________________________________________________________________________________
19
MAX8798
Linear Regulator (VL)
The MAX8798 includes an internal 3.3V linear regulator.
BOOST is the input of the linear regulator. The input voltage range is between 5V and 18V. The regulator powers
all the internal circuitry including the MOSFET gate driver. Bypass the VL pin to AGND with a 0.22µF or greater
ceramic capacitor. BOOST should be directly connected
to the output of the step-up regulator. This feature significantly improves the efficiency at low input voltages.
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
CKV
CPV
SYSTEM
VIDEO
TIMING
OE
CKVB
MAX8798
SCAN DRIVER
STV
STVP
HIGH-VOLTAGE SHIFT REGISTER
COLUMN DRIVER
PANEL GLASS
CKV = SCAN CLK ODD
CKVB = SCAN CLK EVEN
STVP = HIGH-VOLTAGE STV
Figure 5. Scan Driver System Diagram
High-Voltage Level-Shifting Scan Driver
The MAX8798 includes a 3-channel high-voltage (60V)
level-shifting scan driver, which includes logic functions
necessary to drive row driver functions on the panel
glass (Figure 5). The driver outputs (CKV, CKVB, STVP)
swing between their power-supply rails (GON and
GOFF) according to the input logic levels on the block’s
Table 3. STVP Logic
SIGNAL
LOGIC STATE
STV
H
H
H
L
OECON
X
X
X
X
CPV
L
H
X
X
OE
L
X
H
X
H
Hi-Z
Hi-Z
L
STVP
X = Don’t care.
inputs (STV, CPV, OE, and OECON) and the internal
logic of the block (Tables 3, 4). STV is the vertical sync
signal. CPV is the horizontal sync signal. OE is the output
enable signal. OECON is a timing signal derived from
OE that blanks OE if it stays high too long. These signals
have CMOS input logic levels set by the IN supply voltage. CKV and CKVB are complementary scan clock outputs. STVP is the output scan start signal. These output
signals swing from GON to GOFF, which have a maximum range of +35V and -25V. Their 10Ω (typ) output
impedance enables them to swiftly drive capacitive
loads. The complementary CKV and CKVB outputs feature power-saving, charge-sharing inputs (CKVCS,
CKVBCS) that can be used to save power by shorting
each output to its complement during transitions, making
a portion of the transition “lossless.”
Table 4. CKV, CKVB Logic
SIGNAL
LOGIC STATE
STV
H
H
H
L
L
L
L
L
OECON
X
X
X
L
L
L
H
H
CPV
L
H
X
L
—
X
L
—
OE
L
X
H
L
X
—
X
X
CKV
L
H
H
CS
Toggle
Toggle
CS
Toggle
CKVB
H
L
L
CS
Toggle
Toggle
CS
Toggle
X = Don’t care. CS = Charge share state.
20
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
The resistive voltage-divider and the BOOST supply set
the maximum value of VCOM. OUT sinks current from the
voltage-divider to reduce the POS voltage level and
VCOM output. The external resistor at SET (RSET) sets the
full-scale sink current and the minimum value of VCOM.
The DISH input controls a switch between GOFF and
PGND. When DISH is pulled below ground by at least
1V, GOFF is rapidly discharged to GND. Typically,
DISH is capacitively coupled to IN so that if IN falls
suddenly, GOFF is discharged to blank the display
(Figure 3).
The GON input provides the high voltage required to
program the EEPROM. To allow programming, GON is
connected to the TFT LCD VGON supply. VGON
should be between 16.1V and 35V. EEPROM programming is disabled when GON is below 15.5V (typ).
Bypass GON to GND or BOOST (which is bypassed to
GND) with a 0.1µF or greater capacitor.
VCOM Calibrator
The VCOM calibrator is a solid-state alternative to
mechanical potentiometers used for adjusting the LCD
backplane voltage (VCOM) in TFT LCD displays. OUT
attaches to the external resistive voltage-divider at POS
and sinks a programmable current (IOUT), which sets the
VCOM level (Figure 6). An internal 7-bit DAC controls the
sink current and allows the user to increase or decrease
the VCOM level. The DAC is ratiometrically relative to
VBOOST and is monotonic over all operating conditions.
The user can store the DAC setting in an internal
EEPROM. On power-up, the EEPROM presets the DAC
to the last stored setting. The 2-wire I 2 C interface
between the system controller and the programming circuit adjusts the DAC and programs the EEPROM when
WPN is high.
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the device. When
the junction temperature exceeds TJ = +160°C, a thermal sensor immediately activates the fault protection,
which shuts down the step-up regulator, switch control
block, operational amplifier, and the internal linear regulator, allowing the device to cool down. Once the
device cools down by approximately 15°C, cycle the
input voltage (below the UVLO falling threshold) to
clear the fault latch and reactivate the device.
The thermal-overload protection protects the controller
in the event of fault conditions. For continuous operation, do not exceed the absolute maximum junction
temperature rating of TJ = +150°C.
VDD
VDD
VMAIN
VDD
BOOST
LINEAR
REGULATOR
NEG
19R
SDA
VCOM
SCL
WPN
I2C
CONTROL
INTERFACE
VCOM
R
DAC
R3
7
I2C BUS
POS
OUT
SCLS
R4
EEPROM
BLOCK
WPP
7
SET
GON
RSET
VGON
Figure 6. VCOM Calibrator Functional Diagram
______________________________________________________________________________________
21
MAX8798
GOFF Rapid Discharge Function
(DISH Input)
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Design Procedure
Main Step-Up Regulator
Inductor Selection
The minimum inductance value, peak current rating, and
series resistance are factors to consider when selecting
the inductor. These factors influence the converter’s efficiency, maximum output-load capability, transient
response time, and output-voltage ripple. Physical size
and cost are also important factors to be considered.
The maximum output current, input voltage, output voltage, and switching frequency determine the inductor
value. Very high inductance values minimize the current ripple, and therefore, reduce the peak current,
which decreases core losses in the inductor and I2R
losses in the entire power path. However, large inductor values also require more energy storage and more
turns of wire, which increase physical size and can
increase I2R losses in the inductor. Low inductance values decrease the physical size, but increase the current ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size, and cost.
The equations used here include a constant called LIR,
which is the ratio of the inductor peak-to-peak ripple
current to the average DC inductor current at the fullload current. The best trade-off between inductor size
and circuit 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
resistances, the best LIR can shift up or down. If the
inductor resistance is relatively high, more ripple can
be accepted to reduce the number of turns required
and increase the wire diameter. If the inductor resistance is relatively low, increasing inductance to lower
the peak current can decrease losses throughout the
power 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.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency improvements in typical operating regions.
In Figure 2, the LCD’s gate-on and gate-off supply voltages are generated from two unregulated charge
pumps driven by the step-up regulator’s LX node. The
additional load on LX must therefore be considered in
the inductance and current calculations. The effective
maximum output current, IMAIN(EFF) becomes the sum
of the maximum load current of the step-up regulator’s
output plus the contributions from the positive and negative charge pumps:
22
IMAIN(EFF) = IMAIN(MAX) + nNEG × INEG + (nPOS + 1) × IPOS
where IMAIN(MAX) is the maximum step-up output current, n NEG is the number of negative charge-pump
stages, nPOS is the number of positive charge-pump
stages, INEG is the negative charge-pump output current, and IPOS is the positive charge-pump output current, assuming the initial pump source for I POS is
VMAIN.
Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current
(IMAIN(EFF)), the expected efficiency (ηTYP) taken from
an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above
discussion:
2
⎛ V
⎞ ⎛ VMAIN − VIN ⎞ ⎛ ηTYP ⎞
L = ⎜ IN ⎟ ⎜
⎜
⎟
⎝ VMAIN ⎠ ⎝ IMAIN(EFF) × fOSC ⎟⎠ ⎝ LIR ⎠
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current at the minimum input voltage VIN(MIN) using conservation of energy and the expected efficiency at that
operating point (ηMIN) taken from an appropriate curve
in the Typical Operating Characteristics:
IIN(DCMAX
,
)=
IMAIN(EFF) × VMAIN
VIN(MIN) × ηMIN
Calculate the ripple current at that operating point and
the peak current required for the inductor:
IRIPPLE =
VIN(MIN) × (VMAIN − VIN(MIN) )
L × VMAIN × fOSC
IRIPPLE
IPEAK = IIN(DCMAX
,
)+
2
The inductor’s saturation current rating and the
MAX8798’s LX current limit (ILIM) should exceed IPEAK
and the inductor’s DC current rating should exceed
IIN(DC,MAX). For good efficiency, choose an inductor
with less than 0.1Ω series resistance.
Considering Figure 2, the maximum load current
(IMAIN(MAX)) is 300mA, with an 8V output and a typical
input voltage of 3.3V. The effective full-load step-up
current is:
IMAIN(EFF) = 300mA + 2 × 20mA + (2 + 1) × 20mA = 400mA
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
2
⎛ 3.3V ⎞ ⎛ 8V − 3.3V ⎞ ⎛ 0.85 ⎞
L=⎜
⎟ ⎜
⎟⎜
⎟ ≈ 2.8μH
⎝ 8V ⎠ ⎝ 0.4A × 1.2MHz ⎠ ⎝ 0.5 ⎠
A 2.6µH inductor is chosen. Then, using the circuit’s
minimum input voltage (3.0V) and estimating efficiency
of 80% at that operating point:
IIN(DCMAX
,
)=
0.4A × 8V
≈ 1.33A
3V × 0.8
The ripple current and the peak current at that input
voltage are:
IRIPPLE =
3V × (8V − 3V )
2.6μH × 8V × 1.2MHz
IPEAK = 1.33A +
≈ 0.6A
0.6A
= 1.53A
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):
VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR)
I
⎛V
−V ⎞
VRIPPLE(C) ≈ MAIN ⎜ MAIN IN ⎟
COUT ⎝ VMAINfOSC ⎠
and:
VRIPPLE(ESR) ≈ IPEAKRESR(COUT)
where I PEAK is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors, the
output-voltage ripple is typically dominated by
VRIPPLE(C). The voltage rating and temperature characteristics of the output capacitor must also be considered.
Input Capacitor Selection
The input capacitor (CIN) reduces the current peaks
drawn from the input supply and reduces noise injection into the IC. A 10µF ceramic capacitor is used in
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 regulator often runs directly from the output of another regulated supply. Typically, CIN can be reduced below the
values used in Figure 2. Ensure a low noise supply at
IN by using adequate CIN. Alternatively, greater voltage
variation can be tolerated on CIN if IN is decoupled
from CIN using an RC lowpass filter (seen in Figure 2).
Rectifier Diode
The MAX8798’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
diode complements the internal MOSFET well.
Output Voltage Selection
The output voltage of the main step-up regulator is
adjusted by connecting a resistive voltage-divider from
the output (VMAIN) to AGND with the center tap connected to FB (see Figure 2). Select R2 in the 10kΩ to
50kΩ range. Calculate R1 with the following equation:
⎛V
⎞
R1 = R2 × ⎜ MAIN − 1⎟
⎝ VREF
⎠
where VREF, the step-up regulator’s feedback set point,
is 1.235V (typ). Place R1 and R2 close to the IC.
Loop Compensation
Choose RCOMP to set the high-frequency integrator
gain for fast transient response. Choose CCOMP to set
the integrator zero to maintain loop stability.
For low-ESR output capacitors, use the following equations to obtain stable performance and good transient
response:
RCOMP ≈
1000 × VIN × VOUT × COUT
L × IMAIN(MAX)
CCOMP ≈
VOUT × COUT
10 × IMAIN(MAX) × RCOMP
To further optimize transient response, vary RCOMP in
20% steps and CCOMP in 50% steps while observing
transient response waveforms.
______________________________________________________________________________________
23
MAX8798
Choosing an LIR of 0.5 and estimating efficiency of
85% at this operating point:
MAX8798
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. RSET sets the
full-scale sink current, IOUT, which determines the minimum value of the VCOM adjustment range. Large RSET
values increase resolution, but decrease the VCOM
adjustment range. Calculate R3, R4, and RSET using
the following procedure:
1) Choose the maximum VCOM level (VMAX), the minimum VCOM level (V MIN), and the V MAIN supply
voltage.
2) Select R3 between 10kΩ and 500kΩ based on the
acceptable power loss from the VMAIN supply rail
connected to BOOST.
3) Calculate R4:
R4 ≅
VMAX
× R3
V
( BOOST - VMAX )
4) Calculate RSET:
RSET =
VMAX
× R3
20 × (VMAX − VMIN )
5) Verify that ISET does not exceed 120µA:
ISET =
VBOOST
20 × RSET
6) If ISET exceeds 120µA, return to step 2 and choose
a larger value for R1.
7) The resulting resolution is:
(VMAX − VMIN )
127
A complete design example is given below:
VMAX = 4V, VMIN = 2.4V, VBOOST = 8V
If R3 = 200kΩ, then R4 = 200kΩ and R SET =
24.9kΩ.
Resolution = 12.5mV
Applications Information
Power Dissipation
An IC’s maximum power dissipation depends on the
thermal resistance from the die to the ambient environment and the ambient temperature. The thermal resistance depends on the IC package, PCB copper area,
other thermal mass, and airflow.
24
The MAX8798, with its exposed backside paddle soldered to 1in 2 of PCB copper and a large internal
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 dissipation capability. The major components of power
dissipation are the power dissipated in the step-up regulator 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.
Step-Up Regulator
The largest portions of the power dissipated by the
step-up regulator are the internal MOSFET, the inductor, 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.
Operational Amplifier
The power dissipated in the operational amplifier
depends on the output current, the output voltage, and
the supply voltage:
PDSOURCE = IVCOM _ SOURCE × (VBOOST − VVCOM )
PDSINK = IVCOM _ SINK × VVCOM
where IVCOM_SOURCE is the output current sourced by
the operational amplifier, and IVCOM_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:
PDSCAN = 3 × fSCAN × CPANEL × ( VGON − VGOFF )
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.
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
RPULLUP ≤
tR
CBUS
where tR is the rise time in the Electrical Characteristics
table, and CBUS is the total capacitance on the bus.
The MAX8798 uses a nonstandard I2C interface protocol with mostly standard voltage and timing parameters, as defined in the following subsections.
Bus Free
Both data and clock lines remain HIGH. Data transfers
can be initiated only when the bus is not busy (Figure 7).
START Condition (S)
Starting from an idle bus state (both SDA and SCL are
high), a HIGH to LOW transition of the SDA line while
the clock (SCL) is HIGH determines a START condition.
All commands must be preceded by a START condition
from a master device on the bus.
STOP Condition (P)
A LOW to HIGH transition of the SDA line while the
clock (SCL) is HIGH determines a STOP condition. All
operations must be ended with a STOP condition from
the master device.
Data Valid
The state of the data line represents valid data when,
after a START condition, the data line is stable for the
duration of the HIGH period of the clock signal. The
data on the line must be changed during the LOW period of the clock signal. The master generates one clock
pulse per bit of data during write operations and the
slave device outputs 1 data bit per clock pulse during
read operations. Each data transfer is initiated with a
START condition and terminated with a STOP condition.
Two bytes are transferred between the START and
STOP conditions.
Slave Address
After generating a START condition, the bus master
transmits the slave address consisting of the 7-bit device
code (0b1001110 or 9Eh) for the MAX8798 (Figure 8).
For a read operation the 8th bit is 1 and for write operations it is 0. The MAX8798 monitors the bus for its corresponding slave address continuously. It generates an
acknowledge bit if it recognizes its slave address and it
is not busy programming the EEPROM.
SDA
SCL
S
BUS
FREE
DATA LINE
STABLE
DATA VALID
START
CONDITION
P
CHANGE
OF DATA
ALLOWED
STOP
CONDITION
BUS
FREE
Figure 7. I2C Bus Start, Stop, and Data Change Conditions
S
T
A
R
T
SLAVE ADDRESS
1001111
R/W
A
C
K
DATA BYTE
D6
D5
D4
D3
D2
D1
D0
P
R
O
G
A
C
K
S
T
O
P
READ BYTE: R/W = 1, MAX8798 OUTPUTS D6–D0 FOLLOWED BY PROG = 0
WRITE BYTE: R/W = 2, DATA = D6–D0, PROG = 1
PROGRAM EEPROM: R/W = 0, D6–D0 = DON'T CARE, PROG = 0
Figure 8. I2C Slave Address and Data Byte
______________________________________________________________________________________
25
MAX8798
VCOM Calibrator Interface
The MAX8798 is a slave-only device with an I2C address
of 9Eh. The 2-wire I2C-bus-like serial interface (pins SCL
and SDA) is designed to attach to a 1.8V to 4V I2C bus.
Connect both SCL and SDA lines to the VDD supply
through individual pullup resistors. Calculate the
required value of the pullup resistors using:
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Data Byte
The data byte follows successful transmission of the
MAX8798’s slave address (Figure 8). For a read operation, the MAX8798 outputs the 7 bits corresponding to
the current DAC setting followed by a 0 bit. For a write
operation, the bus master must provide the 7-bit data
corresponding to the desired DAC setting followed by a
1 bit. To program the IC’s EEPROM, the master must
make the last bit a zero, in which case the other 7 bits
of data are ignored. For programming, GON must
exceed its programming threshold. Otherwise, programming does not occur and the MAX8798 does not
acknowledge the programming command.
DAC Values
Table 5 lists the DAC values and the corresponding
ISET, VSET, and VOUT values.
Table 5. DAC Settings
7-BIT
DATA BYTE
ISET
VSET (V)
VOUT (V)
0000000
ISET(MAX)
VSET(MAX)
VMIN
0000001
ISET(MAX) 1-LSB
VSET(MAX) 1-LSB
VMIN +
1-LSB
.
.
.
.
.
.
.
.
.
.
.
.
1111110
ISET(MIN) +
1-LSB
VSET(MIN) +
1-LSB
VMAX 1-LSB
1111111
ISET(MIN)
VSET(MIN)
VMAX
Acknowledge/Polling
The MAX8798, when addressed, generates an acknowledge pulse after the reception of each byte (Figure 9).
The master device must generate an extra clock pulse,
which is associated with this acknowledge bit. The
device that acknowledges has to pull down the SDA line
during the acknowledge clock pulse so that the SDA line
is stable LOW during the HIGH period of the acknowledge-related clock pulse. Of course, setup and hold
times must be taken into account. The master signals an
end of data to the slave by not generating an acknowledge bit on the last byte that has been clocked out of the
slave. In this case, the slave leaves the data line HIGH to
enable the master to generate the STOP condition.
The MAX8798 does not generate an acknowledge
while an internal programming cycle is in progress.
Once the internally timed write cycle has started and
the EEPROM inputs are disabled, acknowledge polling
can be initiated. This involves sending a START condition followed by the device address byte. Only if the
internal write cycle has completed does the MAX8798
respond with an acknowledge pulse, allowing the read
or write sequence to continue.
The MAX8798 does not acknowledge a command to
program the EEPROM if VGON is not high enough to
properly program the device. Also, a program command must be preceded by a write command. The IC
does not acknowledge a program command or program the EEPROM unless the DAC data has been
modified since the most recent program command.
DATA OUTPUT
BY MASTER
D7
D6
D0
NOT ACKNOWLEDGE
DATA OUTPUT
BY MAX8798
ACKNOWLEDGE
SCL FROM
MASTER
S
START
CONDITION
1
2
8
9
CLK1
CLK2
CLK8
CLK9
ACKNOWLEDGE
CLOCK PULSE
Figure 9. I2C Bus Acknowledge
26
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
BOOST and VL bypass capacitor ground connections, and the device’s exposed backside paddle.
Connect the AGND and PGND islands by connecting the PGND pin directly to the exposed backside
paddle. Make no other connections between these
separate ground planes.
•
•
Minimize the area of high-current loops by placing
the inductor, output diode, and output capacitors
near the input capacitors and near the LX and
PGND pins. The high-current input loop goes from
the positive terminal of the input capacitor to the
inductor, to the IC’s LX pin, out of PGND, and to the
input capacitor’s negative terminal. The high-current output loop is from the positive terminal of the
input capacitor to the inductor, to the output diode
(D1), to the positive terminal of the output capacitors, reconnecting between the output capacitor
and input capacitor ground terminals. Connect
these loop components with short, wide connections. Avoid using vias in the high-current paths. If
vias are unavoidable, use many vias in parallel to
reduce resistance and inductance.
Create a power ground island (PGND) consisting of
the input and output capacitor grounds, PGND pin,
and any charge-pump components. Connect all
these together with short, wide traces or a small
ground plane. Maximizing the width of the power
ground traces improves efficiency and reduces output-voltage ripple and noise spikes. Create an analog ground plane (AGND) consisting of the AGND
pin, all the feedback-divider ground connections, the
operational-amplifier-divider ground connections,
the COMP capacitor ground connection, the
•
Place the feedback-voltage-divider resistors as
close as possible to the feedback pin. The divider’s
center trace should be kept short. Placing the resistors far away causes the FB trace to become an
antenna that can pick up switching noise. Care
should be taken to avoid running the feedback
trace near LX or the switching nodes in the charge
pumps.
•
Place the IN pin and VL pin bypass capacitors as
close as possible to the device. The ground connections of the IN and VL bypass capacitors should
be connected directly to the AGND pin with a wide
trace.
•
Minimize the length and maximize the width of the
traces between the output capacitors and the load
for best transient responses.
•
Minimize the size of the LX node while keeping it
wide and short. Keep the LX node away from the
feedback node and analog ground. Use DC traces
as shield if necessary.
Refer to the MAX8798 evaluation kit for an example of
proper board layout.
______________________________________________________________________________________
27
MAX8798
PCB Layout and Grounding
Careful PCB layout is important for proper operation.
Use the following guidelines for good PCB layout:
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Chip Information
TRANSISTOR COUNT: 15,227
PROCESS: BiCMOS
NEG
POS
OUT
BOOST
BGND
VL
IN
TOP VIEW
SHDN
VCOM
MAX8798
Pin Configuration
27 26 25 24 23 22 21 20 19
SET
WPP
LX
LX
28
18
29
17
PGND
PGND
30
16
31
15
FB
COMP
32
AGND
GOFF
GON
34
12
35
11
SCLS
WPN
VDD
DISH
36
10
GND
14
MAX8798
4
5
6
7
8
9
OE
2
CPV
1
CKV
3
13
CKVCS
CKVBCS
CKVB
STVP
STV
OECON
33
SDA
SCL
THIN QFN
28
______________________________________________________________________________________
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
QFN THIN.EPS
______________________________________________________________________________________
29
MAX8798
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
MAX8798
Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
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