MAX8752 [MAXIM]
TFT LCD Step-Up DC-DC Converter; TFT LCD升压型DC -DC转换器
| 型号： | MAX8752 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | TFT LCD Step-Up DC-DC Converter |
| 文件： | 总12页 (文件大小：240K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3793; Rev 0; 8/05
TFT LCD Step-Up DC-DC Converter
General Description
Features
The MAX8752 is a high-performance, step-up DC-DC
converter that provides a regulated supply voltage for
active-matrix thin-film transistor (TFT) liquid-crystal dis-
plays (LCDs). The MAX8752 incorporates current-mode,
fixed-frequency, pulse-width modulation (PWM) circuitry
with a built-in n-channel power MOSFET to achieve high
efficiency and fast transient response. The input supply
voltage of the MAX8752 is from 1.8V to 5.5V.
♦ 1.8V to 5.5V Input Supply Range
♦ Built-In 14V, 2.2A, 0.2Ω n-Channel MOSFET
♦ High Efficiency (> 85%)
♦ Fast Transient Response to Pulsed Load
♦ High-Accuracy Output Voltage (1.5%)
♦ Internal Digital Soft-Start
The MAX8752 operates with a switching frequency of
1.2MHz, allowing the use of ultrasmall inductors and low-
ESR ceramic capacitors. The current-mode architecture
provides fast transient response to the pulsed loads typi-
cal of LCD source-driver applications. A compensation
pin (COMP) gives users flexibility in adjusting loop
dynamics. The 14V internal MOSFET can generate output
voltages up to 13V. The internal digital soft-start and cur-
rent limit effectively control inrush and fault currents.
♦ Input Supply Undervoltage Lockout
♦ 1.2MHz Switching Frequency
♦ 0.1µA Shutdown Current
♦ Small 8-Pin TDFN Package
The MAX8752 is available in a 3mm x 3mm 8-pin TDFN
package with a maximum height of 8mm.
Ordering Information
Applications
Notebook Computer Displays
TEMP
PIN-
PKG
PART
RANGE
PACKAGE
CODE
LCD Monitor Panels
8 TDFN
3mm x 3mm
MAX8752ETA -40°C to +85°C
T833-2
Automotive Displays
Typical Operating Circuit
Pin Configuration
V
IN
V
+1.8V TO +5.5V
MAIN
TOP VIEW
LX
IN
FB
MAX8752
MAX8752
GND
SUP
COMP
LDO
SHDN
IN
TDFN
3mm x 3mm
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
TFT LCD Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
LX, SUP to GND .....................................................-0.3V to +14V
Continuous Power Dissipation (T = +70°C)
A
IN, SHDN, LDO to GND............................................-0.3V to +6V
10-Pin TDFN (derate 18.2mW/°C above +70°C) .......1454mW
FB to GND ...................................................-0.3V to (V + 0.3V)
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10s) .................................+300°C
IN
LDO
COMP to GND..........................................-0.3V to (V
+ 0.3V)
LX Switch Maximum Continuous RMS Current.....................1.6A
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
(V = V
IN
= 2.5V, T = 0°C to +85°C. Typical values are at T = +25°C, unless otherwise noted.)
SHDN
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
5.5
UNITS
Input Supply Range
1.8
V
V
Output Voltage Range
13
IN Undervoltage Lockout
Threshold
V
rising, typical hysteresis is 200mV
0.90
1.30
1.75
V
IN
V
V
= 1.3V, not switching
= 1.0V, switching
0.18
2
0.35
5
FB
FB
IN Quiescent Current
mA
IN Shutdown Current
SHDN = GND
6V ≤ V ≤ 13V, I
0.1
5.0
2.7
10.0
5.4
3.0
µA
V
LDO Output Voltage
= 12.5mA
LDO
4.6
2.4
15
SUP
LDO Undervoltage Lockout
LDO Output Current
V
rising, typical hysteresis is 200mV
V
LDO
mA
V
SUP Supply Voltage Range
4.5
13.0
14.0
SUP Overvoltage-Lockout
Threshold
V
V
rising, typical hysteresis is 200mV (Note 1)
rising, typical hysteresis is 200mV (Note 2)
13.2
13.6
V
V
SUP
SUP
SUP Undervoltage-Lockout
Threshold
1.4
LX not switching
LX switching
1.5
4
2.0
8
SUP Supply Current
ERROR AMPLIFIER
FB Regulation Voltage
mA
I
I
= 200mA, T = 0°C to +25°C
= 200mA, T = +25°C to +85°C
1.218
1.223
1.240
1.240
0
1.262
1.257
40
LX
V
LX
FB Input Bias Current
FB Line Regulation
Transconductance
Voltage Gain
V
V
= 1.24V
nA
%/V
µS
FB
IN
= 1.8V to 5.5V
0.05
180
700
0.15
280
70
V/V
OSCILLATOR
Frequency
1000
88
1220
92
1500
96
kHz
%
Maximum Duty Cycle
2
_______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V = V
IN
= 2.5V, T = 0°C to +85°C. Typical values are at T = +25°C, unless otherwise noted.)
SHDN
A
A
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
n-CHANNEL MOSFET
Current Limit
V
V
= 1V, 65% duty cycle
1.8
2.2
0.2
0.1
0.3
2.6
0.4
10
A
Ω
FB
LX
On-Resistance
Leakage Current
= 12V
µA
V/A
Current-Sense Transresistance
SOFT-START
0.2
0.4
Soft-Start Period
13
ms
A
Soft-Start Step Size
CONTROL INPUTS
SHDN Input Low Voltage
SHDN Input High Voltage
SHDN Input Current
0.275
V
V
= 1.8V to 5.5V
= 1.8V to 5.5V
0.6
V
V
IN
IN
0.7 × V
IN
0.001
1.000
µA
ELECTRICAL CHARACTERISTICS
(V = V
IN
= 2.5V, T = -40°C to +85°C. unless otherwise noted.)
A
SHDN
PARAMETER
CONDITIONS
MIN
TYP
MAX
5.5
13
UNITS
Input Supply Range
1.8
V
V
V
Output Voltage Range
IN Undervoltage-Lockout Threshold
V
V
V
rising, typical hysteresis is 200mV
= 1.3V, not switching
0.90
1.75
0.35
5
IN
FB
FB
IN Quiescent Current
mA
= 1.0V, switching
LDO Output Voltage
6V ≤ V
≤ 13V, I
= 12.5mA
4.6
2.4
15
5.4
3.0
V
V
SUP
LDO
LDO Undervoltage Lockout
LDO Output Current
V
rising, typical hysteresis is 200mV
LDO
mA
V
SUP Supply Voltage Range
SUP Overvoltage-Lockout Threshold
SUP Undervoltage-Lockout Threshold
4.5
13.2
13.0
14.0
1.4
2.0
8
V
V
rising, typical hysteresis is 200mV (Note 1)
rising, typical hysteresis is 200mV (Note 2)
V
SUP
SUP
V
LX not switching
LX switching
SUP Supply Current
mA
ERROR AMPLIFIER
FB Regulation Voltage
OSCILLATOR
I
= 200mA
1.210
940
1.7
1.270
1560
V
LX
Frequency
kHz
n-CHANNEL MOSFET
Current Limit
V
= 1V, 65% duty cycle
2.7
0.4
0.4
A
Ω
FB
On-Resistance
Current-Sense Transresistance
0.2
V/A
_______________________________________________________________________________________
3
TFT LCD Step-Up DC-DC Converter
ELECTRICAL CHARACTERISTICS (continued)
(V = V
IN
= 2.5V, T = -40°C to +85°C. unless otherwise noted.)
A
SHDN
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
CONTROL INPUTS
SHDN Input Low Voltage
V
V
= 1.8V to 5.5V
= 1.8V to 5.5V
0.6
V
V
IN
IN
0.7 ×
SHDN Input High Voltage
V
IN
Note 1: Step-up regulator inhibited when VSUP exceeds this threshold.
Note 2: Step-up regulator inhibited until VSUP exceeds this threshold.
Note 3: Specifications to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, V = 2.5V, V
= 10V, T = +25°C, unless otherwise noted.)
A
IN
MAIN
OUTPUT VOLTAGE ERROR
vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
0.5
0
90
85
80
75
70
65
60
55
50
95
90
85
80
75
70
65
60
55
50
V
= 5V
IN
L1 = 3.3μH
L1 = 2.6μH
V
= 5V
IN
V
= 5V
IN
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
V
= 3.3V
IN
V
= 1.8V
IN
V
= 1.8V
IN
V
= 3.3V
IN
V
= 3.3V
IN
V
= 1.8V
IN
1
10
100
1000
10,000
10
100
LOAD CURRENT (mA)
1000
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
IN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SWITCHING FREQUENCY ERROR
vs. INPUT VOLTAGE
IN SUPPLY CURRENT
vs. TEMPERATURE
50
40
30
20
10
0
0.2
0.1
50
40
30
20
10
0
NO LOAD
NORMAL FB
V
= 1.8V
IN
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
V
= 3.3V
IN
V
= 1.3V
FB
V
= 5V
IN
1.5
2.5
3.5
4.5
5.5
1.8
2.8
3.8
4.8
5.8
-40
-20
0
20
40
60
80
SUPPLY VOLTAGE (V)
INPUT VOLTAGE (V)
TEMPERATURE (°C)
4
_______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V = 2.5V, V
IN
= 10V, T = +25°C, unless otherwise noted.)
MAIN
A
SOFT-START (HEAVY LOAD)
LOAD TRANSIENT RESPONSE
PULSED-LOAD TRANSIENT RESPONSE
MAX8752 toc08
MAX8752 toc07
MAX8752 toc09
I
I
MAIN
MAIN
1A/div
200mA/div
100mA
0A
0A
INDUCTOR
CURRENT
1A/div
INDUCTOR
CURRENT
1A/div
INDUCTOR
CURRENT
1A/div
0A
0A
0V
V
MAIN
5V/div
10V
10V
V
V
MAIN
MAIN
500mA/div
200mV/div
10V OFFSET
10V OFFSET
2ms/div
100μs/div
10μs/div
SUP SUPPLY CURRENT
vs. SUP VOLTAGE
SUP SUPPLY CURRENT
vs. TEMPERATURE
SWITCHING WAVEFORMS
MAX8752 toc10
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
I
= 140mA
LOAD
NO LOAD
LX
5V/div
4.2
3.8
3.4
3.0
V
= 3.3V
IN
0V
V
= 1.8V
IN
V
= 1.8V
IN
V
= 5V
IN
INDUCTOR
CURRENT
500mA/div
V
= 5V
IN
V
= 3.3V
12
IN
I
= 300mA
LOAD
0A
4
6
8
10
14
-40
-20
0
20
40
60
80
1μs/div
SUP VOLTAGE (V)
TEMPERATURE (°C)
LDO OUTPUT VOLTAGE
vs. LDO CURRENT
LDO OUTPUT VOLTAGE
vs. TEMPERATURE
5.08
5.06
5.04
5.02
5.00
4.98
4.96
4.94
5.08
5.06
5.04
5.02
5.00
4.98
0
10
20
30
40
50
-40
-20
0
20
40
60
80
LDO CURRENT (mA)
TEMPERATURE (°C)
_______________________________________________________________________________________
5
TFT LCD Step-Up DC-DC Converter
Pin Description
PIN
NAME
FUNCTION
Compensation Pin for Error Amplifier. Connect a series resistance and capacitor from COMP to GND.
See the Loop Compensation section for component selection guidelines.
1
COMP
Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltage-
divider between the step-up regulator’s output (V
FB. Place the divider close to the IC and minimize the trace area to reduce noise coupling. Set V
) and GND, with the center tap connected to
MAIN
2
FB
MAIN
according to the Output Voltage Selection section.
3
4
SHDN
Shutdown Control Input. Drive SHDN low to turn off the MAX8752.
GND
Ground
Switching Node. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction
to LX and minimize the trace area for lower EMI.
5
6
7
LX
IN
Supply Pin. Connect IN to the input supply through a series 100Ω resistor and bypass it to GND with
0.1µF or greater ceramic capacitor.
Internal 5V Linear-Regulator Output. This regulator powers all internal circuitry. Bypass LDO to GND
with a 0.22µF or greater ceramic capacitor.
LDO
Linear-Regulator Supply Input. SUP is the supply input of the internal 5V linear regulator. Connect
SUP to the step-up regulator output and bypass SUP to GND with a 0.1µF capacitor.
8
SUP
—
BP
Backside Paddle. Connect the backside paddle to analog ground.
C11
V
GON
28V/10mA
D4
0.1μ
F
C13
C10
0.1μF
V
C9
GOFF
0.1μF
D2
D3
0.1μ
F
-9V/20mA
C12
0.1
C8
μ
F
0.1μF
V
V
IN
MAIN
L1
2.6μH
+1.8V TO +5.5V
+10V/240mA
D1
C2
C1
10μF
R1
90.9k
1%
10
μ
F
R4
16V
Ω
6.3V
LX
100Ω
IN
FB
C3
0.1μF
R2
13k
Ω
1%
R3
40.2k
MAX8752
Ω
COMP
LDO
GND
SUP
C6
20pF
C4
1.2nF
C14
C7
0.22μF
0.1μF
SHDN
Figure 1. Typical Applications Circuit
_______________________________________________________________________________________
6
TFT LCD Step-Up DC-DC Converter
current trip point each time the internal MOSFET turns
on. As the load changes, the error amplifier sources or
sinks current to the COMP output to set the inductor
peak current necessary to service the load. To maintain
stability at high duty cycles, a slope-compensation sig-
nal is summed with the current-sense signal.
CLOCK
LX
LOGIC
AND DRIVER
GND
CURRENT LIMIT
STARTUP
OSC
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 cur-
rent through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the cur-
rent-feedback signal and the slope compensation
exceed the COMP voltage, the controller resets the flip-
flop and turns off the MOSFET. Since the inductor cur-
rent is continuous, a transverse potential develops
across the inductor that turns on the diode (D1). The
voltage across the inductor then becomes the differ-
ence 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.
IN
SOFT-
START
I
LIMIT
SLOPE COMP
CURRENT
SENSE
∑
OSCILLATOR
PWM
COMPARATOR
SHDN
SUP
ERROR AMP
LINEAR
REGULATOR
AND
FB
1.24V
MAX8752
LDO
BOOTSTRAP
COMP
Figure 2. MAX8752 Functional Diagram
Detailed Description
At light loads, this architecture allows the MAX8752 to
“skip” cycles to prevent overcharging the output
capacitor voltage.
The MAX8752 is a highly efficient, step-up power sup-
ply designed for TFT-LCD panels. The typical circuit
shown in Figure 1 operates from an input voltage as
low as 1.8V, and produces a MAIN output of 10V at
220mA from 2.5V input while supporting discrete
diode-capacitor charge pumps that produce
-9V at 20mA and +28V at 10mA. If the charge-pump
outputs are not required, the diodes and capacitors
associated with them may be eliminated and the main
output increased to 270mA.
In this region of operation, the inductor ramps up to a
peak value of approximately 250mA, discharges to the
output, and waits until another pulse is needed.
Output-Current Capability
The output-current capability of the MAX8752 is a func-
tion of current limit, input voltage, operating frequency,
and inductor value. Because of the slope compensa-
tion used to stabilize the feedback loop, the inductor
current limit depends on the duty cycle. The current
limit is determined by the following equation:
The MAX8752 employs a current-mode, fixed-frequen-
cy, pulse-width modulation (PWM) architecture for fast
transient response and low-noise operation. 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 function reduce the number of external com-
ponents required. The output voltage can be set from
I
= (1.162 - 0.361 x D) x I
LIM_EC
LIM
where I
_
is the current limit specified at 65% duty
LIM EC
cycle (see the Electrical Characteristics) and D is the
duty cycle.
The output current capability depends on the current-
limit value and is governed by the following equation:
V
IN
to 13V with an external resistive voltage-divider.
The MAX8752 regulates the output voltage through a
combination of an error amplifier, two comparators, and
several signal generators (Figure 2). The error amplifier
compares the signal at FB to 1.24V and varies the
COMP output. The voltage at COMP determines the
⎡
⎤
⎥
⎦
0.5 x D V
V
V
OUT
IN
IN
I
= I
−
x
x η
⎢
⎣
OUT(MAX)
LIM
f
x L
OSC
_______________________________________________________________________________________
7
TFT LCD Step-Up DC-DC Converter
where I
is the current limit calculated above, η is the
LIM
Table 1. Component List
regulator efficiency (85% nominal), and D is the duty
cycle. The duty cycle when operating at the current
limit is:
DESIGNATION
DESCRIPTION
10µF 10%, 4V X5R ceramic capacitor
(0603)
TDK C1608X5R0G106K
Murata GRM188R60G106M
C1
V
− V + V
IN DIODE
OUT
D =
V
− I
× R
+ V
OUT
LIM
ON DIODE
10µF 10%, 16V X5R ceramic capacitor
(1206)
TDK C3216X5R1C106K
Murata GRM319R61A106K
C2
where V
ON
is the rectifier diode forward voltage and
DIODE
R
is the on-resistance of the internal MOSFET.
3A, 30V Schottky diode (M-flat)
Toshiba CRS02
D1
L1
Bootstrapping and Soft-Start
The MAX8752 features bootstrapping operation. In nor-
mal operation, the internal linear regulator supplies
power to the internal circuitry. The input of the linear
regulator (SUP) should be directly connected to the
output of the step-up regulator. After the input voltage
at SUP is above 1.75V, the regulator starts open-loop
switching to generate the supply voltage for the linear
regulator. The internal reference block turns on when
the LDO voltage exceeds 2.7V (typ).
2.6µH, 2.1A power inductor
3.3µH, 1.7A power inductor
Sumida CDRH6D12-3R3
Applications Information
Step-up regulators using the MAX8752 can be
designed by performing simple calculations for a first
iteration. All designs should be prototyped and tested
prior to production. Table 1 provides a list of power
components for the typical applications circuit. Table 2
lists component suppliers.
When the reference voltage reaches regulation, the
PWM controller and the current-limit circuit are enabled
and the step-up regulator enters soft-start. During the
soft-start, the main step-up regulator directly limits the
peak inductor current, allowing from zero up to the full
current limit in eight equal current steps. The maximum
load current is available after the output voltage reach-
es regulation (which terminates soft-start), or after the
soft-start timer expires (13ms typ). The soft-start routine
minimizes the inrush current and voltage overshoot and
ensures a well-defined startup behavior.
External component value choice is primarily dictated
by the output voltage and the maximum load current,
as well as maximum and minimum input voltages.
Begin by selecting an inductor value. Once the induc-
tor value and peak current are known, choose the
diode and capacitors.
Inductor Selection
The minimum inductance value, peak current rating,
and series resistance are factors to consider when
selecting the inductor. These factors influence the con-
verter’s efficiency, maximum output load capability,
transient response time, and output voltage ripple.
Physical size and cost are also important factors to
consider.
Shutdown
The MAX8752 shuts down to reduce the supply current
to 0.1µA when SHDN is low. In this mode, the internal ref-
erence, error amplifier, comparators, and biasing circuit-
ry turn off and the n-channel MOSFET is turned off. In
shutdown, the step-up regulator’s output is connected to
IN through the external inductor and rectifier diode.
Table 2. Component Suppliers
SUPPLIER
PHONE
FAX
WEBSITE
www.murata.com
Murata
Sumida
TDK
770-436-1300
847-545-6700
847-803-6100
949-455-2000
770-436-3030
847-545-6720
847-803-6296
949-859-3963
www.sumida.com
www.component.tdk.com
www.toshiba.com/taec
Toshiba
8
_______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-
rent ripple and therefore reduce the peak current,
which decreases core losses in the inductor and I2R
losses in the entire power path. However, large induc-
tor 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 val-
ues 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.
I
is the positive charge-pump output current,
POS
assuming the pump source for I
is V
MAIN.
POS
Calculate the approximate inductor value using the typ-
ical input voltage (V ), the maximum output current
IN
(I
), the expected efficiency (η
) taken from
MAIN(MAX)
TYP
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
2
⎛
⎜
⎞
⎟
⎛
⎞
V
V
− V
× f
η
TYP
LIR
⎛
⎞
IN
MAIN
IN
L =
⎜
⎝
⎟
⎠
⎜
⎟
V
I
⎝
⎠
MAIN
⎝ MAIN(MAX)
OSC ⎠
The equations used here include a constant, LIR, which
is the ratio of the inductor peak-to-peak ripple current
to the average DC inductor current at the full-load cur-
rent. The best trade-off between inductor size and cir-
cuit 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 resis-
tances, the best LIR can shift up or down. If the induc-
tor 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.
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage V
using con-
IN(MIN)
servation of energy and the expected efficiency at that
operating point (η ) taken from an appropriate curve
MIN
in the Typical Operating Characteristics:
I
× V
MAIN
MAIN(MAX)
I
=
IN(DC,MAX)
V
× η
MIN
IN(MIN)
Calculate the ripple current at that operating point and
the peak current required for the inductor:
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficien-
cy improvements in typical operating regions.
V
× (V
− V
)
IN(MIN)
MAIN
IN(MIN)
× f
OSC
I
I
=
RIPPLE
L × V
MAIN
In Figure 1, the LCD’s gate-on and gate-off voltages
are generated from two unregulated charge pumps dri-
ven by the step-up regulator’s LX node. The additional
load on LX must therefore be considered in the induc-
tance calculation. The effective maximum output cur-
RIPPLE
I
= I
+
PEAK
IN(DC,MAX)
2
The inductor’s saturation current rating and the
MAX8752’s LX current limit (I ) should exceed I
rent I
becomes the sum of the maximum load
MAIN(EFF)
LIM
PEAK
current on the step-up regulator’s output plus the con-
tributions from the positive and negative charge
pumps:
and the inductor’s DC current rating should exceed
I
. For good efficiency, choose an inductor
IN(DC,MAX)
with less than 0.1Ω series resistance.
Considering the Typical Applications Circuit (Figure 1),
I
= I
I
+ η
x I
+ (η
+ 1) x
MAIN(EFF)
MAIN(MAX)
NEG NEG
POS
the maximum load current (I
) is 180mA with a
MAIN(MAX)
10V output and a typical input voltage of 2.5V:
POS
I
= 180mA + 1 x 20mA + 3 x 10mA = 230mA
MAIN(EFF)
where I
is the maximum main output current,
MAIN(MAX)
n
n
is the number of negative charge-pump stages,
is the number of positive charge-pump stages,
is the negative charge-pump output current, and
NEG
POS
NEG
I
_______________________________________________________________________________________
9
TFT LCD Step-Up DC-DC Converter
Choosing an LIR of 0.5 and estimating efficiency of
80% at this operating point:
Input Capacitor Selection
The input capacitor (C ) reduces the current peaks
IN
drawn from the input supply and reduces noise injec-
tion into the IC. A 10µF ceramic capacitor is used in the
Typical 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.
2
⎛
⎞
⎛
⎞ ⎛
⎞
2.5V
10V
10V − 2.5V
0.80
0.50
L =
≈ 2.6μH
⎜
⎟
⎜
⎟ ⎜
⎟
0.23A × 1.2MHz
⎝
⎠
⎝
⎠ ⎝
⎠
Using the circuit’s minimum input voltage (2.2V) and
estimating efficiency of 75% at that operating point:
Typically, C can be reduced below the values used in
IN
0.23A × 10V
the Typical Applications Circuit. Ensure a low noise
I
=
≈ 1.4A
IN(DC,MAX)
2.2V × 0.75
supply at IN by using adequate C . Alternatively,
IN
greater voltage variation can be tolerated on C if IN is
IN
The ripple current and the peak current are:
decoupled from C using an RC lowpass filter (see R3
IN
and C3 in Figure 1).
2.2V × (10V − 2.2V)
I
=
≈ 0.55A
RIPPLE
Rectifier Diode Selection
The MAX8752’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. The diode should be
rated to handle the output voltage and the peak switch
current. Make sure that the diode’s peak current rating
2.6μH × 10V × 1.2MHz
0.55A
I
= 1.4A +
≈ 1.7A
PEAK
2
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):
is at least I
calculated in the Inductor Selection
PEAK
section and that its breakdown voltage exceeds the
output voltage.
Output Voltage Selection
The MAX8752 operates with an adjustable output from
V
= V
+ V
RIPPLE
RIPPLE(C) RIPPLE(ESR)
V
to 13V. Connect a resistive voltage-divider from the
IN
output (V
) to GND with the center tap connected to
MAIN
FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ range.
Calculate R1 with the following equation:
⎛
⎞
I
C
V
V
− V
MAIN
MAIN IN
V
≈
, and
RIPPLE(C)
⎜
⎟
f
⎝
⎠
OUT
MAIN OSC
⎛
⎞
V
V
MAIN
R1 = R2 ×
− 1
⎟
⎜
⎝
⎠
FB
V
≈ I
R
RIPPLE(ESR)
PEAK ESR(COUT)
where V , the step-up regulator’s feedback set point,
FB
is 1.24V (typ). Place R1 and R2 close to the IC.
where I
is the peak inductor current (see the
PEAK
Inductor Selection section). For ceramic capacitors, the
output voltage ripple is typically dominated by
V
. The voltage rating and temperature charac-
RIPPLE(C)
teristics of the output capacitor must also be considered.
10 ______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
capacitor and input-capacitor ground terminals.
Connect these loop components with short, wide
connections. Avoid using vias in the high-current
paths, especially the ground paths. If vias are
unavoidable, use many vias in parallel to reduce
resistance and inductance.
Loop Compensation
The voltage-feedback loop needs proper compensa-
tion to prevent excessive output ripple and poor effi-
ciency caused by instability. This is done by
connecting a resistor (R
) and capacitor (C
)
COMP
COMP
in series from COMP to GND, and another capacitor
(C ) from COMP to GND. R is chosen to set
COMP2
COMP
2) Create a power ground island (PGND) consisting of
the input and output capacitor grounds and GND.
Connect all of 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 feedback divider’s ground, the COMP capac-
itor’s ground, and the IC’s exposed backside pad
near pin 1. Connect the AGND and PGND islands
by connecting the GND pin directly to the exposed
backside pad. Make no other connections between
these separate ground planes.
the high-frequency integrator gain for fast transient
response, while C is chosen to set the integrator
COMP
zero to maintain loop stability. The second capacitor,
, is chosen to cancel the zero introduced by
C
COMP2
output-capacitance ESR. For optimal performance,
choose the components using the following equations:
264 × V × V
× C
OUT
IN
OUT
R
≈
COMP
L × I
MAIN(EFF)
V
× C
OUT
OUT
C
≈
COMP
10 × I
× R
MAIN(MAX)
COMP
3) Place the feedback voltage-divider resistors as
close to FB as possible. 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. Avoid running the feed-
back trace near LX.
0.02 × R
× L × I
MAIN(EFF)
ESR
C
≈
COMP2
V
× V
OUT
IN
For the ceramic output capacitor, where ESR is small,
is optional. The best gauge of correct loop
4) Place the SUP and LDO bypass capacitors and the
IN bypass capacitors (C3 in Figure 1) if within 5mm
of their respective pins. Connect their ground termi-
nals to GND through the IC’s exposed back paddle
near GND (pin4).
C
COMP2
compensation is by inspecting the transient response
of the MAX8752. Adjust R and C as neces-
COMP
COMP
sary to obtain optimal transient performance.
PC Board Layout and Grounding
Careful PC board layout is important for proper opera-
tion. Use the following guidelines for good PC board
layout:
5) Minimize the length and maximize the width of the
traces between the output capacitors and the load
for best transient responses.
6) Minimize the size of the LX node while keeping it
wide and short. Keep the LX node away from the
feedback node and other sensitive nodes. Use DC
traces as shield if necessary.
1) Minimize the area of high-current loops by placing
the inductor, rectifier diode, and output capacitors
near the input capacitors and near the LX and GND
pins. The high-current input loop goes from the
positive terminal of the input capacitor to the induc-
tor, to the IC’s LX pin, out the IC’s GND pin, 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 rectifier
diode (D1), to the positive terminal of the output
capacitors, reconnecting between the output-
Refer to the MAX8752 evaluation kit for an example of
proper board layout.
Chip Information
TRANSISTOR COUNT: 3091
PROCESS: BiCMOS
______________________________________________________________________________________ 11
TFT LCD Step-Up DC-DC Converter
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.)
D2
D
A2
PIN 1 ID
N
0.35x0.35
b
[(N/2)-1] x e
REF.
PIN 1
INDEX
AREA
E
E2
DETAIL A
e
A1
k
C
C
L
L
A
L
L
e
e
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
1
-DRAWING NOT TO SCALE-
21-0137
G
2
COMMON DIMENSIONS
SYMBOL
MIN.
0.70
2.90
2.90
0.00
0.20
MAX.
0.80
3.10
3.10
0.05
0.40
A
D
E
A1
L
k
0.25 MIN.
0.20 REF.
A2
PACKAGE VARIATIONS
DOWNBONDS
ALLOWED
PKG. CODE
T633-1
N
6
D2
E2
e
JEDEC SPEC
b
[(N/2)-1] x e
1.90 REF
1.90 REF
1.95 REF
1.95 REF
1.95 REF
2.00 REF
2.40 REF
2.40 REF
1.50±0.10 2.30±0.10 0.95 BSC
1.50±0.10 2.30±0.10 0.95 BSC
1.50±0.10 2.30±0.10 0.65 BSC
1.50±0.10 2.30±0.10 0.65 BSC
1.50±0.10 2.30±0.10 0.65 BSC
MO229 / WEEA
MO229 / WEEA
MO229 / WEEC
MO229 / WEEC
MO229 / WEEC
0.40±0.05
0.40±0.05
0.30±0.05
0.30±0.05
0.30±0.05
NO
NO
T633-2
6
T833-1
8
NO
T833-2
8
NO
T833-3
8
YES
NO
T1033-1
T1433-1
T1433-2
10
14
14
1.50±0.10 2.30±0.10 0.50 BSC MO229 / WEED-3 0.25±0.05
1.70±0.10 2.30±0.10 0.40 BSC
1.70±0.10 2.30±0.10 0.40 BSC
- - - -
- - - -
0.20±0.05
0.20±0.05
YES
NO
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
2
-DRAWING NOT TO SCALE-
21-0137
G
2
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2005 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.
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