MAXIM MAX8740ETB

19-3698; Rev 0; 5/05
KIT
ATION
EVALU
LE
B
A
IL
A
AV
TFT-LCD Step-Up DC-DC Converter
The MAX8740 is a high-performance, step-up DC-DC
converter that provides a regulated supply voltage for
active-matrix, thin-film transistor (TFT), liquid-crystal
displays (LCDs). The MAX8740 incorporates currentmode, fixed-frequency, pulse-width modulation (PWM)
circuitry with a built-in n-channel power MOSFET to
achieve high efficiency and fast transient response.
Users can select 640kHz or 1.2MHz operation using a
logic input pin (FREQ). The high switching frequencies
allow the use of ultra-small inductors and low-ESR
ceramic capacitors. The current-mode architecture provides fast transient response to pulsed loads. A compensation pin (COMP) gives users flexibility in adjusting
loop dynamics. The 30V internal MOSFET can generate
output voltages up to 28V from a 2.6V and 5.5V input
voltage range. Soft-start slowly ramps the input current
and is programmed with an external capacitor.
The MAX8740 is available in a 10-pin thin DFN package.
Features
♦ 90% Efficiency
♦ Adjustable Output from VIN to 28V
♦ 2.6V to 5.5V Input Supply Range
♦ Input Supply Undervoltage Lockout
♦ Pin-Programmable 640kHz/1.2MHz Switching
Frequency
♦ Programmable Soft-Start
♦ 0.1µA Shutdown Current
♦ Small, 10-Pin Thin DFN Package
Applications
Notebook Computer Displays
Ordering Information
PART
LCD Monitor Panels
MAX8740ETB
Pin Configuration
TEMP RANGE
PIN-PACKAGE
-40°C to +85°C
10 TDFN 3mm x 3mm
Minimal Operating Circuit
VIN
2.6V TO 5.5V
SS
FREQ
IN
LX
LX
TOP VIEW
10
9
8
7
6
VOUT
6
LX
8
7
LX
FB
IN
2
MAX8740
9
MAX8740
4
5
GND
FB
3
GND
2
SHDN
1
COMP
3
10
FREQ
GND 5
SHDN
GND
SS
4
COMP 1
THIN DFN
3mm x 3mm
________________________________________________________________ Maxim Integrated Products
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.
1
MAX8740
General Description
MAX8740
TFT-LCD Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
LX to GND ..............................................................-0.3V to +30V
IN, SHDN, FREQ, FB to GND ...................................-0.3V to +6V
COMP, SS to GND .......................................-0.3V to (VIN + 0.3V)
LX Switch Maximum Continuous RMS Current .....................2.4A
Continuous Power Dissipation (TA = +70°C)
10-Pin TDFN (derate 24.1mW/°C above +70°C) .......1481.5mW
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
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 = V SHDN = 3V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
Input Voltage Range
CONDITIONS
MIN
TYP
2.6
5.5
18V < VOUT < 24V
4.0
5.5
Output Voltage Range
IN Undervoltage-Lockout
Threshold
IN Quiescent Current
IN Shutdown Current
MAX
VOUT < 18V
VIN rising, typical hysteresis is 50mV; LX remains off
below this level
UNITS
V
28
V
2.38
2.57
V
0.22
0.44
2
5
0.1
10.0
1.22
1.24
1.26
V
50
125
250
nA
0.05
0.15
%/V
200
315
2.20
VFB = 1.3V, not switching
VFB = 1.0V, switching, FREQ = GND
SHDN = GND
mA
µA
ERROR AMPLIFIER
FB Regulation Voltage
Level to produce VCOMP = 1.24V
FB Input Bias Current
VFB = 1.24V
FB Line Regulation
Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V
Transconductance
100
Voltage Gain
2400
µS
V/V
OSCILLATOR
Frequency
FREQ = GND
540
640
740
FREQ = IN
1000
1220
1500
88
91
94
%
3.9
4.6
5.3
A
VIN = 3V (typ value at TA = +25°C)
0.11
0.17
VIN = 5V (typ value at TA = +25°C)
0.095
0.15
Maximum Duty Cycle
kHz
n-CHANNEL MOSFET
Current Limit
On-Resistance
Leakage Current
VFB = 1V, 71% duty cycle
VLX = 28V
Current-Sense Transresistance
Ω
30
55
µA
0.09
0.15
0.25
V/A
2.5
4.5
SOFT-START
Reset Switch Resistance
Charge Current
2
VSS = 1.2V
_______________________________________________________________________________________
100
Ω
7.5
µA
TFT-LCD Step-Up DC-DC Converter
(VIN = V SHDN = 3V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.3 x
VIN
V
CONTROL INPUTS
SHDN, FREQ Input Low Voltage
VIN = 2.6V to 5.5V
SHDN, FREQ Input High Voltage
VIN = 2.6V to 5.5V
SHDN, FREQ Input Hysteresis
VIN = 2.6V to 5.5V
0.7 x
VIN
0.1 x
VIN
FREQ Pulldown Current
SHDN Input Current
V
2.3
SHDN = GND
V
6.0
9.5
µA
0.001
1
µA
TYP
MAX
UNITS
ELECTRICAL CHARACTERISTICS
(VIN = V SHDN = 3V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETER
Input Voltage Range
CONDITIONS
MIN
VOUT < 18V
2.6
5.5
18V < VOUT < 28V
4.0
5.5
Output Voltage Range
IN Quiescent Current
IN Shutdown Current
28
VFB = 1.3V, not switching
0.44
VFB = 1.0V, switching, FREQ = GND
5
SHDN = GND
10
V
V
mA
µA
ERROR AMPLIFIER
FB Regulation Voltage
Level to produce VCOMP = 1.24V
FB Line Regulation
Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V
1.260
V
0.15
%/V
100
330
µS
FREQ = GND
490
770
FREQ = IN
900
1600
VFB = 1V, 71% duty cycle
3.9
5.3
A
0.09
0.25
V/A
0.3 x
VIN
V
Transconductance
1.215
OSCILLATOR
Frequency
kHz
n-CHANNEL MOSFET
Current Limit
Current-Sense Transresistance
CONTROL INPUTS
SHDN, FREQ Input Low Voltage
VIN = 2.6V to 5.5V
SHDN, FREQ Input High Voltage
VIN = 2.6V to 5.5V
0.7 x
VIN
V
Note 1: -40°C specifications are guaranteed by design, not production tested.
_______________________________________________________________________________________
3
MAX8740
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(1.2MHz OPERATION)
70
60
50
VIN = 3.3V
60
1
10
10
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
SUPPLY CURRENT vs. SUPPLY VOLTAGE
0.6
800
FREQ = GND
600
11.9
fOSC = 1.2MHz
L = 2.7µH
SWITCHING
0.5
0.4
NONSWITCHING
0.3
10
100
1000
LOAD CURRENT (mA)
10,000
SUPPLY CURRENT vs. TEMPERATURE
(SWITCHING)
0.60
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
1000
VIN = 3.3V
1
MAX8740 toc05
0.7
MAX8740 toc04
FREQ = IN
12.1
1000
100
LOAD CURRENT (mA)
1200
VIN = 5.0V
12.3
11.5
1
LOAD CURRENT (mA)
1400
12.5
11.7
40
1000
100
MAX8740 toc03
12.7
50
40
VIN = 5.0V
0.55
VIN = 3.3V
0.50
0.2
400
0.1
2.5
3.0
3.5
4.0
4.5
INPUT VOLTAGE (V)
5.0
5.5
SOFT-START
(RLOAD = 30Ω)
2ms/div
4
80
70
12.9
MAX8740 toc06
VIN = 3.3V
VIN = 5.0V
OUTPUT VOLTAGE (V)
80
L = 5.6µH
fOSC = 640kHz
90
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 5.0V
MAX8740 toc02
L = 2.7µH
fOSC = 1.2MHz
90
OUTPUT VOLTAGE vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
100
MAX8740 toc01
100
SWITCHING FREQUENCY (kHz)
MAX8740
TFT-LCD Step-Up DC-DC Converter
0.45
2.5
3.0
MAX8740 toc07
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
-40
-20
0
20
40
60
TEMPERATURE (°C)
SWITCHING WAVEFORMS
(ILOAD = 800mA)
MAX8740 toc08
400ns/div
_______________________________________________________________________________________
80
100
TFT-LCD Step-Up DC-DC Converter
PIN
NAME
1
COMP
2
FB
FUNCTION
Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop
Compensation section for component selection guidelines.
Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltage-divider
between the step-up regulator’s output (VOUT) and GND, with the center tap connected to FB. Place the
divider close to the IC and minimize the trace area to reduce noise coupling. Set VOUT according to the
Output Voltage Selection section.
3
SHDN
Shutdown Control Input. Drive SHDN low to turn off the MAX8740.
4, 5
GND
Ground. Connect pins 4 and 5 directly together.
6, 7
LX
Switch Pin. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and
minimize the trace area for lower EMI. Connect pins 6 and 7 directly together.
8
IN
Supply Pin. Bypass IN with a minimum 1µF ceramic capacitor directly to GND.
9
FREQ
Frequency-Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high, the
frequency is 1.2MHz. This input has a 5µA pulldown current.
SS
Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The softstart capacitor is charged with a constant current of 4.5µA. Full current limit is reached after t = 2.5 x 105 CSS.
The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start
capacitor is charged to 0.4V, after which soft-start begins.
10
L1
2.7µH
VIN
4.5V TO 5.5V
C1
10µF
6.3V
R3
10Ω
D1
6
LX
8
C3
1µF
R1
196kΩ
1%
7
LX
FB
IN
3
10
C6
33nF
VOUT
13.5V/800mA
R2
20kΩ
1%
FREQ
GND 5
SHDN
GND
SS
C7
10µF
20V
2
MAX8740
9
C2
10µF
20V
4
COMP 1
R4
47kΩ
1%
C5
68pF
C4
560pF
Figure 1. Typical Operating Circuit
_______________________________________________________________________________________
5
MAX8740
Pin Description
MAX8740
TFT-LCD Step-Up DC-DC Converter
SKIP
COMPARATOR
SHDN
4µA
IN
BIAS
SOFTSTART
SKIP
COMP
ERROR
AMPLIFIER
SS
ERROR
COMPARATOR
FB
∞
LX
CONTROL
AND DRIVER
LOGIC
1.24V
N
CLOCK
GND
OSCILLATOR
FREQ
SLOPE
COMPENSATION
Σ
CURRENT
SENSE
5µA
MAX8740
Figure 2. Functional Diagram
Detailed Description
The MAX8740 is a highly efficient power supply that
employs a current-mode, fixed-frequency, PWM architecture for fast transient response and low-noise operation. The device 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
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 command 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.
At light loads, this architecture allows the MAX8740 to
“skip” cycles to prevent overcharging the output voltage.
In this region of operation, the inductor ramps up to a
peak value of approximately 150mA, discharges to the
output, and waits until another pulse is needed again.
Output Current Capability
The output current capability of the MAX8740 is a function of current limit, input voltage, operating frequency,
and inductor value. Because of the slope compensation used to stabilize the feedback loop, the inductor
6
current limit depends on the duty cycle. The current
limit is determined by the following equation:
ILIM = (1.26 - 0.35 x D) x ILIM_EC
where ILIM_EC is the current limit specified at 71% duty
cycle (see the Electrical Characteristics table) and D is
the duty cycle.
The output current capability depends on the currentlimit value and is governed by the following equation:

0.5 × D × VIN 
VIN
IOUT(MAX) = ILIM −
× η
 ×
fOSC × L 
VOUT

where ILIM is the current limit calculated above, η is the
regulator efficiency (85% nominal), and D is the duty
cycle. The duty cycle when operating at the current
limit is:
D =
VOUT − VIN + VDIODE
VOUT − ILIM × RON + VDIODE
where VDIODE is the rectifier diode forward voltage and
RON is the on-resistance of the internal MOSFET.
_______________________________________________________________________________________
TFT-LCD Step-Up DC-DC Converter
Shutdown
The MAX8740 shuts down to reduce the supply current
to 0.1µA when SHDN is low. In this mode, the internal
reference, error amplifier, comparators, and biasing circuitry turn off, and the n-channel MOSFET is turned off.
The step-up regulator’s output is connected to IN by
the external inductor and rectifier diode.
Frequency Selection
The MAX8740’s frequency can be user selected to operate at either 640kHz or 1.2MHz. Connect FREQ to GND
for 640kHz operation. For a 1.2MHz switching frequency,
connect FREQ to IN. This allows the use of small, minimum-height external components while maintaining low
output noise. FREQ has an internal pulldown, allowing
the user the option of leaving FREQ unconnected for
640kHz operation.
Step-up regulators using the MAX8740 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.
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 L is known,
choose the diode and capacitors.
Inductor Selection
Table 1. Component List
DESIGNATION
Applications Information
DESCRIPTION
C1
10µF ±10%, 6.3V X5R ceramic capacitor
(0805)
Murata GRM21BR60J106K
Taiyo Yuden JMK212BJ106KD
C2, C7
10µF ±20%, 25V X5R ceramic capacitors
(1210)
TDK C3225X5R1E106M,
Taiyo Yuden TMK325BJ106MM
D1
3A, 40V Schottky diode (SM8)
Central Semiconductor CMSH3-40M
L1
3.3µH ±30%, 4.0A power inductor
Sumida CDRH8D28-3R3, 3.3µH
(alternate : Sumida CDRH103R-3R3, 3.3µH)
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, transientresponse 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 I 2R
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.
Table 2. Component Suppliers
PHONE
FAX
Murata
SUPPLIER
770-436-1300
770-436-3030
www.murata.com
WEBSITE
Sumida
847-545-6700
847-545-6720
www.sumida.com
Taiyo Yuden
800-348-2496
847-925-0899
www.t-yuden.com
TDK
847-803-6100
847-390-4405
www.component.tdk.com
Toshiba
949-455-2000
949-859-3963
www.toshiba.com/taec
_______________________________________________________________________________________
7
MAX8740
Soft-Start
The MAX8740 can be programmed for soft-start upon
power-up with an external capacitor. When the shutdown
pin is taken high, the soft-start capacitor (CSS) is immediately charged to 0.4V. Then the capacitor is charged at a
constant current of 4.5µA (typ). During this time, the SS
voltage directly controls the peak inductor current, allowing 0A at VSS = 0.4V to the full current limit at VSS = 1.5V.
The maximum load current is available after the soft-start
is completed. When the SHDN pin is taken low, the softstart capacitor is discharged to ground.
MAX8740
TFT-LCD Step-Up DC-DC Converter
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 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 the
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.
Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current
(IOUT(MAX)), 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
 η
 V  
VOUT − VIN
TYP 
L =  IN  

 
 VOUT   IOUT(MAX) × 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(DC, MAX) =
VIN(MIN) × ηMIN
VIN(MIN) × (VOUT − VIN(MIN) )
L × VOUT × fOSC
I
IPEAK = IIN(DC, MAX) + RIPPLE
2
8
Considering the typical operating circuit, the maximum
load current (IOUT(MAX)) is 900mA with a 13.5V output
and a 5V typical input voltage. Choosing an LIR of 0.35
and estimating efficiency of 85% at this operating point:
2
 5V   13.5V − 5V   0.85 
L = 
 
 
 ≈ 2.7µH
 13.5V   0.9A × 1.2MHz   0.35 
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 85% at that operating point:
IIN(DC, MAX) =
0.9A × 3.5V
≈ 3.2A
4.5V × 0.85
The ripple current and the peak current are:
IRIPPLE =
4.5V × (12.5V − 4.5V)
≈ 0.93A
2.7µH × 13.5V × 1.2MHz
IPEAK = 3.2A +
0.93A
≈ 3.7A
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)
IOUT(MAX) × VOUT
Calculate the ripple current at that operating point and
the peak current required for the inductor:
IRIPPLE =
The inductor’s saturation current rating and the
MAX8740’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.
VRIPPLE(C) ≈
IOUT
COUT
 VOUT − VIN 
 V
, and
 OUT fOSC 
VRIPPLE(ESR) ≈ IPEAK RESR(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.
_______________________________________________________________________________________
TFT-LCD Step-Up DC-DC Converter
sen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations:
RCOMP ≈
CCOMP ≈
CCOMP2 ≈
315 × VIN × VOUT × COUT
L × IOUT(MAX)
VOUT × COUT
10 × IOUT(MAX) × RCOMP
0.0036 × RESR × L × IOUT(MAX)
VIN × VOUT
Rectifier Diode Selection
The MAX8740’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. The diode should be
rated to handle the output voltage and the peak switch
current. Make sure that the diode’s peak current rating
is at least IPEAK calculated in the Inductor Selection
section and that its breakdown voltage exceeds the
output voltage.
The soft-start capacitor should be large enough that it
does not reach final value before the output has
reached regulation. Calculate CSS to be:
Output Voltage Selection
The MAX8740 operates with an adjustable output from
VIN to 28V. Connect a resistive voltage-divider from the
output (VOUT) to GND with the center tap connected to
FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ range.
Calculate R1 with the following equation:
CSS > 21 × 10 −6 × COUT ×


2
VOUT − VIN × VOUT


 V × I

−
I
×
V
INRUSH
OUT
OUT 
 IN
V

R1 = R2 ×  OUT − 1
 VFB

where COUT is the total output capacitance including
any bypass capacitor on the output bus, VOUT is the
maximum output voltage, IINRUSH is the peak inrush
current allowed, IOUT is the maximum output current
during power-up, and VIN is the minimum input voltage.
where VFB, the step-up regulator’s feedback set point,
is 1.28V (typ). Place R1 and R2 close to the IC.
Loop Compensation
The voltage feedback loop needs proper compensation
to prevent excessive output ripple and poor efficiency
caused by instability. This is done by connecting a
resistor (RCOMP) and capacitor (CCOMP) in series from
COMP to GND, and another capacitor (CCOMP2) from
COMP to GND. RCOMP is chosen to set the high-frequency integrator gain for fast transient response, while
CCOMP is chosen to set the integrator zero to maintain
loop stability. The second capacitor, CCOMP2, is cho-
For the ceramic output capacitor, where ESR is small,
CCOMP2 is optional. The best gauge of correct loop
compensation is by inspecting the transient response
of the MAX8740. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance.
Soft-Start Capacitor
The load must wait for the soft-start cycle to finish
before drawing a significant amount of load current.
The duration after which the load can begin to draw
maximum load current is:
tMAX = 6.77 x 105 x CSS
_______________________________________________________________________________________
9
MAX8740
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 the typical operating 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. Typically, CIN
can be reduced below the values used in the typical
operating circuit. 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 (see R3 and C3 in Figure 1).
MAX8740
TFT-LCD Step-Up DC-DC Converter
D2
V3
-10V
D3
C7
C8
0.1µF 0.1µF
C10
0.22µF
V2
+28V C9
1µF
L1
2.7µH
VIN
4.5V TO 5.5V
C1
10µF
6.3V
R4
10Ω
D1
6
LX
8
C5
1µF
R1
196kΩ
1%
7
LX
FB
IN
3
10
VOUT
13.5V/800mA
R2
20kΩ
1%
FREQ
GND 5
SHDN
GND
SS
C7
10µF
25V
2
MAX8740
9
C2
10µF
25V
4
COMP 1
C4
33nF
R3
47kΩ
1%
C6
68pF
C3
560pF
Figure 3. Multiple-Output TFT-LCD Power Supply
Multiple-Output Power Supply for TFT LCD
Figure 3 shows a power supply for active-matrix TFTLCD flat-panel displays. Output-voltage transient performance is a function of the load characteristic. Add or
remove output capacitance (and recalculate compensation-network component values) as necessary to meet
the required transient performance. Regulation performance for secondary outputs (V2 and V3) depends on
the load characteristics of all three outputs.
PC Board Layout and Grounding
Careful PC board layout is important for proper operation.
Use the following guidelines for good PC board layout:
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 inductor, to the IC’s LX pin, out of GND, 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),
10
and 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.
2) Create a power ground island (PGND) consisting of
the input and output capacitor grounds and GND
pins. 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 ground connection, the COMP and SS capacitor ground connections, and the device’s exposed backside pad.
Connect the AGND and PGND islands by connecting the GND pins directly to the exposed backside
pad. Make no other connections between these
separate ground planes.
______________________________________________________________________________________
TFT-LCD Step-Up DC-DC Converter
4) Place the IN pin bypass capacitor as close to the
device as possible. The ground connection of the
IN bypass capacitor should be connected directly
to GND pins with a wide trace.
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 analog ground. Use DC traces
as a shield if necessary.
Refer to the MAX8740 evaluation kit for an example of
proper board layout.
Chip Information
TRANSISTOR COUNT: 2746
PROCESS: BiCMOS
______________________________________________________________________________________
11
MAX8740
3) Place the feedback voltage-divider-resistors as
close to the FB pin 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
feedback trace near LX.
TFT-LCD Step-Up DC-DC Converter
6, 8, &10L, DFN THIN.EPS
D2
D
A2
PIN 1 ID
N
0.35x0.35
b
PIN 1
INDEX
AREA
E
[(N/2)-1] x e
REF.
E2
DETAIL A
e
k
A1
CL
A
CL
L
L
e
e
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
1
2
COMMON DIMENSIONS
MIN.
MAX.
D
0.70
2.90
0.80
3.10
E
A1
2.90
0.00
3.10
0.05
L
k
0.20
0.40
0.25 MIN.
A2
0.20 REF.
SYMBOL
A
PACKAGE VARIATIONS
PKG. CODE
N
D2
E2
e
JEDEC SPEC
b
[(N/2)-1] x e
DOWNBONDS
ALLOWED
T633-1
6
1.50±0.10
2.30±0.10
0.95 BSC
MO229 / WEEA
0.40±0.05
1.90 REF
NO
T633-2
6
1.50±0.10
2.30±0.10
0.95 BSC
MO229 / WEEA
0.40±0.05
1.90 REF
NO
T833-1
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
NO
T833-2
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
NO
T833-3
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
YES
T1033-1
10
1.50±0.10
2.30±0.10
0.50 BSC
MO229 / WEED-3
0.25±0.05
2.00 REF
NO
T1433-1
14
1.70±0.10
2.30±0.10
0.40 BSC
----
0.20±0.05
2.40 REF
YES
T1433-2
14
1.70±0.10
2.30±0.10
0.40 BSC
----
0.20±0.05
2.40 REF
NO
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
2
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 12
© 2005 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.
MAX8740
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.)