General Description
The MAX8727 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 MAX8727 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 24V from an input voltage
between 2.6V and 5.5V. Soft-start slowly ramps the input
current and is programmed with an external capacitor.
The MAX8727 is available in a 10-pin thin DFN package.
Applications
Notebook Computer Displays
LCD Monitor Panels
Features
♦ 90% Efficiency
♦ Adjustable Output from V
IN
to 24V
♦ 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
MAX8727
TFT-LCD Step-Up DC-DC Converter
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
19-3480; Rev 0; 10/04
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.
EVALUATION KIT
AVAILABLE
Pin Configuration
LX LX
FB
GND
GND
FREQ
IN
COMP
SS
1
4
5
2
3
9
8
67
10
V
OUT
V
IN
2.6V TO 5.5V
SHDN
MAX8727
Minimal Operating Circuit
PART TEMP RANGE PIN-PACKAGE
MAX8727ETB -40°C to +85°C 10 Thin DFN 3mm x 3mm
TOP VIEW
COMP 1
FB 2
3
SHDN
GND 4
GND 5
MAX8727
THIN DFN
3mm x 3mm
10
SS
FREQ
9
IN
8
LX
7
LX
6
MAX8727
TFT-LCD Step-Up DC-DC Converter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= V
SHDN
= 3V, FREQ = GND, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
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.
LX to GND ..............................................................-0.3V to +26V
IN, SHDN, FREQ, FB to GND ...................................-0.3V to +6V
COMP, SS to GND .........................................-0.3V to V
IN
+ 0.3V
LX Switch Maximum Continuous RMS Current .....................2.4A
Continuous Power Dissipation (T
A
= +70°C)
10-Pin Thin DFN (derate 24.4mW/°C above +70°C) ....1951mW
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
Input Voltage Range
Output Voltage Range 24 V
IN Undervoltage-Lockout
Threshold
IN Quiescent Current
IN Shutdown Current SHDN = GND 0.1 10.0 µA
ERROR AMPLIFIER
FB Regulation Voltage Level to produce V
FB Input Bias Current VFB = 1.24V 50 125 250 nA
FB Line Regulation Level to produce V
Transconductance 100 200 300 µS
Voltage Gain 700 V/V
Shutdown FB Input Voltage SHDN = GND 0.05 0.10 0.15 V
OSCILLATOR
Frequency
Maximum Duty Cycle 87 90 93 %
n-CHANNEL MOSFET
Current Limit VFB = 1V, 75% duty cycle 3.0 3.8 4.6 A
On-Resistance 125 250 Ω
Leakage Current VLX = 24V 30 45 µA
Current-Sense Transresistance 0.11 0.21 0.31 V/A
SOFT-START
Reset Switch Resistance 100 Ω
Charge Current VSS = 1.2V 2.5 4.5 7.5 µA
PARAMETER CONDITIONS MIN TYP MAX UNITS
V
< 18V 2.6 5.5
OUT
18V < V
rising, typical hysteresis is 50mV 2.20 2.38 2.57 V
V
IN
VFB = 1.3V, not switching 0.225 0.440
= 1.0V, switching 2 5
V
FB
FREQ = GND 540 640 740
FREQ = IN 1000 1220 1500
< 24V 4.0 5.5
OUT
= 1.24V 1.22 1.24 1.26 V
COMP
= 1.24V, VIN = 2.6V to 5.5V 0.05 0.15 %/V
COMP
V
mA
kHz
MAX8727
TFT-LCD Step-Up DC-DC Converter
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN= V
SHDN
= 3V, FREQ = GND, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)
ELECTRICAL CHARACTERISTICS
(VIN= V
SHDN
= 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)
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
FREQ Pulldown Current 2.3 6.0 9.5 µA
SHDN Input Current SHDN = GND 0.001 1 µA
PARAMETER CONDITIONS MIN TYP MAX UNITS
0.3 ×
V
IN
0.7 ×
V
IN
0.1 ×
V
IN
Input Voltage Range
Output Voltage Range 24 V
IN Undervoltage-Lockout
Threshold
IN Quiescent Current
IN Shutdown Current SHDN = GND 10 µA
ERROR AMPLIFIER
FB Regulation Voltage Level to produce V
FB Input Bias Current VFB = 1.24V 250 nA
FB Line Regulation Level to produce V
Transconductance 100 300 µS
Shutdown FB Input Voltage SHDN = GND 0.05 0.15 V
OSCILLATOR
Frequency
Maximum Duty Cycle 86 94 %
n-CHANNEL MOSFET
Current Limit VFB = 1V, 75% duty cycle 3.0 5.1 A
On-Resistance 250 mΩ
Current-Sense Transresistance 0.11 0.31 V/A
SOFT-START
Reset Switch Resistance 100 Ω
Charge Current VSS = 1.2V 2.5 7.5 µA
PARAMETER CONDITIONS MIN TYP MAX UNITS
V
< 18V 2.6 5.5
OUT
18V < V
rising, typical hysteresis is 50mV 2.20 2.57 V
V
IN
VFB = 1.3V, not switching 0.44
= 1.0V, switching 5
V
FB
FREQ = GND 490 770
FREQ = IN 900 1600
< 24V 4.0 5.5
OUT
= 1.24V 1.215 1.260 V
COMP
= 1.24V, VIN = 2.6V to 5.5V 0.15 %/V
COMP
V
V
V
V
mA
kHz
MAX8727
TFT-LCD Step-Up DC-DC Converter
4 _______________________________________________________________________________________
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
ELECTRICAL CHARACTERISTICS (continued)
(VIN= V
SHDN
= 3V, FREQ = GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)
Typical Operating Characteristics
(Circuit of Figure 1. VIN= 5V, V
MAIN
= 15V, TA= +25°C unless otherwise noted.)
CONTROL INPUTS
SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V
SHDN, FREQ Input High Voltage VIN = 2.6V to 5.5V
PARAMETER CONDITIONS MIN TYP MAX UNITS
EFFICIENCY vs. LOAD CURRENT
(1.2MHz OPERATION)
100
L = 3.6µH
90
EFFICIENCY vs. LOAD CURRENT
(640kHz OPERATION)
100
L = 6.8µH
MAX8727 toc01
90
MAX8727 toc02
0.7 ×
V
IN
0.3 ×
V
IN
V
V
LOAD REGULATION
0.5
0
MAX8727 toc03
80
VIN = 5.0V
70
EFFICIENCY (%)
60
50
11000
VIN = 3.3V
10010
LOAD CURRENT (mA)
80
70
EFFICIENCY (%)
60
50
1 1000
SWITCHING FREQUENCY
1400
vs. INPUT VOLTAGE
1300
1200
1100
1000
900
800
SWITCHING FREQUENCY (kHz)
700
600
500
2.5 5.5
FREQ = IN
FREQ = GND
INPUT VOLTAGE (V)
4.0
3.5
MAX8727 toc04
3.0
2.5
2.0
1.5
SUPPLY CURRENT (mA)
1.0
0.5
0
5.04.54.03.53.0
2.5 5.5
vs. SUPPLY VOLTAGE
VIN = 5.0V
VIN = 3.3V
10010
LOAD CURRENT (mA)
SUPPLY CURRENT
SWITCHING
NONSWITCHING
SUPPLY VOLTAGE (V)
-0.5
-1.0
LOAD REGULATION (%)
-1.5
-2.0
1 1000
LOAD CURRENT (mA)
VIN = 5.0V
VIN = 3.3V
10010
SUPPLY CURRENT vs. TEMPERATURE
(SWITCHING)
5
MAX8727 toc05
5.04.54.03.53.0
4
3
2
SUPPLY CURRENT (mA)
1
0
-40 100
VIN = 5.0V
VIN = 3.3V
TEMPERATURE (°C)
MAX8727 toc06
806040200-20
MAX8727
TFT-LCD Step-Up DC-DC Converter
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VIN= 5V, V
MAIN
= 15V, TA= +25°C unless otherwise noted.)
SOFT-START
= 30Ω)
(R
LOAD
1ms/div
MAX8727 toc07
PULSED LOAD-TRANSIENT RESPONSE
= 100mA TO 1.1A)
(I
LOAD
MAX8727 toc09
V
OUT
5V/div
OV
INDUCTOR
CURRENT
1A/div
OA
15V
V
OUT
5mV/div
AC-COUPLED
I
OUT
1A/div
0.1A
INDUCTOR
CURRENT
1A/div
OA
LOAD-TRANSIENT RESPONSE
= 50mA TO 550mA)
(I
LOAD
1µs/div
MAX8727 toc08
SWITCHING WAVEFORMS
= 600mA)
(I
LOAD
MAX8727 toc10
15V
V
OUT
5mV/div
AC-COUPLED
I
OUT
500mA/div
50mA
INDUCTOR
CURRENT
1A/div
OA
LX
10V/div
OV
INDUCTOR
CURRENT
1A/div
OA
1µs/div
1µs/div
MAX8727
TFT-LCD Step-Up DC-DC Converter
6 _______________________________________________________________________________________
Pin Description
Figure 1 Typical Operating Circuit
PIN NAME FUNCTION
1 COMP
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
2FB
between the step-up regulator’s output (V
divider close to the IC and minimize the trace area to reduce noise coupling. Set V
OUT
Output Voltage Selection section.
3 SHDN Shutdown Control Input. Drive SHDN low to turn off the MAX8727.
4 GND Ground. Connect pins 4 and 5 directly together.
5 GND Ground. Connect pins 4 and 5 directly together.
6LX
7LX
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.
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.
Soft-Start Control Pin. Connect a soft-start capacitor (C
10 SS
start capacitor is charged with a constant current of 4.5µA. Full current limit is reached after t = 2.5 × 10
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.
) and GND, with the center tap connected to FB. Place the
according to the
OUT
) to this pin. Leave open for no soft-start. The soft-
SS
5
CSS.
V
4.5V TO 5.5V
IN
10µF
6.3V
L1
3.6µH
C1
10Ω
C3
1µF
R3
67
LX LX
8
IN
MAX8727
9
FREQ
3
SHDN
10
C6
33nF
SS
D1
GND
GND
COMP
R1
309kΩ
1%
2
FB
R2
28.0kΩ
5
4
1
1%
R4
100kΩ
C4
330pF
C2
4.7µF
25V
C5
39pF
C7
4.7µF
25V
C8
4.7µF
25V
V
OUT
15V/600mA
Detailed Description
The MAX8727 is a highly efficient power supply that
employs a current-mode, fixed-frequency, pulse-width
modulation (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 MAX8727 to
“skip” cycles to prevent overcharging the output voltage.
In this region of operation, the inductor ramps up to a
peak value of approximately 50mA, discharges to the
output, and waits until another pulse is needed again.
Output Current Capability
The output current capability of the MAX8727 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
current limit depends on the duty cycle. The current
limit is determined by the following equation:
I
LIM
= (1.26 - 0.35 x D) x I
LIM_EC
where I
LIM_EC
is the current limit specified at 75% duty
cycle (see the Electrical Characteristics) and D is the
duty cycle.
The output current capability depends on the currentlimit value and is governed by the following equation:
where I
LIM
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:
where V
DIODE
is the rectifier diode forward voltage and
RONis the on-resistance of the internal MOSFET.
MAX8727
TFT-LCD Step-Up DC-DC Converter
_______________________________________________________________________________________ 7
SHDN
Figure 2. MAX8727 Functional Diagram
COMP
FB
FREQ
IN
SS
LX
GND
1.24V
OSCILLATOR
ERROR
AMPLIFIER
∞
SLOPE
COMPEN-
SATION
BIAS
Σ
SKIP
COMPARATOR
ERROR
COMPARATOR
CLOCK
SKIP
CONTROL
AND DRIVER
LOGIC
CURRENT
SENSE
4µA
SOFTSTART
N
5µA
MAX8727
II
OUT MAX LIM
()
⎡
⎢
⎢
⎣
DV
.
××
05
fLVV
OSC
⎤
IN
⎥
×
⎥
⎦
IN
=−
××
OUT
D
=
VVV
−+
OUT IN DIODE
VIRV
−× +
OUT LIM ON DIODE
η
MAX8727
Soft-Start
The MAX8727 can be programmed for soft-start upon
power-up with an external capacitor. When the shutdown
pin is taken high, the soft-start capacitor (C
SS
) 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 V
SS
= 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.
Frequency Selection
The MAX8727’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.
Shutdown
The MAX8727 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.
Applications Information
Step-up regulators using the MAX8727 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
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 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.
TFT-LCD Step-Up DC-DC Converter
8 _______________________________________________________________________________________
Table 1. Component List
Table 2. Component Suppliers
DESIGNATION DESCRIPTION
10µF ±10%, 6.3V X5R ceramic capacitor
C1
C2, C7, C8
D1
L1
(0805)
Murata GRM21BR60J106K
Taiyo Yuden JMK212BJ106KD
4.7µF±20%, 25V X7R ceramic capacitors
(1206)
Murata GRM31CR71E475M
3A, 30V Schottky diode (M-Flat)
Toshiba CMS02
3.6µH ±30% power inductor
Sumida CDRH6D26-3R6NC
Murata 770-436-1300 770-436-3030 www.murata.com
Sanyo 619-661-4143 619-661-1055 www.sanyovideo.com
Sumida 847-545-6700 847-545-6720 www.sumida.com
Taiyo Yuden 800-348-2496 847-925-0899 www.t-yuden.com
Toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec
SUPPLIER PHONE FAX WEBSITE
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
(I
MAIN(MAX)
), the expected efficiency (η
TYP
) taken from
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current at the minimum input voltage V
IN(MIN)
using con-
servation of energy and the expected efficiency at that
operating point (η
MIN
) taken from an appropriate curve
in the Typical Operating Characteristics:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX8727’s LX current limit (I
LIM
) should exceed I
PEAK
,
and the inductor’s DC current rating should exceed
I
IN(DC,MAX)
. For good efficiency, choose an inductor
with less than 0.1Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I
MAIN(MAX)
) is 600mA with a 15V output and
a typical input voltage of 5V. Choosing an LIR of 0.35 and
estimating efficiency of 85% at this operating point:
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 85% at that operating point:
The ripple current and the peak current are:
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):
where I
PEAK
is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors,
the output voltage ripple is typically dominated by
V
RIPPLE(C)
. The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
MAX8727
TFT-LCD Step-Up DC-DC Converter
_______________________________________________________________________________________ 9
L
⎛
=
⎜
V
⎝
V
IN
MAIN
2
⎛
⎞
⎟
⎠
VV
−
MAIN IN
⎜
⎜
I f LIR
MAIN MAX OSC
⎝
×
()
⎞
⎛
⎞
η
TYP
⎟
⎜
⎟
⎠
⎟
⎝
⎠
L
2
V
5
⎛
⎞
⎜
⎟
⎝
⎠
V
15
VV
15 5
⎛
⎜
⎝
A MHz
. .
×
06 12
−
.
085
⎞
⎛
⎞
⎟
⎜
⎠
⎝
.
035
.=
⎟
⎠
H
≈µ
36
AV
.
×
I
IN DC MAX(, )
I
RIPPLE
IA
PEAK
45 15 45
=
. .
36 15 12
. . .
=+ ≈
06 15
V
. .
×
45 085
VV V
. ( .)
×−
H V MHz
µ× ×
235
073
.=
≈
A
2
270
A
235
.
≈
073
A
A
I
IN DC MAX
(, )
IV
MAIN MAX MAIN
=
V
IN MIN MIN
×
()
×η
()
I
RIPPLE
VVV
IN MIN MAIN IN MIN
=
II
PEAK IN DC MAX
=+
( )
×−
() ()
LV f
××
MAIN OSC
I
(, )
RIPPLE
2
VV V
RIPPLE RIPPLE C RIPPLE ESR
V
RIPPLE C
=+
≈
()
VIR
RIPPLE ESR PEAK ESR COUT
() ( )
() ( )
⎛
I
MAIN
C
VV
MAIN IN
⎜
Vf
⎝
OUT
≈
MAIN OSC
−
⎞
and
,
⎟
⎠
MAX8727
Input Capacitor Selection
The input capacitor (C
IN
) 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, C
IN
can be reduced below the values used in the typical
operating circuit. Ensure a low noise supply at IN by
using adequate C
IN
. Alternatively, greater voltage varia-
tion can be tolerated on CINif IN is decoupled from C
IN
using an RC lowpass filter (see R3 and C3 in Figure 1).
Rectifier Diode Selection
The MAX8727’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 I
PEAK
calculated in the Inductor Selection
section and that its breakdown voltage exceeds the
output voltage.
Output Voltage Selection
The MAX8727 operates with an adjustable output from
V
IN
to 24V. Connect a resistive voltage-divider from the
output (V
MAIN
) 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:
where VFB, the step-up regulator’s feedback set point,
is 1.24V (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 (R
COMP
) and capacitor (C
COMP
) in series from
COMP to GND, and another capacitor (C
COMP2
) from
COMP to GND. R
COMP
is chosen to set the high-frequency integrator gain for fast transient response, while
C
COMP
is chosen to set the integrator zero to maintain
loop stability. The second capacitor, C
COMP2
, is cho-
sen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations:
For the ceramic output capacitor, where ESR is small,
C
COMP2
is optional. The best gauge of correct loop
compensation is by inspecting the transient response
of the MAX8727. Adjust R
COMP
and C
COMP
as neces-
sary to obtain optimal transient performance.
Soft-Start Capacitor
The soft-start capacitor should be large enough that it
does not reach final value before the output has
reached regulation. Calculate CSSto be:
where C
OUT
is the total output capacitance including
any bypass capacitor on the output bus, V
OUT
is the
maximum output voltage, I
INRUSH
is the peak inrush
current allowed, I
OUT
is the maximum output current
during power-up, and VINis the minimum input voltage.
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:
t
MAX
= 6.77 x 105x C
SS
TFT-LCD Step-Up DC-DC Converter
10 ______________________________________________________________________________________
RR
12 1 =× −
⎛
⎜
⎝
V
MAIN
V
FB
⎞
⎟
⎠
R
COMP
C
COMP
C
COMP
2
315
≈
≈
.
0 0036
≈
VV C
×× ×
IN OUT OUT
LI
×
VC
OUT OUT
10
IR
××
MAIN MAX COMP
RLI
×××
ESR MAIN MAX
VV
IN OUT
()
MAIN MAX
×
()
×
()
CC
>× × ×
21 10
SS OUT
⎛
VVV
OUT IN OUT
VI I V
×−×
IN INRUSH OUT OUT
⎜
⎜
⎜
⎝
−
6
2
−×
⎞
⎟
⎟
⎟
⎠
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),
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.
MAX8727
TFT-LCD Step-Up DC-DC Converter
______________________________________________________________________________________ 11
Figure 3. Multiple-Output TFT-LCD Power Supply
V
4.5V TO 5.5V
IN
10µF
6.3V
V3
C10
1µF
C6
39pF
-14V
C2
4.7µF
25V
C7
4.7µF
25V
C8
4.7µF
25V
V
OUT
15V/600mA
D1
GND
GND
COMP
D3
R1
309kΩ
1%
2
FB
R2
28.0kΩ
5
4
1
1%
R3
100kΩ
C3
330pF
C7
0.1µFC80.1µF
V2
+28V
C9
1µF
C1
R4
10Ω
C5
1µF
D2
L1
3.6µH
67
LX LX
8
IN
MAX8727
9
FREQ
3
SHDN
10
C4
33nF
SS
MAX8727
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.
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 MAX8727 evaluation kit for an example of
proper board layout.
Chip Information
TRANSISTOR COUNT: 2746
PROCESS: BiCMOS
TFT-LCD Step-Up DC-DC Converter
12 ______________________________________________________________________________________
MAX8727
TFT-LCD Step-Up DC-DC Converter
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 ____________________ 13
© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
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
.)
PIN 1
INDEX
AREA
-DRAWING NOT TO SCALE-
D
N
E
A
DETAIL A
E2
C
L
L
e
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
C
L
e
21-0137
6, 8, &10L, DFN THIN.EPS
L
1
G
2
COMMON DIMENSIONS
SYMBOL MIN. MAX.
A
0.70 0.80
2.90 3.10
D
E
2.90 3.10
A1
0.00 0.05
L
0.20 0.40
0.25 MIN.k
A2 0.20 REF.
PACKAGE VARIATIONS
PKG. CODE
T633-2 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF
T833-2 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF
T833-3 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF
-DRAWING NOT TO SCALE-
N
D2 E2 e JEDEC SPEC b
2.30±0.101.50±0.106T633-1 0.95 BSC MO229 / WEEA 1.90 REF0.40±0.05
2.30±0.108T833-1
1.50±0.10
1.50±0.10
0.65 BSC
0.40 BSC - - - - 0.20±0.05 2.40 REFT1433-2 14 2.30±0.101.70±0.10
MO229 / WEEC
MO229 / WEED-3
DOWNBONDS
[(N/2)-1] x e
1.95 REF0.30±0.05
2.00 REF0.25±0.050.50 BSC2.30±0.1010T1033-1
2.40 REF0.20±0.05- - - - 0.40 BSC1.70±0.10 2.30±0.1014T1433-1
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
ALLOWED
NO
NO
NO
NO
YES
NO
YES
NO
21-0137
2
G
2
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