MAXIM MAX8752

19-3793; Rev 0; 8/05
KIT
ATION
EVALU
LE
B
A
IL
A
AV
TFT LCD Step-Up DC-DC Converter
♦ 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
♦ Input Supply Undervoltage Lockout
♦ 1.2MHz Switching Frequency
♦ 0.1µA Shutdown Current
♦ Small 8-Pin TDFN Package
Ordering Information
Applications
PART
TEMP
RANGE
PINPACKAGE
PKG
CODE
MAX8752ETA
-40°C to +85°C
8 TDFN
3mm x 3mm
T833-2
LCD Monitor Panels
IN
LX
5
LX
TOP VIEW
IN
VMAIN
8
VIN
+1.8V TO +5.5V
Pin Configuration
SUP
Typical Operating Circuit
6
Automotive Displays
LDO
Notebook Computer Displays
7
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 displays (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.
The MAX8752 operates with a switching frequency of
1.2MHz, allowing the use of ultrasmall inductors and lowESR ceramic capacitors. The current-mode architecture
provides fast transient response to the pulsed loads typical 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 current limit effectively control inrush and fault currents.
The MAX8752 is available in a 3mm x 3mm 8-pin TDFN
package with a maximum height of 8mm.
Features
FB
MAX8752
MAX8752
GND
COMP
LDO
2
3
4
SHDN
GND
1
FB
IN
SUP
COMP
SHDN
TDFN
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
MAX8752
General Description
MAX8752
TFT LCD Step-Up DC-DC Converter
ABSOLUTE MAXIMUM RATINGS
LX, SUP to GND .....................................................-0.3V to +14V
IN, SHDN, LDO to GND............................................-0.3V to +6V
FB to GND ...................................................-0.3V to (VIN + 0.3V)
COMP to GND ..........................................-0.3V to (VLDO + 0.3V)
LX Switch Maximum Continuous RMS Current .....................1.6A
Continuous Power Dissipation (TA = +70°C)
10-Pin TDFN (derate 18.2mW/°C above +70°C) .......1454mW
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 = VSHDN = 2.5V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
CONDITIONS
Input Supply Range
MIN
TYP
MAX
UNITS
5.5
V
13
V
1.30
1.75
V
0.18
0.35
2
5
1.8
Output Voltage Range
IN Undervoltage Lockout
Threshold
IN Quiescent Current
VIN rising, typical hysteresis is 200mV
0.90
VFB = 1.3V, not switching
VFB = 1.0V, switching
mA
IN Shutdown Current
SHDN = GND
0.1
10.0
µA
LDO Output Voltage
6V ≤ VSUP ≤ 13V, ILDO = 12.5mA
4.6
5.0
5.4
V
LDO Undervoltage Lockout
VLDO rising, typical hysteresis is 200mV
2.4
2.7
3.0
V
LDO Output Current
15
SUP Supply Voltage Range
4.5
SUP Overvoltage-Lockout
Threshold
VSUP rising, typical hysteresis is 200mV (Note 1)
SUP Undervoltage-Lockout
Threshold
VSUP rising, typical hysteresis is 200mV (Note 2)
SUP Supply Current
13.2
LX not switching
LX switching
mA
13.6
13.0
V
14.0
V
1.4
V
1.5
2.0
4
8
mA
ERROR AMPLIFIER
FB Regulation Voltage
ILX = 200mA, T = 0°C to +25°C
1.218
1.240
1.262
ILX = 200mA, T = +25°C to +85°C
1.223
1.240
1.257
0
40
nA
0.05
0.15
%/V
180
280
FB Input Bias Current
VFB = 1.24V
FB Line Regulation
VIN = 1.8V to 5.5V
Transconductance
70
Voltage Gain
700
V
µS
V/V
OSCILLATOR
Frequency
Maximum Duty Cycle
2
1000
1220
1500
kHz
88
92
96
%
_______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
MAX8752
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSHDN = 2.5V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
n-CHANNEL MOSFET
Current Limit
VFB = 1V, 65% duty cycle
1.8
On-Resistance
Leakage Current
VLX = 12V
Current-Sense Transresistance
0.2
2.2
2.6
A
0.2
0.4
Ω
0.1
10
µA
0.3
0.4
V/A
SOFT-START
Soft-Start Period
Soft-Start Step Size
13
ms
0.275
A
CONTROL INPUTS
SHDN Input Low Voltage
VIN = 1.8V to 5.5V
SHDN Input High Voltage
VIN = 1.8V to 5.5V
0.6
V
0.001
1.000
µA
TYP
MAX
0.7 × VIN
SHDN Input Current
V
ELECTRICAL CHARACTERISTICS
(VIN = VSHDN = 2.5V, TA = -40°C to +85°C. unless otherwise noted.)
PARAMETER
CONDITIONS
Input Supply Range
MIN
1.8
Output Voltage Range
IN Undervoltage-Lockout Threshold
IN Quiescent Current
VIN rising, typical hysteresis is 200mV
0.90
VFB = 1.3V, not switching
V
13
V
1.75
V
0.35
VFB = 1.0V, switching
5
LDO Output Voltage
6V ≤ VSUP ≤ 13V, ILDO = 12.5mA
4.6
5.4
LDO Undervoltage Lockout
VLDO rising, typical hysteresis is 200mV
2.4
3.0
LDO Output Current
15
SUP Supply Voltage Range
mA
V
V
mA
4.5
13.0
V
13.2
14.0
V
VSUP rising, typical hysteresis is 200mV (Note 2)
1.4
V
LX not switching
2.0
SUP Overvoltage-Lockout Threshold
VSUP rising, typical hysteresis is 200mV (Note 1)
SUP Undervoltage-Lockout Threshold
SUP Supply Current
UNITS
5.5
LX switching
8
mA
ERROR AMPLIFIER
FB Regulation Voltage
ILX = 200mA
1.210
1.270
V
940
1560
kHz
1.7
2.7
A
OSCILLATOR
Frequency
n-CHANNEL MOSFET
Current Limit
VFB = 1V, 65% duty cycle
On-Resistance
Current-Sense Transresistance
0.2
0.4
Ω
0.4
V/A
_______________________________________________________________________________________
3
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSHDN = 2.5V, TA = -40°C to +85°C. unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0.6
V
CONTROL INPUTS
SHDN Input Low Voltage
VIN = 1.8V to 5.5V
SHDN Input High Voltage
0.7 ×
VIN
VIN = 1.8V to 5.5V
V
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, VIN = 2.5V, VMAIN = 10V, TA = +25°C, unless otherwise noted.)
EFFICIENCY (%)
VIN = 3.3V
75
70
VIN = 1.8V
65
60
80
75
70
VIN = 3.3V
65
60
55
0.5
1000
VIN = 1.8V
-1.0
VIN = 3.3V
-1.5
-2.0
-3.0
10
100
LOAD CURRENT (mA)
1
1000
10
IN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SWITCHING FREQUENCY ERROR
vs. INPUT VOLTAGE
50
MAX8752 toc04
0.1
40
IN SUPPLY CURRENT (μA)
0
NORMAL FB
-0.1
-0.2
-0.3
-0.4
NO LOAD
30
20
VFB = 1.3V
10
VIN = 1.8V
40
30
VIN = 3.3V
20
10
VIN = 5V
-0.5
0
-0.6
1.8
2.8
3.8
4.8
INPUT VOLTAGE (V)
5.8
10,000
50
IN SUPPLY CURRENT (μA)
0.2
100
1000
LOAD CURRENT (mA)
IN SUPPLY CURRENT
vs. TEMPERATURE
MAX8752 toc05
100
LOAD CURRENT (mA)
-0.5
VIN = 1.8V
50
10
VIN = 5V
0
-2.5
55
50
4
VIN = 5V
85
80
EFFICIENCY (%)
L1 = 3.3μH
MAX8752 toc06
90
OUTPUT VOLTAGE ERROR (%)
VIN = 5V
MAX8752 toc02
85
95
MAX8752 toc01
90
L1 = 2.6μH
OUTPUT VOLTAGE ERROR
vs. LOAD CURRENT
EFFICIENCY vs. LOAD CURRENT
MAX8752 toc03
EFFICIENCY vs. LOAD CURRENT
SWITCHING FREQUENCY ERROR (%)
MAX8752
TFT LCD Step-Up DC-DC Converter
0
1.5
2.5
3.5
4.5
SUPPLY VOLTAGE (V)
5.5
-40
-20
0
20
40
TEMPERATURE (°C)
_______________________________________________________________________________________
60
80
TFT LCD Step-Up DC-DC Converter
(Circuit of Figure 1, VIN = 2.5V, VMAIN = 10V, TA = +25°C, unless otherwise noted.)
SOFT-START (HEAVY LOAD)
LOAD TRANSIENT RESPONSE
PULSED-LOAD TRANSIENT RESPONSE
MAX8752 toc08
MAX8752 toc07
MAX8752 toc09
IMAIN
1A/div
IMAIN
200mA/div
INDUCTOR
CURRENT
1A/div
100mA
0A
VMAIN
5V/div
INDUCTOR
CURRENT
1A/div
0A
0A
0A
10V
10V
VMAIN
500mA/div
10V OFFSET
0V
INDUCTOR
CURRENT
1A/div
VMAIN
200mV/div
10V OFFSET
100μs/div
10μs/div
SWITCHING WAVEFORMS
SUP SUPPLY CURRENT
vs. SUP VOLTAGE
SUP SUPPLY CURRENT
vs. TEMPERATURE
INDUCTOR
CURRENT
500mA/div
ILOAD = 300mA
SUP SUPPLY CURRENT (mA)
0V
3.5
3.0
VIN = 1.8V
2.5
2.0
VIN = 5V
1.5
1.0
VIN = 3.3V
4.2
VIN = 3.3V
VIN = 1.8V
3.8
VIN = 5V
3.4
0.5
0
0A
4
1μs/div
6
8
10
SUP VOLTAGE (V)
12
3.0
14
-40
-20
0
20
40
TEMPERATURE (°C)
60
80
LDO OUTPUT VOLTAGE
vs. LDO CURRENT
LDO OUTPUT VOLTAGE
vs. TEMPERATURE
5.06
MAX8752 toc14
5.08
MAX8752 toc13
5.08
5.06
5.04
LDO VOLTAGE (V)
OUTPUT VOLTAGE (V)
ILOAD = 140mA
SUP SUPPLY CURRENT (mA)
NO LOAD
4.0
LX
5V/div
MAX8752 toc11
4.5
MAX8752 toc12
2ms/div
MAX8752 toc10
5.04
5.02
5.02
5.00
4.98
5.00
4.96
4.94
4.98
-40
-20
0
20
40
TEMPERATURE (°C)
60
80
0
10
MAX8752
Typical Operating Characteristics (continued)
20
30
LDO CURRENT (mA)
40
50
_______________________________________________________________________________________
5
TFT LCD Step-Up DC-DC Converter
MAX8752
Pin Description
PIN
NAME
FUNCTION
COMP
Compensation Pin for Error Amplifier. Connect a series resistance and capacitor from COMP to GND.
See the Loop Compensation section for component selection guidelines.
2
FB
Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltagedivider between the step-up regulator’s output (VMAIN) 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 VMAIN
according to the Output Voltage Selection section.
3
SHDN
Shutdown Control Input. Drive SHDN low to turn off the MAX8752.
4
GND
Ground
5
LX
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.
6
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.
7
LDO
Internal 5V Linear-Regulator Output. This regulator powers all internal circuitry. Bypass LDO to GND
with a 0.22µF or greater ceramic capacitor.
8
SUP
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.
BP
—
1
Backside Paddle. Connect the backside paddle to analog ground.
C11
0.1μF
VGOFF
-9V/20mA
VIN
L1
2.6μH
+1.8V TO +5.5V
VMAIN
+10V/240mA
D1
R4
100Ω
R1
90.9kΩ
1%
LX
IN
R3
40.2kΩ
C2
10μF
16V
FB
C3
0.1μF
R2
13kΩ
1%
MAX8752
COMP
C4
1.2nF
C13
0.1μF
D3
C12
0.1μF
C8
0.1μF
C1
10μF
6.3V
C10
0.1μF
C9
0.1μF
D2
VGON
28V/10mA
D4
GND
C6
20pF
SUP
C14
0.22μF
LDO
C7
0.1μF
SHDN
Figure 1. Typical Applications Circuit
6
_______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
LOGIC
AND DRIVER
IN
STARTUP
OSC
GND
CURRENT LIMIT
SOFTSTART
ILIMIT
SLOPE COMP
∑
OSCILLATOR
SHDN
SUP
LDO
CURRENT
SENSE
PWM
COMPARATOR
ERROR AMP
LINEAR
REGULATOR
AND
BOOTSTRAP
FB
MAX8752
1.24V
COMP
Figure 2. MAX8752 Functional Diagram
Detailed Description
The MAX8752 is a highly efficient, step-up power supply 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.
The MAX8752 employs a current-mode, fixed-frequency, pulse-width modulation (PWM) architecture for fast
transient response and low-noise operation. The high
switching frequency (1.2MHz) allows the use of lowprofile 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 components required. The output voltage can be set from
VIN 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
At light loads, this architecture allows the MAX8752 to
“skip” cycles to prevent overcharging the output
capacitor voltage.
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 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:
ILIM = (1.162 - 0.361 x D) x ILIM_EC
where ILIM_EC is the current limit specified at 65% 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:
⎡
0.5 x D VIN ⎤
VIN
IOUT(MAX) = ⎢ILIM −
xη
⎥x
f
x
L
V
OSC
OUT
⎣
⎦
_______________________________________________________________________________________
7
MAX8752
LX
CLOCK
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 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.
MAX8752
TFT LCD Step-Up DC-DC Converter
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
Table 1. Component List
DESIGNATION
DESCRIPTION
C1
10µF ±10%, 4V X5R ceramic capacitor
(0603)
TDK C1608X5R0G106K
Murata GRM188R60G106M
C2
10µF ±10%, 16V X5R ceramic capacitor
(1206)
TDK C3216X5R1C106K
Murata GRM319R61A106K
D1
3A, 30V Schottky diode (M-flat)
Toshiba CRS02
L1
2.6µH, 2.1A power inductor
3.3µH, 1.7A power inductor
Sumida CDRH6D12-3R3
where VDIODE is the rectifier diode forward voltage and
RON is the on-resistance of the internal MOSFET.
Bootstrapping and Soft-Start
The MAX8752 features bootstrapping operation. In normal 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).
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 reaches 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.
Shutdown
The MAX8752 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. In
shutdown, the step-up regulator’s output is connected to
IN through the external inductor and rectifier diode.
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.
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 inductor 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 converter’s efficiency, maximum output load capability,
transient response time, and output voltage ripple.
Physical size and cost are also important factors to
consider.
Table 2. Component Suppliers
SUPPLIER
PHONE
FAX
Murata
770-436-1300
770-436-3030
www.murata.com
Sumida
847-545-6700
847-545-6720
www.sumida.com
TDK
847-803-6100
847-803-6296
www.component.tdk.com
Toshiba
949-455-2000
949-859-3963
www.toshiba.com/taec
8
WEBSITE
_______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
In Figure 1, the LCD’s gate-on and gate-off 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 calculation. The effective maximum output current IMAIN(EFF) becomes the sum of the maximum load
current on the step-up regulator’s output plus the contributions from the positive and negative charge
pumps:
IMAIN(EFF) = IMAIN(MAX) + ηNEG x INEG + (ηPOS + 1) x
IPOS
I POS is the positive charge-pump output current,
assuming the pump source for IPOS is VMAIN.
Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current
(IMAIN(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
⎞ ⎛
VMAIN − VIN
TYP ⎞
L = ⎜ IN ⎟ ⎜
⎟
⎟ ⎜
⎝ VMAIN ⎠ ⎝ IMAIN(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) =
IMAIN(MAX) × 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
I
IPEAK = IIN(DC, MAX) + RIPPLE
2
The inductor’s saturation current rating and the
MAX8752’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 the Typical Applications Circuit (Figure 1),
the maximum load current (IMAIN(MAX)) is 180mA with a
10V output and a typical input voltage of 2.5V:
IMAIN(EFF) = 180mA + 1 x 20mA + 3 x 10mA = 230mA
where IMAIN(MAX) is the maximum main output current,
nNEG 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
_______________________________________________________________________________________
9
MAX8752
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.
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
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.
MAX8752
TFT LCD Step-Up DC-DC Converter
Choosing an LIR of 0.5 and estimating efficiency of
80% at this operating point:
2
⎛ 2.5V ⎞ ⎛ 10V − 2.5V ⎞ ⎛ 0.80 ⎞
L = ⎜
⎟ ⎜
⎟ ≈ 2.6μH
⎟ ⎜
⎝ 10V ⎠ ⎝ 0.23A × 1.2MHz ⎠ ⎝ 0.50 ⎠
Using the circuit’s minimum input voltage (2.2V) and
estimating efficiency of 75% at that operating point:
IIN(DC, MAX) =
0.23A × 10V
2.2V × 0.75
≈ 1.4 A
The ripple current and the peak current are:
IRIPPLE =
2.2V × (10V − 2.2V)
2.6μH × 10V × 1.2MHz
IPEAK = 1.4 A +
0.55A
2
≈ 0.55A
≈ 1.7A
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)
VRIPPLE(C) ≈
IMAIN
COUT
⎛ VMAIN − VIN ⎞
⎜ V
⎟ , and
⎝ MAIN 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.
10
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 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.
Typically, CIN can be reduced below the values used in
the Typical Applications Circuit. Ensure a low noise
supply at IN by using adequate C IN . 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).
Rectifier Diode Selection
The MAX8752’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.
Output Voltage Selection
The MAX8752 operates with an adjustable output from
VIN to 13V. Connect a resistive voltage-divider from the
output (VMAIN) 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:
⎛V
⎞
R1 = R2 × ⎜ MAIN − 1⎟
⎝ VFB
⎠
where VFB, the step-up regulator’s feedback set point,
is 1.24V (typ). Place R1 and R2 close to the IC.
______________________________________________________________________________________
TFT LCD Step-Up DC-DC Converter
RCOMP ≈
CCOMP ≈
CCOMP2 ≈
264 × VIN × VOUT × COUT
L × IMAIN(EFF)
VOUT × COUT
10 × IMAIN(MAX) × RCOMP
0.02 × RESR × L × IMAIN(EFF)
VIN × VOUT
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 MAX8752. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance.
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 the IC’s GND pin, and to
the input capacitor’s negative terminal. The highcurrent 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-
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.
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 capacitor’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.
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 feedback trace near LX.
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 terminals to GND through the IC’s exposed back paddle
near GND (pin4).
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.
Refer to the MAX8752 evaluation kit for an example of
proper board layout.
Chip Information
TRANSISTOR COUNT: 3091
PROCESS: BiCMOS
______________________________________________________________________________________
11
MAX8752
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 chosen to cancel the zero introduced by
output-capacitance ESR. For optimal performance,
choose the components using the following equations:
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.)
6, 8, &10L, DFN THIN.EPS
MAX8752
TFT LCD Step-Up DC-DC Converter
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
CL
A
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.
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.