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19-3329; Rev 3; 3/10
KIT ATION EVALU LE B AVAILA
High-Efficiency LCD Boost with True Shutdown
General Description Features
o 15V or Adjustable Output Voltage Up to 28V o Safety Features Protect Against Output Faults o 20mA at 20V from a Single Li+ Battery o o o o True Shutdown 87% Efficiency Up to 800kHz Switching Frequency Small, 6-Pin SOT23 and DFN (MAX8570 Only) Packages
MAX8570-MAX8575
The MAX8570 family of LCD step-up converters uses an internal n-channel switch and an internal p-channel output isolation switch. These converters operate from a 2.7V to 5.5V supply voltage and deliver up to 28V at the output. A unique control scheme provides the highest efficiency over a wide range of load conditions. The internal MOSFET switch reduces external component count and a high switching frequency (up to 800kHz) allows for tiny surface-mount components. Three current-limit options are available. The MAX8570 and MAX8572 use a 110mA current limit to reduce ripple and component size in low-current applications. For high-power requirements, the MAX8574 and MAX8575 use a 500mA current limit and supply up to 20mA at 20V. The MAX8571 and MAX8573 use a 250mA current limit for a compromise between ripple and power. Built-in safety features protect the internal switch and down-stream components from fault conditions. Additional features include a low quiescent current and a True ShutdownTM mode to save power. The MAX8570/ MAX8571/MAX8574 allow the user to set the output voltage between 3V and 28V, and the MAX8572/ MAX8573/MAX8575 have a preset 15V output. These step-up converters are ideal for small LCD panels with low current requirements, but can also be used in other applications. The MAX8571 evaluation kit is available to help reduce design time.
Ordering Information
PART MAX8570ELT+T MAX8570EUT+T MAX8571EUT+T MAX8572EUT+T MAX8573EUT+T MAX8574EUT+T TEMP RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C PINPACKAGE 6L DFN 6 SOT23 6 SOT23 6 SOT23 6 SOT23 6 SOT23 TOP MARK ACW ABTJ ABTK ABTL ABTM ABTN ABTO
MAX8575EUT+T -40C to +85C 6 SOT23 +Denotes a lead(Pb)-free RoHS-compliant package. T = Tape and reel.
Applications
LCD Bias Generators Polymer LEDs (OLED) Cellular or Cordless Phones Palmtop Computers Personal Digital Assistants (PDAs) Organizers Handy Terminals
PART MAX8570 MAX8571 MAX8572 MAX8573 MAX8574 MAX8575 110mA 250mA 110mA 250mA 500mA 500mA
Selector Guide
CURRENT LIMIT OUTPUT VOLTAGE Adjustable Adjustable 15V 15V Adjustable 15V
Pin Configurations
TOP VIEW +
FB 1 GND 2 SHDN 3 6 VCC SW LX
Typical Operating Circuit
VOUT = VCC TO 28V SW LX
MAX8570 MAX8571 MAX8574
5 4
VCC = 2.7V TO 5.5V
VCC
MAX8572 MAX8573 MAX8575 OUT
SOT23
Pin Configurations continued at end of data sheet. True Shutdown is a trademark of Maxim Integrated Products, Inc.
ON OFF
SHDN
GND
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com
High-Efficiency LCD Boost with True Shutdown MAX8570-MAX8575
ABSOLUTE MAXIMUM RATINGS
VCC, SHDN to GND ..................................................-0.3V to +6V SW to GND .................................................-0.3V to (VCC + 0.3V) FB to GND (MAX8570/MAX8571/ MAX8574)...............................................-0.3V to (VCC + 0.3V) OUT to GND (MAX8572/MAX8573/MAX8575) .......-0.3V to +30V LX to GND ..............................................................-0.3V to +30V ILX, ICC ..............................................................................600mA Continuous Power Dissipation (TA = +70C) DFN (derate 4.5mW/C above +70C)....................357.8mW SOT23-6 (derate 8.7mW/C above +70C)...............695.7mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C Soldering Temperature (reflow) .......................................+260C
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
(VCC = V SHDN = 3.6V, SW open, VFB = 1.3V (MAX8570/MAX8571/MAX8574) or VOUT = 16V (MAX8572/MAX8573/MAX8575), TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER VCC Input Voltage Range VCC Undervoltage Lockout VCC Supply Current VCC Shutdown Current Line Regulation Load Regulation FB Regulation Voltage FB Input Bias Current OUT Regulation Voltage OUT Input Bias Current LX Voltage Range MAX8571/MAX8573 LX Switch Current Limit (Note 2) MAX8570/MAX8572 MAX8574/MAX8575 LX On-Resistance LX Leakage Current Maximum LX On-Time Minimum LX Off-Time Current-Limit Propagation Delay SHDN Low Level (VIL) SHDN High Level (VIH) SHDN Leakage Current 2.7V VCC 5.5V 4.2V VCC 5.5V 2.7V VCC < 4.2V 1.5 1.4 -1 +1 VFB > 1V or VOUT > 12.2V VFB = 0.25V or VOUT = 3.4V MAX8571/MAX8573/MAX8574/MAX8575, ILX = 100mA MAX8570/MAX8572, ILX = 50mA VLX = 28V TA = +25C TA = -40C to +85C 8 0.8 4.0 0.217 0.088 0.425 0.241 0.101 0.484 0.9 1.5 0.01 0.05 11 1 5 55 0.7 14 1.2 6.0 TA = 0C to +85C TA = -40C to +85C VOUT = 15V SHDN = GND, VCC = 5.5V TA = +25C TA = -40C to +85C VCC rising, 50mV typical hysteresis CONDITIONS MIN 2.70 2.33 2.5 25 0.05 0.05 0.1 0.1 1.216 1.2137 -50 14.85 14.813 2.4 -4 15 1.226 1.236 1.2383 +50 15.15 15.187 4.4 28 0.267 0.108 0.540 1.5 2.4 2 A s s ns V V A A TYP MAX 5.50 2.65 35 1 UNITS V V A A %/V %/mA V nA V A V
Circuit of Figure 3, VOUT = 15V, ILOAD = 5mA, VCC = 2.7V to 5.5V Circuit of Figure 3, VOUT = 15V, ILOAD = 0 to 5mA TA = 0C to +85C TA = -40C to +85C
2
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High-Efficiency LCD Boost with True Shutdown
ELECTRICAL CHARACTERISTICS (continued)
(VCC = V SHDN = 3.6V, SW open, VFB = 1.3V (MAX8570/MAX8571/MAX8574) or VOUT = 16V (MAX8572/MAX8573/MAX8575), TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 1)
PARAMETER SW PMOS Current Limit SW PMOS On-Resistance SW PMOS Leakage Current SW Soft-Start Time CONDITIONS VCC = 3.6V, VSW = 0V, VFB = 0V, ICC (peak) VCC = 3.6V, VSW = 0V, VFB = 0V, ICC (average) VCC = 2.7V, VFB = 0V, ISW = 100mA SW = GND, VCC = 5.5V, VFB = 0V VCC = 2.7V, CSW = 4.7F TA = +25C TA = -40C to +85C MIN 0.45 0.15 TYP 0.75 0.30 1.5 0.01 0.02 0.2 1 MAX 1.10 0.60 2.5 1 UNITS A A ms
MAX8570-MAX8575
Note 1: Parameters are production tested at TA = +25C. Limits over temperature are guaranteed by design. Note 2: Specified currents are measured at DC. Actual LX current limits are slightly higher in circuit due to current-limit comparator delay. Actual currents (with 2H) are 110mA (MAX8570/MAX8572), 250mA (MAX8571/MAX8573), and 500mA (MAX8574/MAX8575).
Typical Operating Characteristics
(MAX8571, VCC = 3.6V, VOUT = 18V, Circuit of Figure 2, TA = +25C, unless otherwise noted.)
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE (MAX8571)
MAX8570/71/73/74/75 toc01
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE (FIGURE 3, MAX8573)
MAX8570/71/73/74/75 toc02
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE (MAX8574)
18.8 18.6 OUTPUT VOLTAGE (V) 18.4 18.2 18.0 17.8 17.6 17.4 L1 = TOKO S1024-100M R1 = 1.1M, R2 = 75k, C4 = 4.7pF 3.1 3.4 3.7 4.0 4.3 4.6 4.9 20mA LOAD
MAX8570/71/73/74/75 toc03
18.5 18.4 18.3 OUTPUT VOLTAGE (V) 18.2 18.1 18.0 17.9 17.8 17.7 17.6 17.5 L1 = MURATA LQH32CN220K23 R1 = 3.9M, R2 = 287k 2.7 3.1 3.5 3.9 4.3 4.7 5.1 1mA LOAD 5mA LOAD
15.5 15.4 15.3 OUTPUT VOLTAGE (V) 15.2 15.1 15.0 14.9 14.8 14.7 14.6 14.5 L1 = MURATA LQH32CN220K23 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5mA LOAD 1mA LOAD
19.0 5mA LOAD
17.2 17.0 5.5
5.5
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX8570/71/73/74/75 toc04
OUTPUT VOLTAGE vs. TEMPERATURE
MAX8570/71/73/74/75 toc05
EFFICIENCY vs. SUPPLY VOLTAGE
MAX8570/71/73/74/75 toc06
19.0 18.8 OUTPUT VOLTAGE (V) 18.6 18.4 18.2 18.0 17.8 17.6 17.4 0
L1 = MURATA LQH32CN220K23 MAX8574, R1 = 1.1M, R2 = 75k, C4 = 4.7pF
18.4 18.3 18.2 OUTPUT VOLTAGE (V) 18.1 18.0 17.9 17.8 17.7
100 95 EFFICIENCY (%) 90 85 80 22H, 5mA LOAD 75 22H, 1mA LOAD L1 = TOKO A914BYW-470M 47H, 5mA LOAD 47H, 1mA LOAD
MAX8570
MAX8571 17.6 1mA LOAD -40 -15 10 35 60 85
L1 = MURATA LQH32CN220K23
R1 = 3.9M, R2 = 287k, C4 = 10pF 5 10 15 20 25
70 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 TEMPERATURE (C) SUPPLY VOLTAGE (V)
LOAD CURRENT (mA)
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3
High-Efficiency LCD Boost with True Shutdown MAX8570-MAX8575
Typical Operating Characteristics (continued)
(MAX8571, VCC = 3.6V, VOUT = 18V, Circuit of Figure 2, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. BATTERY VOLTAGE (FIGURE 4)
MAX8570/71/73/74/75 toc07
EFFICIENCY vs. LOAD CURRENT WITH 22H INDUCTOR
MAX8570/71/73/74/75 toc08
EFFICIENCY vs. LOAD CURRENT WITH 47H INDUCTOR
MAX8570, L1 = MURATA LQH32CN470K23 90 EFFICIENCY (%) 80 70 60 50 40 MAX8571, L1 = TOKO A914BYW-470M
MAX8570/71/73/74/75 toc09
100 5mA LOAD 90 EFFICIENCY (%) 80 1mA LOAD 70 60 50 40 0 VCC = 3.6V L1 = MURATA LQH32CN220K23
100 90 EFFICIENCY (%) 80 70 60 50 40 MAX8571, MURATA LQH32CN220K23 MAX8574, TOKO A914BYW-220M MAX8570, MURATA LQH32CN220K23
100
2
4
6
8
10
12
0.1
1
10
100
0.1
1
10
100
BATTERY VOLTAGE (V)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
PEAK INDUCTOR CURRENT LIMIT vs. SUPPLY VOLTAGE
MAX8570/71/73/74/75 toc10
SUPPLY CURRENT vs. LOAD CURRENT
MAX8570/71/73/74/75 toc11
NO-LOAD CURRENT vs. SUPPLY VOLTAGE
70 SUPPLY CURRENT (A) 60 50 40 30 20 MAX8573, FIGURE 3 R1 = 7.87M R2 = 576k R1 = 3.9M R2 = 287k
MAX8570/71/73/74/75 toc12
700 600 CURRENT LIMIT (mA) 500 400 300 200 100 0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 MAX8570 MAX8571 MAX8574
120 100 SUPPLY CURRENT (mA) 80 60 40 20 0 L1 = MURATA LQH32CN220K23 0 5 10
80
10 0 15 0
L1 = MURATA LQH32CN220K23 1 2 3
NO SWITCHING
5.5
4
5
6
SUPPLY VOLTAGE (V)
LOAD CURRENT (mA)
SUPPLY VOLTAGE (V)
LINE TRANSIENT 3V TO 5.5V (MAX8571)
MAX8570/71/73/74/75 toc13
LINE TRANSIENT 3V TO 5.5V (FIGURE 3, MAX8573)
MAX8570/71/73/74/75 toc14
VOUT
200mV/div (AC-COUPLED)
VOUT
200mV/div (AC-COUPLED)
VCC 2V/div
VCC 2V/div
0 100s/div 3.6k LOAD, R1 = 3.9M, R2 = 287k 3k LOAD 100s/div
0
4
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High-Efficiency LCD Boost with True Shutdown
Typical Operating Characteristics (continued)
(MAX8571, VCC = 3.6V, VOUT = 18V, Circuit of Figure 2, TA = +25C, unless otherwise noted.)
MAX8570-MAX8575
LOAD TRANSIENT
MAX8570/71/73/74/75 toc15
STARTUP AND SHUTDOWN WAVEFORMS
MAX8570/71/73/74/75 toc16
VOUT
VSHDN 100mV/div (AC-COUPLED) BOOST SOFT-START SW TURN-ON VOUT 5mA/div
5V/div
10V/div 0
IOUT
0
ILX 200mA/div 0 400s/div 1.8 LOAD
100s/div
Pin Description
PIN MAX8570 (DFN) MAX8570/ MAX8572/ MAX8571/ MAX8573/ MAX8574 MAX8575 (SOT23) (SOT23) 1 -- NAME FUNCTION
3
FB
Feedback for Setting the Output Voltage. Connect FB to the center of a resistor voltage-divider from the output to GND to set positive output voltages. Output. The output voltage is preset to 15V. Connect a 1F ceramic capacitor from OUT to GND. In shutdown, OUT is pulled to GND by an internal 7.5M resistor. Ground Shutdown Input. A logic-low at SHDN places the part in low-power shutdown mode. Pull SHDN high or connect to VCC for normal operation. Inductor Switching Connection Isolation Switch Output. Internally connected to the drain of a p-channel MOSFET used to isolate the output from the input during shutdown. Connect a 4.7F ceramic capacitor from SW to GND. If true shutdown is not required, SW can be left open with the input supply connected directly to the inductor. Input Voltage Supply. Connect a 2.7V to 5.5V input supply to VCC. Connect a 1F ceramic capacitor from VCC to GND.
-- 2 1 6
-- 2 3 4
1 2 3 4
OUT GND SHDN LX
5
5
5
SW
4
6
6
VCC
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5
High-Efficiency LCD Boost with True Shutdown MAX8570-MAX8575
VCC SHDN SW LX
THERMAL SHUTDOWN
OUT (MAX8572/MAX8573/ MAX8575 ONLY)
ILIM
MAX8570- MAX8575
CONTROL LOGIC
FB (MAX8570/MAX8571/ MAX8574 ONLY) EA 1.226V GND
Figure 1. Functional Diagram
L1 22H D1 C3 4.7F VCC = 2.7V TO 5.5V C1 1F ON OFF SW LX VOUT = VCC TO 28V C2 1F C3 4.7F VCC = 2.7V TO 5.5V R2 SHDN GND ON OFF C1 1F SW LX L1 22H D1 VOUT = 15V C2 1F
VCC
MAX8570 MAX8571 MAX8574
C4 10pF FB
R1
VCC
MAX8572 MAX8573 MAX8575 OUT
SHDN
GND
Figure 2. Typical Application Circuit with Adjustable Output Voltage
L1 22H D1 SW LX VOUT = VBATT TO 28V C2 1F
Figure 3. Typical Application Circuit with 15V Preset Output Voltage
L1 22H D1 C3 4.7F VCC = 2.7V TO 5.5V C1 1F ON SW LX +VOUT C4 10pF FB C6 0.1F D3 -VOUT D2 SHDN OFF GND C5 1F R1 C2 1F R2
VBATT = 0.8V TO 28V C3 4.7F
VCC = 2.7V TO 5.5V C1 1F ON OFF
VCC
MAX8570 MAX8571 MAX8574
C4 10pF FB
R1
VCC
MAX8570 MAX8571 MAX8574
R2 SHDN GND
Figure 4. Using a Separate Input Supply for the Inductor
6
Figure 5. Negative Output Voltage for LCD Bias
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High-Efficiency LCD Boost with True Shutdown
Detailed Description
The MAX8570 family of compact, step-up DC-DC converters operate from a 2.7V to 5.5V supply. Consuming only 25A of supply current, these ICs include an internal MOSFET switch with a low on-resistance. A trueshutdown feature disconnects the battery from the load and reduces the supply current to 0.05A (typ). These DC-DC converters are available with either a fixed 15V output or are adjustable up to 28V. Three current-limit options are available: 110mA, 250mA, and 500mA. See the Selector Guide on page 1. With a typical step-up converter circuit, the output remains connected to the input through the inductor and output rectifier, holding the output voltage to one diode drop below VCC when the converter is shut down and allowing the output to draw power from the input. The MAX8570 family features true-shutdown mode, disconnecting the output from the input with an internal pchannel MOSFET switch when shut down. This eliminates power draw from the input during shutdown.
MAX8570-MAX8575
Soft-Start
The MAX8570 family uses two soft-start mechanisms. When the true-shutdown feature is used (SW is connected as in Figure 2 and Figure 3), the gate of the internal high-side p-channel switch turns on slowly to prevent inrush current. This takes approximately 200s. When SW is fully turned on, the internal n-channel switch begins boosting the input to set the output voltage. When VFB is less than 0.5V (with or without the use of true shutdown), the minimum off-time of the internal n-channel switch increases from 1s to 5s to control inrush current.
Control Scheme
The MAX8570 family features a minimum off-time current-limited control scheme operating in discontinuous mode. An internal p-channel MOSFET switch connects VCC to SW to provide power to the inductor when the converter is operating. When the converter is shut down, this switch disconnects the input supply from the inductor (see Figure 1). To boost the output voltage, an n-channel MOSFET switch turns on and allows current to ramp up in the inductor. Once this current reaches the current limit, the switch turns off and the inductor current flows through D1 to supply the output. The switching frequency varies depending on the load and input voltage and can be up to 800kHz.
Separate Power for Inductor
Separate power supplies can be used for the IC and the inductor. This allows power to be used from a battery or supply with a voltage as low as 0.8V, or higher than the VCC operating range of the converter. When using a separate inductor supply, SW is left unconnected and the supply is connected directly to the inductor (see Figure 4). Note that in this configuration the output is no longer disconnected from the input during shutdown. In shutdown the output voltage goes to a diode drop below the inductor supply voltage.
Setting the Output Voltage
The output voltage of the MAX8570, MAX8571, and MAX8574 is adjustable from VCC to 28V by using a resistor voltage-divider (see Figure 2). Select R2 from 10k to 600k and calculate R1 with the following equation: V R1 = R2 OUT - 1 VFB where VFB = 1.226V and VOUT can range from VCC to 28V. For best accuracy, ensure that the bias current through the feedback resistors is at least 2A. The MAX8572, MAX8573, and MAX8575 have a fixed 15V output. When using these parts, connect OUT directly to the output (see Figure 3).
Protection Features
The MAX8570 family has protection features designed to make it extremely robust to application errors (see Table 1). If the output capacitor in the application is missing, the MAX8570 family protects the internal switch from being damaged. If the top feedback resistor or the external diode is disconnected, the converter stops switching and the output is resistively loaded to ground. Similarly, if the external diode polarity is reversed, the converter discontinues switching. If the bottom feedback resistor is missing, the output stays at a diode drop less than the inductor supply voltage or 1.226V (whichever is greater). In fact, in response to most fault conditions, the MAX8570 family protects not only itself, but also the downstream circuitry.
Shutdown (SHDN)
Drive SHDN low to enter shutdown. During shutdown the supply current drops to 0.05A (typ), the output is disconnected from the input, and LX enters a highimpedance state. The capacitance and load at the output determine the rate at which VOUT decays. SHDN can be pulled as high as 6V regardless of the input and output voltages.
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7
High-Efficiency LCD Boost with True Shutdown MAX8570-MAX8575
Table 1. Protection Features
COMMON APPLICATION FAULTS OUT to FB resistor missing or disconnected. RESULT WITH COMPETING STEP-UP CONVERTERS OUT voltage rises until the output capacitor is destroyed and/or downstream components are damaged. RESULT WITH MAX8570 FAMILY
Converter stops switching. LX may boost one or two times before the FB voltage exceeds the trip point. In the rare case where the capacitive loading and external loading on OUT is small enough that the energy in one cycle can slew it more than 50V, the internal MOSFET will clamp between 35V and 70V (nondestructively). Converter stops switching and OUT is resistively loaded to GND. OUT is resistively loaded to GND and the converter stops switching. FB node driven above its regulation point, the converter stops switching, and OUT is resistively loaded to GND. True off-switch detects short, opens when current reaches pMOS current limit, and restarts soft-start. This protects the inductor and diode.
Output cap missing and FB open.
OUT voltage rises until the output capacitor is destroyed and/or downstream components are damaged.
FB shorted to GND. Diode missing or disconnected. Diode reverse polarity. FB node open.
OUT voltage rises until the output capacitor is destroyed and/or downstream components are damaged. Inductor energy forces LX node high, possibly damaging the internal switch. Unpredictable, possibly boosting output voltage beyond acceptable design range. Current ramps up through inductor and diode, generally destroying one of the devices.
OUT shorted to ground.
Design Procedure
Inductor Selection
Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Circuits using larger inductance values may provide more output power. The inductor's saturation current rating should be greater than the peak switching current. Recommended inductor values range from 10H to 100H.
value. See the Selector Guide on page 1 for selecting the IC with the correct current limit.
Diode Selection
The high switching frequency of up to 800kHz requires a high-speed rectifier. Schottky diodes are recommended due to their low forward-voltage drop. To maintain high efficiency, the average current rating of the diode should be greater than the peak switching current. Choose a reverse breakdown voltage greater than the output voltage.
Selecting the Current Limit
The peak LX current limit (ILX(MAX)) required for the application is calculated from the following equation:
POUT(MAX) POUT(MAX) POUT(MAX) + 1.25 x + 3s x VBATT(MIN) VBATT(MIN) L
2
Capacitors
Small ceramic surface-mount capacitors with X7R or X5R temperature characteristics are recommended due to their small size, low cost, low equivalent series resistance (ESR), and low equivalent series inductance (ESL). If nonceramic capacitors are used, it is important that they have low ESR to reduce the output ripple voltage and peak-peak load-transient voltage. For most applications, use a 1F ceramic capacitor for the output and VCC bypass capacitors. For SW or the inductor supply, a 4.7F or greater ceramic capacitor is recommended.
ILX(MAX) 1.25 x
where P OUT(MAX) is the maximum output power required by the load and VBATT(MIN) is the minimum supply voltage used to supply the inductor (this is VCC unless a separate supply is used for the inductor). The IC current limit must be greater than this calculated
8
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High-Efficiency LCD Boost with True Shutdown
For the MAX8570/MAX8571/MAX8574 a feed-forward capacitor (C4 in Figures 2 and 4) connected from the output to FB improves stability over a wide range of battery voltages. A 10pF capacitor is recommended for the MAX8571 and MAX8574. A 10pF to 47pF capacitor is recommended for the MAX8570. Note that increasing C4 degrades line and load regulation.
PC Board Layout
Careful printed circuit layout is important for minimizing ground bounce and noise. Keep the GND pin and ground pads for the input and output capacitors as close together as possible. Keep the connection to LX as short as possible. Locate the feedback resistors as close as possible to the FB pin and keep the feedback traces routed away from noisy areas such as LX. Refer to the MAX8571EVKIT for a layout example.
MAX8570-MAX8575
Applications Information
Negative Output Voltage for LCD Bias
A negative output voltage can be generated by adding a diode/capacitor charge pump as shown in Figure 5. In this configuration, the negative output is lower in magnitude than the positive output by a forward diode drop. If there is little or no load on the positive output, the negative output drifts from its nominal voltage. To prevent this, it may be necessary to preload the positive output with a few hundred microamps, which can be done by selecting lower than normal values of R1 and R2.
_______________________________________________________________________________________
9
High-Efficiency LCD Boost with True Shutdown MAX8570-MAX8575
Pin Configurations (continued)
TOP VIEW +
SHDN 1 GND 2 FB 3 6 LX SW VCC OUT 1 GND 2 SHDN 3
+
6 VCC SW LX
MAX8570
5 4
MAX8572 MAX8573 MAX8575
5 4
DFN
SOT23
Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 6L DFN 6 SOT23 PACKAGE CODE L622+1 U6SN+1 DOCUMENT NO. 21-0164 21-0058
10
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High-Efficiency LCD Boost with True Shutdown
Revision History
REVISION NUMBER 2 3 REVISION DATE 8/09 3/10 Added DFN package Added soldering temperature, corrected unit of measurement error, and updated figure reference DESCRIPTION PAGES CHANGED 1, 2, 5, 9, 10 2, 5, 9
MAX8570-MAX8575
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 ____________________ 11
(c) 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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