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19-2248; Rev 1; 12/03
KIT ATION EVALU BLE AVAILA
Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs
General Description
The MAX1878 dual power supply contains a step-down and step-up DC-DC converter in a small 12-pin QFN package for use in PDAs. The step-down DC-DC converter delivers over 500mA to an output as low as 1.25V for logic power. The step-up DC-DC converter delivers over 15mA and an output as high as 28V for a liquid crystal display (LCD). With an input voltage from 2.0V to 5.5V the MAX1878 is intended for use in systems powered by a 2-cell alkaline or 1-cell lithium-ion (Li+) battery. Fast switching frequency allows the use of small inductors and capacitors, and the low 19A typical quiescent current allows high efficiency when the system is in standby mode. Each output can be independently enabled. The MAX1878 is available in a small 1mm high 4mm x 4mm 12-pin QFN package and requires no external FETs. The MAX1878 evaluation kit is available to speed designs. Two Output Voltages Main Output: 1.25V to VIN LCD Output: Up to 28V 2.0V to 5.5V Input Range Low 19A Quiescent Supply Current 1A Shutdown Supply Current High Switching Frequency for Small External Components Small 1mm High 4mm x 4mm 12-Pin QFN Package
Features
Evaluation Kit Available to Speed Designs
MAX1878
________________________Applications
Personal Digital Assistants (PDA) Organizers/Translators MP3 Players GPS Receivers
PART MAX1878EGC
Ordering Information
TEMP RANGE -40C to +85C PINPACKAGE 12-QFN TOP MARK AAAO
__________Typical Operating Circuit
INPUT 2.0V TO 5.5V 10H
Pin Configuration
PGNDLCD 11
TOP VIEW
IN LXLCD FBLCD AIN1 AIN2 10H LCD OUTPUT UP TO 28V 0.1F
12 IN 1 2
ONLCD 10 9 8 7 LXLCD AGND ON 6 FBLCD
MAX1878
LX FB
MAIN OUTPUT 1.25V TO VIN
LX AIN1
ON MAIN LCD OFF ON OFF ONLCD ON
3
PGND 4 AIN2
MAX1878EGC
5 FB
PGND
AGND PGNDLCD
4mm x 4mm QFN
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
ABSOLUTE MAXIMUM RATINGS
FB, FBLCD, AIN1, AIN2, ON, ONLCD to AGND ......-0.3V to +6V AIN2 to AIN1..........................................................-0.3V to +0.3V AIN1, AIN2 to IN ....................................................-0.3V to +0.3V IN to PGND...............................................................-0.3V to +6V LX to PGND .................................................-0.3V to (VIN + 0.3V) LXLCD to PGNDLCD..............................................-0.3V to +30V PGND, PGNDLCD to AGND..................................-0.3V to +0.3V LX Current .........................................................................800mA LXLCD Current..................................................................500mA Continuous Power Dissipation (TA = +70C) 12-Pin QFN (derate 16.9mW/C above +70C) .............1.35W Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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 = VAIN = 2.5V, circuit of Figure 1, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER GENERAL Input Voltage Range Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis Quiescent Current Shutdown Quiescent Current MAIN OUTPUT (Step-Down Converter) Output Voltage Adjustment Range FB Regulation Voltage FB Input Bias Current Main Output Current (Note 1) Line Regulation Load Regulation VMAIN VFB IFB IMAIN VIN = VAIN = 2V VIN = VAIN = 2V VMAIN = 1.8V VIN = VAIN = 2.5V VIN = VAIN = 2.0V 250 200 +25C to +85C 0C to +85C 1.25 1.225 1.220 10 500 350 1 1 150 mV 100 100 0.1 0.55 0.42 5 0.95 0.65 % A 1.250 VIN 1.275 1.280 50 V V nA mA % % IAIN1 + IAIN2 VFB = VFBLCD = 1.30V, VONLCD = 0, step-down converter only VFB = VFBLCD = 1.30V VON = VONLCD = 0 VIN, VAIN VUVLO VIN rising VIN falling 1.7 2.0 1.92 1.82 100 19 24 0 30 38 1 A 5.5 2.0 V V mV A SYMBOL CONDITIONS MIN TYP MAX UNITS
ILOAD = 150mA, VIN = VAIN = 2V to 3V, FB = GND VIN = VAIN = 2.5V, ILOAD = 10mA to 150mA VIN = VAIN = 2V, ILOAD = 150mA, VFB = 0.8V VIN = VAIN = 3V, ILOAD = 150mA, VFB = 0.8V
Dropout Voltage
LX Max Duty Cycle LX Leakage Current LX P-Channel On-Resistance
VFB = 0.8V VON = 0, VIN = 5.5V VIN = VAIN = 2V, ILX = 300mA VIN = VAIN = 3V, ILX = 300mA
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Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VAIN = 2.5V, circuit of Figure 1, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER LX N-Channel On-Resistance LX Current Limit Idle Mode Threshold LX Minimum On-Time LX Minimum Off-Time ON Input Low Voltage ON Input High Voltage ON Input Leakage Current LCD OUTPUT (Step-Up Converter) LCD Output Voltage Adjust Range FBLCD Regulation Voltage LXLCD On-Resistance LXLCD Current Limit LXLCD Leakage Current LCD Output Current (Note 2) FBLCD Input Bias Current LCD Line Regulation LCD Load Regulation LXLCD Maximum On-Time LXLCD Minimum Off-Time ONLCD Input Low Voltage ONLCD Input High Voltage ONLCD Input Leakage Current tLXLCDON tLXLCDOFF VFBLCD < 0.9V (soft-start) 2V < VAIN = VIN < 5.5V 2V < VAIN = VIN < 5.5V 1.3 -1 1 ILCD IFBLCD VLXLCD = 28V VAIN = VIN = 2.5V, VLCD = 18V VAIN = VIN = 2V, VLCD = 18V VAIN = VIN = 2V VAIN = VIN = 2V to 3V, ILOAD = 5mA, VLXLCD = 18V VAIN = VIN = 2.5V, ILOAD = 1mA to 5mA, VLXLCD = 18V 5.1 0.5 1.3 1.5 1.4 VLCD VFBLCD VIN = VAIN = 2V +25C to +85C 0C to +85C VIN + 1V 1.225 1.220 2.8 1.7 140 280 0 7.6 6.6 10 1 1.3 9.8 1.0 2.6 17 1.7 4.4 0.4 50 1.250 28 1.275 1.280 5.0 3.0 440 1 V V mA A mA nA % % s s V V A tLXON tLXOFF 2V < VIN < 5.5V 2V < VIN < 5.5V 1.3 -1 1 SYMBOL CONDITIONS VIN = VAIN = 2V, ILX = 300mA VIN = VAIN = 3V, ILX = 300mA 330 70 240 200 MIN TYP 0.62 0.46 550 135 440 390 MAX 0.93 0.65 800 220 740 670 0.4 UNITS mA mA ns ns V V A
MAX1878
VAIN = VIN = 2V, ILXLCD = 150mA VAIN = VIN = 3V, ILXLCD = 150mA
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Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
ELECTRICAL CHARACTERISTICS
(VIN = VAIN = 2.5V, circuit of Figure 1, TA = -40C to +85C, unless otherwise noted.) (Note 3)
PARAMETER GENERAL Quiescent Current from AIN FB Regulation Voltage LX Current Limit LX Minimum On-Time LX Minimum Off-Time LCD OUTPUT (Step-Up Converter) LXLCD Current Limit LXLCD Maximum On-Time LXLCD Minimum Off-Time FBLCD Regulation Voltage tLXLCDON tLXLCDOFF VFBLCD VFBLCD < 0.9V VAIN = VIN = 2V 130 5.1 0.5 1.3 1.212 450 17 1.7 4.5 1.288 mA s s V tLXON tLXOFF IAIN VFB VFB = VFBLCD = 1.30V VAIN = VIN = 2V 1.212 310 240 200 38 1.288 820 740 670 A V mA ns ns MAIN OUTPUT (Step-Down Converter) SYMBOL CONDITIONS MIN MAX UNITS
Note 1: Main output current is guaranteed by LX current limit, LX on resistance, and LX minimum off-time. Note 2: LCD output current is guaranteed by LXLCD current limit, LXLCD on-resistance, and LXLCD minimum off-time, starting into a resistive load. Note 3: Specifications to -40C are guaranteed by design and not production tested.
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Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
Typical Operating Characteristics
(VIN = VAIN = 2.5V, circuit of Figure 1, TA = +25C, unless otherwise noted.)
STEP-DOWN CONVERTER EFFICIENCY vs. LOAD CURRENT (VMAIN = 1.8V)
MAX1878 toc01
STEP-DOWN CONVERTER EFFICIENCY vs. LOAD CURRENT (VMAIN = 1.5V)
MAX1878 toc02
STEP-UP CONVERTER EFFICIENCY vs. LOAD CURRENT (VLCD = 18V)
VIN = 5.0V (22H)
MAX1878 toc03
100 VIN = 2.5V 90 EFFICIENCY (%) 80 VIN = 5.0V 70 60 50 40 0.1 1 10 100 ONLCD = PGNDLCD VIN = 3.6V
100 90 EFFICIENCY (%) 80 70 VIN = 3.6V 60 50 40 ONLCD = PGNDLCD 0.1 1 10 100 VIN = 5.0V VIN = 2.5V
100 90 80 70 60 50 40 30 ON = PGND 0.01 0.1 1 LOAD CURRENT (mA) 10 VIN = 2.5V (10H) VIN = 3.6V (15H)
1000
1000
EFFICIENCY (%)
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
STEP-DOWN CONVERTER OUTPUT VOLTAGE vs. LOAD CURRENT (VMAIN = 1.8V)
MAX1878 toc04
STEP-DOWN CONVERTER OUTPUT VOLTAGE vs. LOAD CURRENT (VMAIN = 1.5V)
MAX1878 toc05
STEP-UP CONVERTER OUTPUT VOLTAGE vs. LOAD CURRENT (VLCD = 18V)
18.8 18.7 18.6 VMAIN (V) 18.5 18.4 18.3 18.2 18.1 18.0 17.9 17.8 VIN = 2.5V (10H) 0.01 0.1 1 LOAD CURRENT (mA) 10 100 VIN = 3.6V (15H) VIN = 5.0V (22H) ONLCD = PGNDLCD
MAX1878 toc06
1.84 VIN = 5.0V 1.83 1.82 VMAIN (V) 1.81 1.80 VIN = 3.6V 1.79 1.78 1.77 ONLCD = PGNDLCD 0.1 1 10 100 VIN = 2.5V
1.535 1.530 1.525 VMAIN (V) 1.520 1.515 1.510 1.505 1.500 1.495
VIN = 5.0V
18.9
VIN = 3.6V VIN = 2.5V ONLCD = PGNDLCD 0.1 1 10 100 1000 LOAD CURRENT (mA)
1000
LOAD CURRENT (mA)
NO LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (VMAIN = 1.8V, VLCD = 18V)
MAX1878 toc07
STEP-DOWN CONVERTER SWITCHING FREQUENCY vs. SUPPLY VOLTAGE
MAX1878 toc08
STEP-UP CONVERTER CURRENT LIMIT vs. INPUT VOLTAGE
MAX1878 toc09
140 STEP-UP AND STEP-DOWN 120 SUPPLY CURRENT (A) 100 STEP-UP 80 60 40 20 0 0 1 2 3 4 5 6 INPUT VOLTAGE (V) STEP-DOWN
1.2 SWITCHING FREQUENCY (MHz) 1.0 0.8 VMAIN = 1.5V 0.6 0.4 0.2 0 2 3 4 SUPPLY VOLTAGE (V) VMAIN = 1.8V
500 450 CURRENT LIMIT (mA) L2 = 22H 400 350 300 L2 = 10H 250 200
L2 = 15H
IMAIN = 150mA ONLCD = PGNDLCD 5 6
2
3
4 5 SUPPLY VOLTAGE (V)
6
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Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
Typical Operating Characteristics (continued)
(VIN = VAIN = 2.5V, circuit of Figure 1, TA = +25C, unless otherwise noted.)
STEP-DOWN LIGHT-LOAD SWITCHING WAVEFORMS
MAX1878 toc10
STEP-DOWN HEAVY-LOAD SWITCHING WAVEFORMS
MAX1878 toc11
STEP-UP LIGHT-LOAD SWITCHING WAVEFORMS
MAX1878 toc12
VMAIN AC-COUPLED 50mV/div
VMAIN AC-COUPLED 20mV/div
VOUT AC-COUPLED 100mV/div
VLX 1V/div 0 4s/div IMAIN = 20mA, VMAIN = +1.8V, VIN = +2.5V, ONLCD = PGNDLCD
VLX 1V/div 0 1s/div IMAIN = 250mA, VMAIN = +1.8V, VIN = +2.5V, ONLCD = PGNDLCD 1s/div ILCD = 2mA, VLCD = +18V, VIN = +2.5V, ON = PGND
VLXLCD 10V/div 0
STEP-UP HEAVY-LOAD SWITCHING WAVEFORMS
MAX1878 toc13
STEP-DOWN LOAD TRANSIENT RESPONSE
MAX1878 toc14
STEP-UP LOAD TRANSIENT RESPONSE
MAX1878 toc15
IMAIN 200mA/div VOUT AC-COUPLED 100mV/div
ILCD 2mA/div
VMAIN AC-COUPLED 100mV/div VLX 2V/div
VLCD AC-COUPLED 200mV/div VLXLCD 10V/div
VLXLCD 10V/div 0 1s/div ILCD = 4.5mA, VLCD = +18V, VIN = +2.5V, ON = PGND 10s/div IMAIN = 10mA to 250mA, VMAIN = +1.8V, VIN = +2.5V, ONLCD = PGNDLCD
20s/div ILCD = 1mA to 4mA, VLCD = +18V, VIN = +2.5V, ON = PGND
LINE TRANSIENT RESPONSE
MAX1878 toc16
SOFT-START AND SHUTDOWN RESPONSE
MAX1878 toc17
VIN 3V TO 2V
IIN 200mA/div
VLCD AC-COUPLED 200mV/div VMAIN AC-COUPLED 20mV/div
VLCD 10V/div VMAIN 1V/div VON = VONLCD 5V/div 400s/div RMAIN = 5.1, RLCD = 9.09k
100s/div VMAIN = +1.8V, IMAIN = 150mA, VLCD =18.0V, ILCD = 2.5mA
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Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs
Pin Description
PIN 1 2 NAME IN LX FUNCTION Step-Down Converter Power Input. Connect IN to the step-down converter power source. Bypass IN to PGND with a 10F or greater low-ESR capacitor. Step-Down Converter Switching Node. Connect LX to the step-down converter output LC filter. LX swings between IN and PGND. Analog Input Power 1. AIN1 supplies power to the MAX1878 internal circuitry. Connect AIN1 to the 2.0V to 5.5V input power source. Bypass AIN1 to AGND with a 1F or greater low-ESR capacitor. Analog Input Power 2. Connect AIN1 and AIN2 together as close to the MAX1878 as possible. Step-Down Converter Feedback Input. Connect a resistive voltage-divider from the step-down converter output voltage to FB. The regulation threshold is 1.25V at FB. LCD Step-Up Converter Feedback Input. Connect a resistive voltage-divider from the step-up converter output voltage to FBLCD. The regulation threshold is 1.25V at FBLCD. Step-Down Converter On/Off Input. Drive ON high to turn on the step-down converter. Drive ON low to turn off the converter. For automatic startup, connect ON to AIN1. Analog (Low-Noise) Ground. The exposed pad and the corner tabs on the QFN package are internally connected to analog ground. See the PC Board Layout and Grounding section. LCD Step-Up Converter Switching Node. Connect LXLCD to the step-up converter inductor and rectifier. LCD Step-Up Converter On/Off Input. Drive ONLCD high to turn on the step-up converter. Drive ONLCD low to turn off the converter. For automatic startup, connect ONLCD to AIN1. LCD Step-Up Converter Power Ground. PGNDLCD is the source of the step-up converter's internal N-channel MOSFET switch. Connect PGNDLCD to PGND as close to the MAX1878 as possible. Power Ground. PGND is the source of the step-down converter's internal N-channel MOSFET synchronous rectifier. Connect PGND to PGNDLCD as close to the MAX1878 as possible.
MAX1878
3 4 5 6 7 8 9 10
AIN1 AIN2 FB FBLCD ON AGND LXLCD ONLCD
11
PGNDLCD
12
PGND
Detailed Description
The MAX1878 step-down and step-up DC-DC converter operates from a 2.0V to 5.5V supply. Consuming only 19A of quiescent supply current, the main stepdown converter delivers over 500mA to an output as low as 1.25V and the LCD step-up converter delivers over 15mA and an output as high as 28V. The MAX1878 uses a unique proprietary current-limited control scheme that provides excellent performance and high efficiency.
Step-Down Converter Control Scheme
The MAX1878 step-down converter uses a proprietary, current-limited control scheme to ensure high efficiency, fast transient response, and physically small external components. This control scheme is simple: when the output voltage is out of regulation, the error comparator
begins a switching cycle by turning on the high-side switch. This switch remains on until the minimum ontime of 440ns expires and the output voltage regulates or the current-limit threshold is exceeded. Once off, the high-side switch remains off until the minimum off-time of 390ns expires and the output voltage falls out of regulation. During this period, the low-side synchronous rectifier turns on and remains on until either the high-side switch turns on again or the inductor current approaches zero. The internal synchronous rectifier eliminates the need for an external Schottky diode. This control scheme allows the MAX1878 step-down converter to provide excellent performance throughout the entire load-current range. When delivering light loads, the high-side switch turns off after the minimum on-time and after the inductor current reaches the 135mA ideal mode threshold to reduce peak inductor
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_______________________________________________________________________________________
Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
current, resulting in increased efficiency and reduced output voltage ripple. When delivering medium and higher output currents, the MAX1878 extends either the on-time or the off-time, as necessary to maintain regulation, resulting in nearly constant frequency operation with high efficiency and low output voltage ripple. enters a high-impedance state and the output remains connected to the input through the inductor and rectifier holding the output voltage to a diode drop below VIN. The LCD output capacitance and load determine the rate at which V LCD decays. Connect ON and ONLCD to IN for normal operation.
Step-Up Converter Control Scheme
The MAX1878 step-up converter features a minimum off-time, current-limited control scheme. The duty cycle is governed by a pair of one-shots that set a minimum off-time and a maximum on-time. The switching frequency can be up to 500kHz and depends upon the load and input voltage. The peak current limit of the internal N-channel MOSFET is 280mA.
Soft-Start
The MAX1878 internal soft-start circuitry limits current drawn at startup, reducing transients on the input source. Soft-start is particularly useful for higher impedance input sources, such as lithium ion and alkaline cells. Step-down converter soft-start is implemented with current limit. At startup the step-down converter current limit is set to 25% of its full current limit. The current limit is increased by 25% every 256 switching cycles until full current limit is reached. Step-up converter soft-start is implemented with LXLCD minimum off-time. At startup the LXLCD minimum off-time is 2.6s allowing the LCD output voltage to build up gradually. When the output reaches approximately 80% of its final output voltage the LXLCD minimum offtime is decreased to its final value of 1s. See Soft-Start and Shutdown Response in the Typical Operating Characteristics section.
On/Off Control
Pulling ON low places the MAX1878 step-down converter in shutdown mode and reduces step-down converter supply current to less than 1A. In shutdown, the internal switching MOSFETs and synchronous rectifier turn off and LX goes high impedance. Pulling ONLCD low places the MAX1878 step-up converter in shutdown mode and reduces step-up converter supply current to less than 1A. In shutdown, LXLCD
VIN 2.0V TO 3.3V* TWO SERIES ALKALINE CELLS C1 10F 1 R5 10 3 4 C2 1F AIN1 AIN2 IN
L2 10H*
D1 LCD OUTPUT UP TO 28V 9 6 R4 270k L1 10H C5 5pF R3 3.6M C3 0.1F
LXLCD FBLCD
MAX1878
LX
2 C6 20pF C4 22F
MAIN OUTPUT 1.25V TO VIN R1 28k
ON MAIN LCD OFF ON OFF
7 10
R6 2M ON FB ONLCD 5
R2 63.4k
PGND 12 *FOR INPUT VOLTAGES GREATER THAN 3.3V USE A HIGHER VALUE INDUCTOR L2. SEE INDUCTOR SELECTION
AGND PGNDLCD 8 11
Figure 1. MAX1878 Standard Application Circuit 8 _______________________________________________________________________________________
Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs
Design Procedure
Setting the Output Voltage
Set the MAX1878 step-down converter output voltage by connecting a resistive voltage-divider from VMAIN to FB (Figure 1). Select an R2 from 30k to 300k. Calculate R1 with the following equation: R1 = R2 xR6(VMAIN - VFB ) VFB (R6 + R2) - VMAIN xR2
MAX1878
V R3 = R4 LCD - 1 VFBLCD where VFBLCD = 1.25V and VLCD may range from (VIN + 1V) to 28V. The FB and FBLCD input bias currents are a maximum of 50nA. These small bias currents allow for large-value feedback resistors that improve light-load efficiency. For less than 1% output voltage error due to bias current, feedback resistors should be chosen such that the current through R2 is 100 times greater than IFB and the current through R4 is 100 times greater than IFBLCD.
where VFB = 1.25V, R6 = 2M and VMAIN may range from 1.25V to VIN. Set the MAX1878 step-up converter output voltage by connecting a resistive voltage-divider from V LCD to FBLCD (Figure 1). Select an R4 from 30k to 300k. Calculate R3 with the following equation:
VIN
1 IN
3 AIN1
4 AIN2
MAX1878
LXLCD 9 CURRENT LIMIT N P VMAIN 2 LX CONTROL LOGIC N TIMERS VLCD
CURRENT LIMIT
5 FB
FBLCD 6
SOFTSTART ON OFF 7 ON ONLCD 10 ON OFF
PGND 12
AGND 8
PGNDLCD 11
Figure 2. Simplified Functional Diagram _______________________________________________________________________________________ 9
Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
Inductor Selection
The MAX1878 is optimized to use a 10H inductor over the entire operating range. Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Circuits using larger inductance values may startup at lower input voltages and exhibit less ripple, but also provide reduced output power. This occurs when the inductance is sufficiently large to prevent the maximum current limit from being reached before the maximum on-time expires. The inductor's saturation current rating should be greater than the peak switching current. However, it is generally acceptable to bias the inductor into saturation by as much as 20%, although this will slightly reduce efficiency. Choose a low DC-resistance inductor to improve efficiency. For the above reasons choose the step-up converter inductor in the range of 10H to 33H depending on the input voltage (4H per volt of VIN). 10F to 47F tantalum capacitor to PGND. Choose a capacitor with 200m to 300m ESR to provide stable switching while minimizing output ripple. A 22F filter capacitor works well for most applications.
Ripple Regulation
For proper switching control the ripple at FB and FBLCD must be greater than 25mV. Use R6 and C6 as shown in Figure 1 to inject ripple into FB. To insure sufficient ripple on FBLCD, connect C5 as shown in Figure 1.
PC Board Layout and Grounding
High switching frequencies make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Connect the inductors, input filter capacitors, and output filter capacitors as close to the device as possible, and keep their traces short, direct, and wide. The external voltage-feedback networks should be very close to the feedback pins, within 0.2 inches (5mm). Keep noisy traces, such as LX and LXLCD, away from the voltage feedback networks; also keep them separate, using grounded copper. The exposed backside pad and corner tabs of the QFN package are internally connected to analog ground. For heat dissipation, connect the exposed backside pad to a large analog ground plane, preferably on a surface of the board that receives good airflow. Connect all power grounds and all analog grounds to separate ground planes in a star ground configuration. Connect the analog ground plane and the power ground plane together at a single point. The MAX1878 evaluation kit data sheet includes a proper PC board layout and routing scheme.
Step-Up Converter Diode Selection
The high maximum switching frequency of 500kHz requires a high-speed rectifier such as the 1N4148. 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. A Schottky diode is not recommended as the lower forward voltage does little to improve efficiency whereas the higher reverse leakage current decreases efficiency.
Input Bypass Capacitors
Bypass VIN with a 10F low-ESR surface-mount ceramic capacitor to PGND and PGNDLCD as close to the IC as possible. This input bypass capacitor reduces peak currents and noise at the input voltage source. Connect AIN1 and AIN2 together and bypass with a low-ESR 1F surface-mount ceramic capacitor to AGND. A low resistance (10) from IN to AIN1 and AIN2 creates a lowpass RC filter and provides low-noise analog input power to the MAX1878.
Chip Information
TRANSISTOR COUNT: 4131 EXPOSED PADDLE CONNECTED TO AGND PROCESS: BiCMOS
Output Filter Capacitors
The MAX1878 is a voltage mode converter and requires ripple at FB and FBLCD for stable regulation. For most applications, bypass VLCD with a 0.1F small ceramic surface-mount capacitor to PGNDLCD. For small ceramic capacitors, the output ripple voltage is dominated by the capacitance value. If tantalum or electrolytic capacitors are used, the higher ESR increases the output ripple voltage. Decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage. Surface-mount capacitors are generally preferred because they lack the inductance and resistance of their through-hole equivalents. Bypass VMAIN with a
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Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs
Package Information
12,16,20, 24L QFN.EPS
MAX1878
PACKAGE OUTLINE 12,16,20,24L QFN, 4x4x0.90 MM
21-0106
E
1 2
______________________________________________________________________________________
11
Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs MAX1878
Package Information (continued)
PACKAGE OUTLINE 12,16,20,24L QFN, 4x4x0.90 MM
21-0106
E
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 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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