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19-2831; Rev 0; 4/03
KIT ATION EVALU ABLE AVAIL
Complete Power IC for Low-Cost PDAs
General Description Features
o Minimum External Components o 4 Linear Regulator Outputs Main LDO 3.3V, 300mA SD Card Output 3.3V, 200mA Core LDO 1.5V, 200mA 2nd Core LDO 1.8V, 30mA o High-Efficiency LCD Step-Up DC-to-DC Output Up to 28V LCD 0V True Shutdown When Off o 50A Quiescent Supply Current o 1.3W Thin QFN Package
MAX1552
The MAX1552 is a complete power-management chip for low-cost personal digital assistants (PDAs) and portable devices operating from a 1-cell lithium-ion (Li+), or 3-cell NiMH, battery. It includes all the regulators, outputs, and voltage monitors necessary for small PDAs while requiring a bare minimum of external components. This device features four linear regulators, a step-up DC-to-DC converter for LCD bias, a microprocessor reset output, and low battery detection in a miniature QFN package. For a compatible Li+ charger for both USB and AC adapter inputs, refer to the MAX1551*. The four linear regulators feature PMOS pass elements for efficient low-dropout operation. The MAIN LDO supplies 3.3V at over 300mA. An SD card slot output supplies 3.3V at 200mA. The COR1 LDO outputs 1.5V at 200mA and the COR2 LDO supplies 1.8V at 20mA. The SD output and COR2 LDOs have pin-controlled shutdown. For other output voltage combinations, contact Maxim. The step-up DC-to-DC converter features an on-board MOSFET and true shutdown when off. This means that during shutdown, input power is disconnected from the inductor so the boost output falls to 0V rather than remaining one diode drop below the input voltage. A P reset output clears when the input voltage rises to 3.4V to ensure an orderly start. A low-battery output warns the system of impending power loss for safe shutdown. Thermal shutdown protects the die from overheating. The MAX1552 operates from a 3.1V to a 5.5V supply voltage and consumes 50A no-load supply current. It is packaged in a 1.3W, 16-pin thin QFN with a power pad on the underside of the package. The MAX1552 is specified for operation from -40C to +85C.
Ordering Information
PART MAX1552ETE TEMP RANGE -40C to +85C PIN-PACKAGE 16 Thin QFN
Typical Application Circuit
VIN IN SWIN SDIG MAX1552 REF COR1 1.5V, 200mA 3.3V, 200mA MAIN 3.3V, 300mA
Applications
SDIG
COR2 OFF ON ENSD ENC2 ENLCD D1 MAIN LX SW
1.8V, 20mA
PDAs Organizers Cellular and Cordless Phones MP3 Players Hand-Held Devices
COR2 OFF ON LCD OFF ON
LCD 20V, 1mA
LFB RESET OUT RS LBO GND
Pin Configuration appears at end of data sheet. *Protected by U.S. Patent #6,507,172.
LOW BATT OUT
________________________________________________________________ Maxim Integrated Products
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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.
Complete Power IC for Low-Cost PDAs MAX1552
ABSOLUTE MAXIMUM RATINGS
IN, SWIN, ENSD, ENC2, ENLCD, RS, LBO, SDIG, to GND.............................................-0.3V to +6V LX to GND ..............................................................-0.3V to +30V MAIN, COR1, COR2, REF, LFB to GND......-0.3V to (VIN + 0.3V) SWIN to IN .............................................................-0.3V to +0.3V Current into LX or SWIN ............................................300mARMS Current Out of SW .....................................................300mARMS Output Short-Circuit Duration.....................................Continuous Continuous Power Dissipation (TA = +70C) 16-Pin Thin QFN (derate 16.9mW/C above +70C) ...1.349W 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 = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER GENERAL IN, SWIN Voltage Range RS and Complete Shutdown Threshold RS Release and Restart Threshold LBO and Sleep Threshold LBO Release and Restart to Full On IN, SWIN Operating Current--All On IN Operating Current--All On Except LCD IN Operating Current--MAIN and COR1 On IN, SWIN Operating Current--Shut Down LDOs MAIN Output Voltage MAIN Current Limit MAIN Dropout Voltage SDIG Output Voltage SDIG Current Limit SDIG Dropout Voltage SDIG Reverse Leakage Current COR1 Output Voltage COR1 Current Limit COR2 Output Voltage COR2 Current Limit ILOAD = 100A to 20mA, VIN = 3.6V to 5.5V ILOAD = 1mA ILOAD = 200mA VSDIG = 5V, ENSD = VIN = GND ILOAD = 100A to 200mA, VIN = 3.6V to 5.5V 1.4625 250 1.755 30 ILOAD = 1mA ILOAD = 300mA ILOAD = 100A to 200mA, VIN = 3.6V to 5.5V 3.2175 250 ILOAD = 100A to 300mA, VIN = 3.6V to 5.5V 3.2175 350 3.3 650 1 210 3.3 310 1 170 7 1.5 450 1.8 50 300 15 1.5375 800 1.845 100 310 3.3825 390 3.3825 1200 V mA mV V mA mV A V mA V mA Operating VIN falling VIN rising VIN falling VIN rising VLFB = 1.3V ENLCD = GND ENLCD = ENC2 = ENSD = GND, LDO loads = 0A VSWIN = VIN = 2.9V 3.1 2.96 3.4025 3.55 3.75 3 3.4 3.6 3.8 100 90 50 2 5.5 3.04 3.4400 3.65 3.85 125 110 65 10 V V V V V A A A A CONDITIONS MIN TYP MAX UNITS
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Complete Power IC for Low-Cost PDAs
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER LCD LX Voltage Range LX Current Limit LX On-Resistance LX Leakage Current Maximum LX On-Time Minimum LX Off-Time LFB Feedback Threshold LFB Input Bias Current SW Off-Leakage Current SW PMOS On-Resistance SW PMOS Peak Current Limit SW PMOS Average Current Limit Soft-Start Time LOGIC IN AND OUT EN_ Input Low Level EN_ Input High Level EN_ Input Leakage Current RS, LBO Output Low Level RS, LBO Output High Leakage THERMAL PROTECTION Thermal-Shutdown Temperature Thermal-Shutdown Hysteresis Rising temperature 160 15 C C Sinking 1mA, VIN = 2.5V VOUT = 5.5V VIN = 3.0V to 5.5V VIN = 3.0V to 5.5V 1.4 0.01 0.25 1 0.4 1 0.4 V V A V A CSW = 1F VLFB = 1.3V SW = GND, VSWIN = 5.5V, ENLCD = GND VLFB > 1.1V VLFB < 0.8V (soft-start) VLX = 28V 8 0.8 4.0 1.23 11 1 5 1.25 5 0.01 1 750 300 0.13 L1 = 10H 210 250 1.7 2 14 1.2 6.0 1.27 100 1 28 275 V mA A s s V nA A mA mA ms CONDITIONS MIN TYP MAX UNITS
MAX1552
ELECTRICAL CHARACTERISTICS
(VIN = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER GENERAL IN, SWIN Voltage Range RS and Complete Shutdown Threshold RS Release and Restart Threshold LBO and Sleep Threshold LBO Release and Restart to Full On IN, SWIN Operating Current--All On IN Operating Current--All On Except LCD IN Operating Current--MAIN and COR1 On IN, SWIN Operating Current--Shut Down Operating VIN falling VIN rising VIN falling VIN rising VLFB = 1.3V ENLCD = GND ENLCD = ENC2 = ENSD = GND, LDO loads = 0A VSWIN = VIN = 2.925V 3.1 2.96 3.36 3.525 3.725 5.5 3.04 3.44 3.675 3.875 125 110 65 10 V V V V V A A A A CONDITIONS MIN MAX UNITS
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Complete Power IC for Low-Cost PDAs MAX1552
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER LDOs MAIN Output Voltage MAIN Current Limit MAIN Dropout Voltage SDIG Output Voltage SDIG Current Limit SDIG Dropout Voltage SDIG Reverse Leakage Current COR1 Output Voltage COR1 Current Limit COR2 Output Voltage COR2 Current Limit LCD LX Voltage Range LX Current Limit LX Leakage Current Maximum LX On-Time Minimum LX Off-Time LFB Feedback Threshold LFB Input Bias Current SW Off-Leakage Current LOGIC IN AND OUT EN_ Input Low Level EN_ Input High Level EN_ Input Leakage Current RS, LBO Output Low Level RS, LBO Output High Leakage Sinking 1mA, VIN = 2.5V VOUT = 5.5V VIN = 3.0V to 5.5V VIN = 3.0V to 5.5V 1.4 1 0.4 1 0.4 V V A V A VLFB = 1.3V SW = GND, VSWIN = 5.5V, ENLCD = GND VLFB > 1.1V VLFB < 0.8V (soft-start) VLX = 28V 8 0.8 4.0 1.22 200 28 275 2 14 1.2 6.0 1.27 100 1 V mA A s s V nA A ILOAD = 100A to 20mA, VIN = 3.6V to 5.5V ILOAD = 200mA VSDIG = 5V, ENSD = VIN = GND ILOAD = 100A to 20mA, VIN = 3.6V to 5.5V 1.4625 250 1.755 30 ILOAD = 300mA ILOAD = 100A to 200mA, VIN = 3.6V to 5.5V 3.2175 250 ILOAD = 100A to 300mA, VIN = 3.6V to 5.5V 3.2175 350 3.3825 1200 310 3.3825 390 300 15 1.5375 800 1.845 100 V mA mV V mA mV A V mA V mA CONDITIONS MIN MAX UNITS
Note 1: Specifications to -40C are guaranteed by design and not production tested.
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Complete Power IC for Low-Cost PDAs
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1552
MAIN DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1552 toc01
SDIG DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1552 toc02
MAIN OUTPUT VOLTAGE vs. LOAD CURRENT
3.25 OUTPUT VOLTAGE (V) 3.00 2.75 2.50 2.25 2.00
MAX1552 toc03
500
250
3.50
DROPOUT VOLTAGE (mV)
DROPOUT VOLTAGE (mV)
400
200
300
150
200
100
100
50 1.75
0 0 100 200 300 400 500 ILOAD (mA)
0 0 50 100 150 200 250 ILOAD (mA)
1.50 0 100 200 300 400 500 600 ILOAD (mA)
SDIG OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1552 toc04
COR1 OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1552 toc05
COR2 OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1552 toc06
3.50 3.25 OUTPUT VOLTAGE (V) 3.00 2.75 2.50 2.25 2.00
1.75
2.00 1.75 OUTPUT VOLTAGE (V) 1.50 1.25 1.00 0.75 0.50
1.50 OUTPUT VOLTAGE (V)
1.25
1.00
0.75 1.75 1.50 0 50 100 150 200 250 300 350 ILOAD (mA) 0.50 0 100 200 ILOAD (mA) 300 400
0
10
20
30 ILOAD (mA)
40
50
LOAD STEP RESPONSE MAIN
MAX1552 toc07
LOAD STEP RESPONSE COR1
MAX1552 toc08
INPUT CURRENT vs. INPUT VOLTAGE
70 60 50 IIN (A) 40 VIN RISING 30 20 10 0 VIN FALLING
MAX1552 toc09
80 VMAIN AC-COUPLED 50mV/div VCOR1 AC-COUPLED 20mV/div
ILOAD 100mA/div
ILOAD 100mA/div
40s/div
40s/div
0
1
2
3 VIN (V)
4
5
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Complete Power IC for Low-Cost PDAs MAX1552
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
LCD SWITCH WAVEFORM
MAX1552 toc10
ENABLE RESPONSE TO ENSD
MAX1552 toc11
VIN AC-COUPLED 20mV/div
RL = 30 CL = 4.7F ENSD 2V/div
LX 10V/div
SDIG 1V/div
LCD AC-COUPLED 20mV/div 2s/div 200s/div
ENABLE RESPONSE TO LCD
MAX1552 toc12
EFFICIENCY vs. LOAD CURRENT
MAX1552 toc13
85
ENLCD 5V/div EFFICIENCY (%)
80 VLCD = 18V 75 VLCD = 15V
LCD BOOST SOFT-START SW TURN-ON LCD 2V/div
70
65
60 400s/div 0 1 2 3 4 5 ILOAD (mA)
LCD OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1552 toc14
LCD OUTPUT VOLTAGE vs. INPUT VOLTAGE
18.75 OUTPUT VOLTAGE (V) 18.50 18.25 18.00 17.75 17.50 17.25 17.00
MAX1552 toc15
19.00 18.75 OUTPUT VOLTAGE (V) 18.50 18.25 18.00 17.75 17.50 17.25 17.00 0 1 2 IIN (mA) 3 4 5
19.00
3.5
4.0
4.5 VIN (V)
5.0
5.5
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Complete Power IC for Low-Cost PDAs
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 NAME COR1 IN SDIG ENSD REF RS LBO GND LX SW SWIN LFB ENLCD ENC2 COR2 MAIN FUNCTION 1.5V, 200mA LDO Output for CPU Core. COR1 turns off when VIN < 3V. Input Voltage to the Device. Bypass IN to GND with a 1F capacitor. 3.3V, 200mA LDO Output for Secure Digital Card Slot. SDIG has reverse-current protection so SDIG can be biased when no power is present at IN. SDIG output turns off when VIN < 3V or when ENSD goes low. SDIG Enable Input. Drive ENSD low to turn off the SDIG output. Drive ENSD high to turn on the SDIG output. 1.25V Reference. Bypass REF with a 0.1F capacitor to GND. Reset Output. RS is an active-low, open-drain output that goes low when VIN falls below 3.0V. RS deasserts when VIN goes above 3.4V. Connect a 1M pullup resistor from RS to MAIN. Low-Battery Output. LBO is an active-low, open-drain output that goes low when VIN falls below 3.6V. LBO deasserts when VIN goes above 3.8V. Connect a 1M pullup resistor from LBO to MAIN. Ground LCD Boost Switch. Connect LX to a boost inductor and a rectifying Schottky diode. See Figure 1. LCD True Shutdown Switch Output. SW is the power source for the boost inductor. SW turns on when ENLCD is high. LCD True Shutdown Switch Input. The SWIN-to-SW switch turns off when ENLCD goes low or when VIN < 3V. Connect SWIN to IN. LCD Feedback Input. Connect LFB to a resistor-divider network between the LCD output and GND. The feedback threshold is 1.25V. Enable Input for LCD (Boost Regulator). Drive ENLCD high to activate the LCD boost. Drive ENLCD low to shut down the LCD output. Enable Input for Secondary Core LDO (COR2). Drive ENC2 high to turn on COR2. Drive low to turn off COR2. 1.8V, 30mA LDO Output for Secondary Core. COR2 output turns off when VIN < 3V or when ENC2 goes low. 3.3V, 300mA LDO Output for Main Supply. MAIN output turns off when VIN < 3V.
MAX1552
Detailed Description
Linear Regulators
The MAX1552 contains all power blocks and voltage monitors for a small PDA. Power for logic and other subsystems is provided by four LDOs: * MAIN--Provides 3.3V at a guaranteed 350mA with a typical current limit of 650mA. * SDIG--Provides 3.3V at a guaranteed 250mA for secure digital cards with a typical current limit at 310mA. COR1--1.5V for CPU core guarantees 250mA and typically current limits at 450mA. COR2--1.8V for CODEC core guarantees 30mA and typically current limits at 50mA.
When SDIG is turned off, reverse current is blocked so the SDIG output can be biased with an external source when no power is present at IN. Leakage current is typically 3A with 3.3V at SDIG.
LCD Boost DC-to-DC
In addition to the LDOs, the MAX1552 also includes a low-current, high-voltage-boost DC-to-DC converter for LCD bias. This circuit can output up to 28V and can be adjusted with either an analog or PWM control signal using external components. SW provides an input-power disconnect for the LCD when ENLCD is low (off). The input-power disconnect function is ideal for applications that require the output voltage to fall to 0V in shutdown (true shutdown). If true shutdown is not required, the SW switch can be bypassed by connecting the boost inductor directly to IN and removing the bypass capacitor on SW (C9 in Figure 1).
* *
MAIN and COR1 regulators are always on as long as the IC is not in low-voltage shutdown (VIN < 3V). COR2 and SDIG can be turned on and off independently through logic signals at ENC2 and ENSD, respectively.
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Complete Power IC for Low-Cost PDAs MAX1552
AC ADAPTER INPUT 3.5V TO 7V DC 1F USB INPUT 3.5V TO 6.0V 1F POK LOW WHEN EITHER USB OR DC IS ABOVE UV AND ABOVE BATT PG POWER PRESENT (EITHER DC OR USB) SDIG OFF ON C8 0.1F MAX1551
BATT C1 1F TO MAIN
VIN
IN SWIN
MAIN C3 4.7F SDIG C5 4.7F C4 4.7F C6 1F
3.3V, 300mA
3.3V, 200mA
USB
MAX1552 REF COR1
1.5V, 200mA
COR2 ENSD ENC2 ENLCD SW
1.8V, 20mA
GND BATTERY CHARGER
COR2 OFF ON LCD OFF ON
L1 10H D1 LX
C9 4.7F LCD 20V, 1mA R1 1.5M C2 1F
MAIN R3 1M RESET OUT LOW BATT OUT R4 1M RS LBO
C7 30pF LFB
GND
R2 100k
CONNECTION FOR PWM-CONTROLLED LCD BIAS RW
RD VWOUT C10
Figure 1. Typical Operating Circuit with Charger and External PWM LCD Control
Voltage Monitors (LBO)--System Sleep
The MAX1552 monitors the battery voltage at IN. When VIN falls below 3.6V, LBO goes low, typically putting the system (P) into a sleep state. The MAX1552 remains fully functional in this state and all outputs maintain normal operation. However, when in sleep mode, the system (P) typically drives ENSD, ENC2, and ENLCD low, turning off COR2, SDIG, and the LCD boost output. Sleep can be set by the system (P) even without a low battery event. The MAX1552 consumes 50A when the system is in sleep mode. The LBO output is deasserted when the battery voltage rises above 3.8V All regulated outputs turn off when VIN falls below 3V. The MAX1552 resumes normal operation when V IN rises above 3.4V.
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Reset Output (RS)
Reset (RS) asserts when VIN goes below 3V. The reset output remains asserted until VIN rises above 3.4V. RS is an open-drain, active-low output. Connect a 1M resistor from RS to MAIN.
Applications Information
LDO Output Capacitors (MAIN, SDIG, COR1, and COR2)
Capacitors are required at each output of the MAX1552 for stable operation over the full load and temperature range. See Figure 1 for recommended capacitor values for each output. To reduce noise and improve load transients, use large output capacitors, up to 10F. Surface-mount ceramic capacitors have very low ESR
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Complete Power IC for Low-Cost PDAs
MAX1552 IN 0.1F Li+ SWIN LDO CONTROL COR1 1.5V, 200mA LDO CONTROL MAIN 3.3V, 300mA
Selecting a Diode Schottky diodes rated at 250mA or more, such as the Motorola MBRS0530 or Nihon EP05Q03L, are recommended. The diode reverse-breakdown voltage rating must be greater than the LCD output voltage. Selecting Capacitors For most applications, use a small 1F LCD output capacitor. This typically provides an output ripple of 30mVP-P. In addition, bypass IN with 1F, and SW with 4.7F ceramic capacitors. An LCD feed-forward capacitor, connected from the output to FB, improves stability over a wide range of battery voltages. A 33pF capacitor is sufficient for most applications; however, this value is also affected by PC board layout.
LCD 20V 1mA
MAX1552
SDIG
OFF
ON
ENSD
LDO CONTROL
SDIG
3.3V, 200mA
COR2 OFF ON LCD ON
ENC2
LDO CONTROL LCD OFF SWITCH
COR2
1.8V, 20mA
OFF
ENLCD
SW
TO MAIN LX RESET OUT LOW BATT OUT RS LBO LFB REF 0.1F BIAS CURRENT REF THSD GND LCD BOOST
Setting the LCD Voltage Adjust the output voltage by connecting a voltagedivider from the output (VOUT) to FB (see Figure 1). Select R2 between 10k and 200k. Calculate R1 with the following equation: R1 = R2 [(VOUT / VFB) - 1] where VFB = 1.25V and VOUT can range from VIN to 28V. The input bias current of FB is typically only 5nA, which allows large-value resistors to be used. For less than 1% error, the current through R2 should be greater than 100 times the feedback input bias current (IFB). LCD Adjustment The LCD boost output can be digitally adjusted by either a DAC or PWM signal. DAC Adjustment Adding a DAC and a resistor, RD, to the divider circuit (Figure 3) provides DAC adjustment of VOUT. Ensure that VOUT(MAX) does not exceed the LCD panel rating. The output voltage (VOUT) as a function of the DAC voltage (V DOUT ) can be calculated using the following formula: R1 (1.25 - VDOUT ) x R1 VOUT = 1.25 x 1 + + RD R2 Using PWM Signals Many microprocessors have the ability to create PWM outputs. These are digital outputs, based on either 16bit or 8-bit counters, with programmable duty cycle. In many applications they are suitable for adjusting the output of the MAX1552 (Figure 1).
Figure 2. Block Diagram
and are commonly available in values up to 10F. X7R and X5R dielectrics are recommended. Note that some ceramic dielectrics, such as Z5U and Y5V, exhibit large capacitance and ESR variation with temperature and require larger than the recommended values to maintain stability over temperature.
LCD Boost Output
Selecting an Inductor The LCD boost is designed to operate with a wide range of inductor values (4.7H to 22H). Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Smaller values make LX switch more frequently for a given load and can reduce efficiency at low load currents. Larger values reduce switching losses due to less frequent switching for a given load, but higher resistance may then reduce efficiency. A 10H inductor provides a good balance and works well for most applications. The inductor's saturation current rating should be greater than the peak switching current (250mA); however, it is generally acceptable to bias some inductors into saturation by as much as 20%, although this slightly reduces efficiency.
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Complete Power IC for Low-Cost PDAs MAX1552
VIN FEEDBACK RESISTORS AVDD DAC VDOUT RD VREF 1.25V R1 i1 ERROR AMP CONTROL VOUT (LCD BIAS)
SIMPLIFIED DC-TO-DC CONVERTER
iD R2
i2
MAX1552
Figure 3. Adjusting the Output Voltage with a DAC
The circuit consists of the PWM source, capacitor C10, and resistors RD and RW. To analyze the transfer function of the PWM circuit, it is easiest to first simplify it to its Thevenin equivalent. The Thevenin voltage can be calculated using the following formula: VTHEV = (D x VOH) + (1 - D) x VOL where D is the duty cycle of the PWM signal, VOH is the PWM output high level (often 3.3V), and V OL is the PWM output low level (usually 0V). For CMOS logic this equation simplifies to: VTHEV = D x VDD where VDD is the I/O voltage of the PWM output. The Thevenin impedance is the sum of resistors RW and RD: RTHEV = RD + RW The output voltage (VOUT) as a function of the PWM average voltage (VTHEV) is: R1 (1.25 - VTHEV ) x R1 VOUT = 1.25 x 1 + + R THEV R2 When using the PWM adjustment method, RD isolates the capacitor from the feedback loop of the MAX1552. The cutoff frequency of the lowpass filter is defined as: fC = 1 2 x x R THEV x C10
An important consideration is the turn-on transient created by the initial charge on filter capacitor C10. This capacitor forms a time constant with R THEV , which causes the output to initialize at a higher-than-intended voltage. This overshoot can be minimized by scaling R D as high as possible compared to R1 and R2. Alternatively, the P can briefly keep the LCD disabled until the PWM voltage has had time to stabilize.
PC Board Layout and Grounding
Careful PC board layout is important for minimizing ground bounce and noise. Keep the MAX1552's ground pin and the ground leads of the input and output capacitors less than 0.2in (5mm) apart. In addition, keep all connections to FB and LX as short as possible. In particular, external feedback resistors should be as close to FB as possible. To minimize output voltage ripple, and to maximize output power and efficiency, use a ground plane and solder GND directly to the ground plane. Refer to the MAX1552 evaluation kit for a layout example.
Thermal Considerations
In most applications, the circuit is located on a multilayer board and full use of the four or more layers is recommended. For heat dissipation, connect the exposed backside pad of the QFN package to a large analog ground plane, preferably on a surface of the board that receives good airflow. Typical applications use multiple ground planes to minimize thermal resistance. Avoid large AC currents through the analog ground plane.
The cutoff frequency should be at least two decades below the PWM frequency to minimize the induced AC ripple at the output.
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Complete Power IC for Low-Cost PDAs
Pin Configuration
ENLCD
Chip Information
TRANSISTOR COUNT: 1872 PROCESS: BiCMOS
MAX1552
COR2 15
MAIN
TOP VIEW
16
14
ENC2
13
COR1 IN SDIG ENSD
1 2
12 LFB 11 SWIN
MAX1552
3 4 5 REF 6 RS 7 LBO 8 GND 10 SW 9 LX
THIN QFN
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Complete Power IC for Low-Cost PDAs MAX1552
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.)
24L QFN THIN.EPS
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
A
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
A
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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