19-2831; Rev 0; 4/03 KIT ATION EVALU E L B A AVAIL Complete Power IC for Low-Cost PDAs Features 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*. ♦ Minimum External Components 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. ♦ 1.3W Thin QFN Package 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 µP 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. ♦ 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 ♦ High-Efficiency LCD Step-Up DC-to-DC Output Up to 28V LCD 0V True Shutdown When Off ♦ 50µA Quiescent Supply Current Ordering Information PART MAX1552ETE PIN-PACKAGE 16 Thin QFN Typical Application Circuit VIN IN The MAX1552 operates from a 3.1V to a 5.5V supply voltage and consumes 50µA 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 -40°C to +85°C. MAIN 3.3V, 300mA SDIG 3.3V, 200mA COR1 1.5V, 200mA COR2 1.8V, 20mA SWIN MAX1552 REF Applications ON ENSD COR2 OFF ON ENC2 SDIG PDAs Organizers Cellular and Cordless Phones MP3 Players Hand-Held Devices TEMP RANGE -40°C to +85°C LCD OFF OFF ON SW ENLCD D1 LCD 20V, 1mA LX MAIN LFB RESET OUT Pin Configuration appears at end of data sheet. LOW BATT OUT RS LBO GND *Protected by U.S. Patent #6,507,172. ________________________________________________________________ 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 MAX1552 General Description MAX1552 Complete Power IC for Low-Cost PDAs 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 = +70°C) 16-Pin Thin QFN (derate 16.9mW/°C above +70°C) ...1.349W Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°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 = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS GENERAL IN, SWIN Voltage Range Operating 3.1 5.5 V RS and Complete Shutdown Threshold VIN falling 2.96 3 3.04 V RS Release and Restart Threshold VIN rising 3.4025 3.4 3.4400 V LBO and Sleep Threshold VIN falling 3.55 3.6 3.65 V LBO Release and Restart to Full On VIN rising 3.75 3.8 3.85 V IN, SWIN Operating Current—All On VLFB = 1.3V 100 125 µA IN Operating Current—All On Except LCD ENLCD = GND 90 110 µA IN Operating Current—MAIN and COR1 On ENLCD = ENC2 = ENSD = GND, LDO loads = 0µA 50 65 µA IN, SWIN Operating Current—Shut Down VSWIN = VIN = 2.9V 2 10 µA 3.2175 3.3 3.3825 V 350 650 1200 mA LDOs MAIN Output Voltage ILOAD = 100µA to 300mA, VIN = 3.6V to 5.5V MAIN Current Limit MAIN Dropout Voltage SDIG Output Voltage ILOAD = 1mA ILOAD = 100µA to 200mA, VIN = 3.6V to 5.5V SDIG Current Limit SDIG Dropout Voltage COR1 Output Voltage ILOAD = 100µA to 200mA, VIN = 3.6V to 5.5V COR1 Current Limit COR2 Current Limit 2 310 3.2175 3.3 3.3825 V 250 310 390 mA 1 ILOAD = 200mA VSDIG = 5V, ENSD = VIN = GND ILOAD = 100µA to 20mA, VIN = 3.6V to 5.5V mV 210 ILOAD = 1mA SDIG Reverse Leakage Current COR2 Output Voltage 1 ILOAD = 300mA mV 170 300 7 15 µA 1.4625 1.5 1.5375 V mA 250 450 800 1.755 1.8 1.845 V 30 50 100 mA _______________________________________________________________________________________ Complete Power IC for Low-Cost PDAs (VIN = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS 28 V 210 250 275 mA 2 µA µs LCD LX Voltage Range LX Current Limit L1 = 10µH LX On-Resistance Ω 1.7 LX Leakage Current VLX = 28V Maximum LX On-Time Minimum LX Off-Time 8 11 14 VLFB > 1.1V 0.8 1 1.2 VLFB < 0.8V (soft-start) 4.0 5 6.0 1.23 1.25 1.27 V 5 100 nA 0.01 1 µA LFB Feedback Threshold LFB Input Bias Current VLFB = 1.3V SW Off-Leakage Current SW = GND, VSWIN = 5.5V, ENLCD = GND µs 1 Ω SW PMOS Peak Current Limit 750 mA SW PMOS Average Current Limit 300 mA 0.13 ms SW PMOS On-Resistance Soft-Start Time CSW = 1µF LOGIC IN AND OUT EN_ Input Low Level VIN = 3.0V to 5.5V EN_ Input High Level VIN = 3.0V to 5.5V Sinking 1mA, VIN = 2.5V RS, LBO Output High Leakage VOUT = 5.5V V 0.01 1 µA 0.25 0.4 V 1 µA 1.4 EN_ Input Leakage Current RS, LBO Output Low Level 0.4 V THERMAL PROTECTION Thermal-Shutdown Temperature Rising temperature Thermal-Shutdown Hysteresis 160 °C 15 °C ELECTRICAL CHARACTERISTICS (VIN = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MIN MAX UNITS GENERAL IN, SWIN Voltage Range Operating 3.1 5.5 V RS and Complete Shutdown Threshold VIN falling 2.96 3.04 V RS Release and Restart Threshold VIN rising 3.36 3.44 V LBO and Sleep Threshold VIN falling 3.525 3.675 V LBO Release and Restart to Full On VIN rising 3.725 3.875 V IN, SWIN Operating Current—All On VLFB = 1.3V 125 µA IN Operating Current—All On Except LCD ENLCD = GND 110 µA IN Operating Current—MAIN and COR1 On ENLCD = ENC2 = ENSD = GND, LDO loads = 0µA 65 µA IN, SWIN Operating Current—Shut Down VSWIN = VIN = 2.925V 10 µA _______________________________________________________________________________________ 3 MAX1552 ELECTRICAL CHARACTERISTICS (continued) MAX1552 Complete Power IC for Low-Cost PDAs ELECTRICAL CHARACTERISTICS (continued) (VIN = VSWIN = VENSD = VENC2 = VENLCD = 4.0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MIN MAX UNITS 3.2175 3.3825 V 350 1200 mA 310 mV 3.2175 3.3825 V 250 390 mA LDOs MAIN Output Voltage ILOAD = 100µA to 300mA, VIN = 3.6V to 5.5V MAIN Current Limit MAIN Dropout Voltage ILOAD = 300mA SDIG Output Voltage ILOAD = 100µA to 200mA, VIN = 3.6V to 5.5V SDIG Current Limit SDIG Dropout Voltage ILOAD = 200mA 300 mV SDIG Reverse Leakage Current VSDIG = 5V, ENSD = VIN = GND 15 µA COR1 Output Voltage ILOAD = 100µA to 20mA, VIN = 3.6V to 5.5V 1.4625 1.5375 V 250 800 mA ILOAD = 100µA to 20mA, VIN = 3.6V to 5.5V 1.755 1.845 V 30 100 mA 28 V 200 275 mA 2 µA 8 14 µs VLFB > 1.1V 0.8 1.2 VLFB < 0.8V (soft-start) 4.0 6.0 1.22 1.27 V 100 nA 1 µA 0.4 V COR1 Current Limit COR2 Output Voltage COR2 Current Limit LCD LX Voltage Range LX Current Limit LX Leakage Current VLX = 28V Maximum LX On-Time Minimum LX Off-Time LFB Feedback Threshold LFB Input Bias Current VLFB = 1.3V SW Off-Leakage Current SW = GND, VSWIN = 5.5V, ENLCD = GND µs LOGIC IN AND OUT EN_ Input Low Level VIN = 3.0V to 5.5V EN_ Input High Level VIN = 3.0V to 5.5V 1.4 EN_ Input Leakage Current 1 RS, LBO Output Low Level Sinking 1mA, VIN = 2.5V RS, LBO Output High Leakage VOUT = 5.5V Note 1: Specifications to -40°C are guaranteed by design and not production tested. 4 V _______________________________________________________________________________________ µA 0.4 V 1 µA Complete Power IC for Low-Cost PDAs SDIG DROPOUT VOLTAGE vs. LOAD CURRENT 300 200 200 3.25 OUTPUT VOLTAGE (V) 400 3.50 MAX1552 toc02 MAX1552 toc01 250 DROPOUT VOLTAGE (mV) DROPOUT VOLTAGE (mV) 500 MAIN OUTPUT VOLTAGE vs. LOAD CURRENT 150 100 MAX1552 toc03 MAIN DROPOUT VOLTAGE vs. LOAD CURRENT 3.00 2.75 2.50 2.25 2.00 50 100 1.75 1.50 0 0 100 200 300 400 0 500 50 SDIG OUTPUT VOLTAGE vs. LOAD CURRENT 150 200 0 250 100 2.50 2.25 600 1.75 OUTPUT VOLTAGE (V) 2.75 500 2.00 MAX1552 toc05 1.50 OUTPUT VOLTAGE (V) 3.00 400 COR2 OUTPUT VOLTAGE vs. LOAD CURRENT 1.75 MAX1552 toc04 3.25 300 200 ILOAD (mA) COR1 OUTPUT VOLTAGE vs. LOAD CURRENT 3.50 1.25 1.00 1.50 1.25 1.00 2.00 0.75 0.75 1.75 1.50 0.50 0.50 50 100 150 200 250 300 350 0 100 ILOAD (mA) 300 200 0 400 10 LOAD STEP RESPONSE MAIN 40 50 ILOAD (mA) INPUT CURRENT vs. INPUT VOLTAGE LOAD STEP RESPONSE COR1 MAX1552 toc07 30 20 ILOAD (mA) MAX1552 toc08 80 VMAIN AC-COUPLED 50mV/div MAX1552 toc09 0 70 VCOR1 AC-COUPLED 20mV/div 60 VIN FALLING 50 IIN (µA) OUTPUT VOLTAGE (V) 100 ILOAD (mA) ILOAD (mA) MAX1552 toc06 0 ILOAD 100mA/div ILOAD 100mA/div 40 VIN RISING 30 20 10 0 40µs/div 40µs/div 0 1 2 3 4 5 VIN (V) _______________________________________________________________________________________ 5 MAX1552 Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) LCD SWITCH WAVEFORM ENABLE RESPONSE TO ENSD MAX1552 toc10 MAX1552 toc11 RL = 30Ω CL = 4.7µF VIN AC-COUPLED 20mV/div ENSD 2V/div LX 10V/div SDIG 1V/div LCD AC-COUPLED 20mV/div 2µs/div 200µs/div EFFICIENCY vs. LOAD CURRENT ENABLE RESPONSE TO LCD MAX1552 toc12 80 LCD 2V/div SW TURN-ON EFFICIENCY (%) LCD BOOST SOFT-START MAX1552 toc13 85 ENLCD 5V/div VLCD = 18V VLCD = 15V 75 70 65 60 0 400µs/div 1 2 3 4 5 ILOAD (mA) LCD OUTPUT VOLTAGE vs. INPUT VOLTAGE LCD OUTPUT VOLTAGE vs. LOAD CURRENT 18.75 OUTPUT VOLTAGE (V) 18.50 18.25 18.00 17.75 18.50 18.25 18.00 17.75 17.50 17.50 17.25 17.25 17.00 MAX1552 toc15 18.75 17.00 0 1 2 3 IIN (mA) 6 19.00 MAX1552 toc14 19.00 OUTPUT VOLTAGE (V) MAX1552 Complete Power IC for Low-Cost PDAs 4 5 3.5 4.0 4.5 5.0 VIN (V) _______________________________________________________________________________________ 5.5 Complete Power IC for Low-Cost PDAs PIN NAME FUNCTION 1 COR1 2 IN 3 SDIG 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. 4 ENSD SDIG Enable Input. Drive ENSD low to turn off the SDIG output. Drive ENSD high to turn on the SDIG output. 5 REF 1.25V Reference. Bypass REF with a 0.1µF capacitor to GND. 6 RS 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 1MΩ pullup resistor from RS to MAIN. 7 LBO 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 1MΩ pullup resistor from LBO to MAIN. 8 GND Ground 9 LX LCD Boost Switch. Connect LX to a boost inductor and a rectifying Schottky diode. See Figure 1. 10 SW LCD True Shutdown Switch Output. SW is the power source for the boost inductor. SW turns on when ENLCD is high. 11 SWIN LCD True Shutdown Switch Input. The SWIN-to-SW switch turns off when ENLCD goes low or when VIN < 3V. Connect SWIN to IN. 12 LFB 13 ENLCD 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 1µF capacitor. 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. 14 ENC2 Enable Input for Secondary Core LDO (COR2). Drive ENC2 high to turn on COR2. Drive low to turn off COR2. 15 COR2 1.8V, 30mA LDO Output for Secondary Core. COR2 output turns off when VIN < 3V or when ENC2 goes low. 16 MAIN 3.3V, 300mA LDO Output for Main Supply. MAIN output turns off when VIN < 3V. 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. 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. 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 3µA 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). _______________________________________________________________________________________ 7 MAX1552 Pin Description MAX1552 Complete Power IC for Low-Cost PDAs AC ADAPTER INPUT 3.5V TO 7V DC MAX1551 IN C1 1µF 1µF USB INPUT 3.5V TO 6.0V VIN BATT MAIN SWIN SDIG TO MAIN USB MAX1552 1µF POK LOW WHEN EITHER USB OR DC IS ABOVE UV AND ABOVE BATT PG GND C8 0.1µF POWER PRESENT (EITHER DC OR USB) REF ON COR2 OFF ON ENC2 SDIG LCD BATTERY CHARGER COR1 COR2 ENSD OFF OFF C5 4.7µF C4 4.7µF C6 1µF 3.3V, 200mA 1.5V, 200mA 1.8V, 20mA SW L1 10µH ENLCD ON 3.3V, 300mA C3 4.7µF D1 R3 1MΩ LCD 20V, 1mA LX MAIN C7 30pF R4 1MΩ C9 4.7µF R1 1.5MΩ C2 1µF LFB RS RESET OUT LBO LOW BATT OUT R2 100kΩ GND 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 Reset Output (RS) The MAX1552 monitors the battery voltage at IN. When VIN falls below 3.6V, LBO goes low, typically putting the system (µP) 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 (µP) typically drives ENSD, ENC2, and ENLCD low, turning off COR2, SDIG, and the LCD boost output. Sleep can be set by the system (µP) even without a low battery event. The MAX1552 consumes 50µA 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. 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 1MΩ resistor from RS to MAIN. 8 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 10µF. Surface-mount ceramic capacitors have very low ESR _______________________________________________________________________________________ Complete Power IC for Low-Cost PDAs MAX1552 LDO CONTROL MAIN LDO CONTROL COR1 LDO CONTROL SDIG ENC2 LDO CONTROL COR2 ENLCD LCD OFF SWITCH IN 0.1µF SDIG Li+ OFF ON COR2 OFF ON LCD OFF ON 3.3V, 300mA SWIN ENSD Selecting Capacitors For most applications, use a small 1µF LCD output capacitor. This typically provides an output ripple of 30mVP-P. In addition, bypass IN with 1µF, and SW with 4.7µF 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. 1.5V, 200mA 3.3V, 200mA 1.8V, 20mA SW TO MAIN LX RS RESET OUT LCD BOOST LCD 20V 1mA LBO LOW BATT OUT LFB REF 0.1µF BIAS CURRENT Setting the LCD Voltage Adjust the output voltage by connecting a voltagedivider from the output (VOUT) to FB (see Figure 1). Select R2 between 10kΩ and 200kΩ. Calculate R1 with the following equation: R1 = R2 [(VOUT / VFB) - 1] REF THSD GND Figure 2. Block Diagram and are commonly available in values up to 10µF. 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 (4.7µH to 22µH). 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 10µH 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. 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 ) × R1 VOUT = 1.25 × 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). _______________________________________________________________________________________ 9 MAX1552 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. MAX1552 Complete Power IC for Low-Cost PDAs VIN FEEDBACK RESISTORS R1 AVDD DAC SIMPLIFIED DC-TO-DC CONVERTER i1 VDOUT RD ERROR AMP i2 iD R2 VREF 1.25V CONTROL VOUT (LCD BIAS) 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 ) × R1 VOUT = 1.25 × 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 × π × R THEV × 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 µP 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. 10 ______________________________________________________________________________________ Complete Power IC for Low-Cost PDAs COR1 1 IN 2 MAIN COR2 ENC2 ENLCD TOP VIEW 16 15 14 13 Chip Information TRANSISTOR COUNT: 1872 PROCESS: BiCMOS 12 LFB 11 SWIN MAX1552 ENSD 4 9 REF 5 6 7 8 GND 10 SW LBO 3 RS SDIG LX THIN QFN ______________________________________________________________________________________ 11 MAX1552 Pin Configuration 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 MAX1552 Complete Power IC for Low-Cost PDAs 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 © 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.