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19-1586; Rev 0; 7/00
Low-Voltage, Step-Down DC-DC Converters in SOT23
General Description
The MAX1733/MAX1734 step-down DC-DC converters deliver over 250mA to outputs as low as 1.25V. These converters use a unique proprietary current-limited control scheme that achieves over 90% efficiency. These devices maintain extremely low quiescent supply current (40A), and their high 1.2MHz (max) operating frequency permits small, low-cost external components. This combination of features makes the MAX1733/ MAX1734 excellent high-efficiency alternatives to linear regulators in space-constrained applications. Internal synchronous rectification greatly improves efficiency and eliminates the external Schottky diode required in conventional step-down converters. Both devices also include internal digital soft-start to limit input current upon startup and reduce input capacitor requirements. The MAX1733 provides an adjustable output voltage (1.25V to 2.0V). The MAX1734 provides factory-preset output voltages (see Selector Guide). Both are available in space-saving 5-pin SOT23 packages. o 250mA Guaranteed Output Current o Synchronous Rectifier for Over 90% Efficiency o Tiny 5-Pin SOT23 Package o 40A Quiescent Supply Current o 0.01A Logic-Controlled Shutdown o Up to 1.2MHz Switching Frequency o Fixed 1.8V or 1.5V Outputs (MAX1734) o Adjustable Output Voltage (MAX1733) o 1.5% Initial Accuracy o 2.7V to 5.5V Input Range o Soft-Start Limits Startup Current
Features
MAX1733/MAX1734
Ordering Information
PART MAX1733EUK-T MAX1734EUK_ _-T TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 5 SOT23-5 5 SOT23-5
Applications
Cellular, PCS, and Cordless Telephones PDAs, Palmtops, and Handy-Terminals Battery-Powered Equipment
Note: The MAX1734 offers two output voltages. See the Selector Guide, then insert the proper designator into the blanks above to complete the part number.
Selector Guide
PART MAX1733EUK MAX1734EUK18 MAX1734EUK15 VOUT (V) Adjustable 1.8 1.5 TOP MARK ADKY ADKW ADKX
Typical Operating Circuit
INPUT +2.7V TO +5.5V IN 2.2F LX IN 1 10H VOUT AT 250mA
Pin Configuration
TOP VIEW
5 LX
MAX1734
SHDN OUT
22F GND 2
MAX1733 MAX1734
4 OUT (FB)
GND
SHDN 3
SOT23-5
( ) ARE FOR MAX1733 ONLY.
________________________________________________________________ Maxim Integrated Products
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Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
ABSOLUTE MAXIMUM RATINGS
IN, SHDN to GND .....................................................-0.3V to +6V OUT, FB, LX to GND ....................................-0.3V to (VIN + 0.3V) OUT Short Circuit to GND ..........................................Continuous Continuous Power Dissipation (TA = +70C) 5-Pin SOT23 (derate 7.1mW/C above +70C)............571mW 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 = +2.7V to +5.5V, SHDN = IN, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER Input Voltage Range Startup Voltage UVLO Threshold UVLO Hysteresis Quiescent Supply Current Shutdown Supply Current Output Voltage Range (MAX1733) Output Voltage Accuracy (MAX1734) Load Regulation Line Regulation OUT Sense Current (MAX1734) FB Feedback Threshold (MAX1733) FB Leakage Current (MAX1733) SHDN Input High Voltage SHDN Input Low Voltage SHDN Leakage Current High-Side Current Limit Low-Side Current Limit High-Side On-Resistance Rectifier On-Resistance Rectifier Off-Current Threshold LX Leakage Current LX Reverse Leakage Current Minimum On-Time Minimum Off-Time VFB IFB VIH VIL ISHDN ILIMP ILIMN RONP RONN ILXOFF ILXLEAK ILXLK,R tON(MIN) tOFF(MIN) VIN = 5.5V, VLX = 0 to VIN IN unconnected, VLX = 5.5V, SHDN = GND VIN = 3.6V VIN = 3.6V 0.28 0.28 ILX = -50mA, VIN = 3.0V ILX = -50mA, VIN = 5.5V ILX = -50mA, VIN = 3.0V ILX = -50mA, VIN = 5.5V IIN ISHDN VOUT IOUT = 0, TA = +25C IOUT = 0 to 250mA IOUT = 0 to 250mA VIN = 2.7V to 5.5V VOUT = VREG, VIN = V SHDN= 5V TA = +25C, VIN = 3.6V VIN = 3.6V VFB = 1.5V 2.7V < VIN < 5.5V 2.7V < VIN < 5.5V SHDN = GND or IN 300 200 0.001 425 325 0.7 0.5 1.0 0.8 40 0.1 0.1 0.4 0.4 5 5 0.5 0.5 1.6 0.4 1 535 430 1.4 1.1 2 1.6 1.231 1.220 0.001 No switching, no load (FB/OUT above trip point) SHDN = GND 1.25 -1.5 -3 0 0 4 1.250 8 1.269 1.280 0.2 SYMBOL VIN VSTART VUVLO VIN rising VIN falling 1.55 1.85 1.65 200 40 0.01 70 4 2.0 +1.5 +3 CONDITIONS MIN 2.7 TYP MAX 5.5 2.0 1.95 UNITS V V V mV A A V % %/mA %/V A V A V V A mA mA mA A A s s
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Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
ELECTRICAL CHARACTERISTICS
(VIN = +2.7V to +5.5V, SHDN = IN, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Startup Voltage UVLO Threshold Quiescent Supply Current Shutdown Supply Current Output Voltage Range (MAX1733) Output Voltage Accuracy (MAX1734) OUT Sense Current (MAX1734) FB Feedback Threshold (MAX1733) FB Leakage Current (MAX1733) SHDN Input High Voltage SHDN Input Low Voltage SHDN Leakage Current High-Side Current Limit Low-Side Current Limit High-Side On-Resistance Rectifier On-Resistance LX Leakage Current LX Reverse Leakage Current Minimum On-Time Minimum Off-Time IOUT VFB IFB VIH VIL ISHDN ILIMP ILIMN RONP RONN ILXLEAK ILXLK,R tON(MIN) tOFF(MIN) ILX = -50mA, VIN = 3.0V ILX = -50mA, VIN = 5.5V ILX = -50mA, VIN = 3.0V ILX = -50mA, VIN = 5.5V VIN = 5.5V, VLX = 0 to VIN IN unconnected, VLX = 5.5V, SHDN = GND 0.25 0.25 SYMBOL VIN VSTART VUVLO IIN ISHDN VOUT IOUT = 0 to 250mA VOUT = VREG, VIN = V SHDN = 5V VIN = 3.6V VFB = 1.5V 2.7V < VIN < 5.5V 2.7V < VIN < 5.5V SHDN = GND or IN 300 200 1.6 0.4 1 565 430 1.4 1.1 2 1.6 5 5 0.55 0.55 1.210 VIN rising, 200mV typical hysteresis VIN falling No switching (FB/OUT above trip point) SHDN = GND 1.25 -3 1.55 70 4 2.0 +3 8 1.280 0.2 CONDITIONS MIN 2.7 MAX 5.5 2.0 1.95 UNITS V V V A A V % A V A V V A mA mA A A s s
Note 1: All devices are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design.
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Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
Typical Operating Characteristics
(CIN = 2.2F ceramic, COUT = 22F tantalum, L = 10H, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VOUT = 1.8V)
MAX1733/4-01
EFFICIENCY vs. LOAD CURRENT (VOUT = 1.5V)
90 80 EFFICIENCY (%) VIN = 2.7V
MAX1733/4-02
NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE
60 SUPPLY CURRENT (A) 58 56 54 52 50 48 46 VOUT = 1.5V, TA = +25C VOUT = 1.5V, TA = -40C 2.5 3.0 3.5 4.0 4.5 5.0 5.5 VOUT = 1.8V, TA = +85C VOUT = 1.8V, TA = +25C
MAX1733/4-03
100 VIN = 2.7V 90 80 EFFICIENCY (%) 70 60 50 40 30 0.1 1 10 100 LOAD CURRENT (mA) VIN = 5.0V VIN = 3.6V
100
62
70 VIN = 3.6V 60 50 40 30
VIN = 5.0V
1000
0.1
1
10 100 LOAD CURRENT (mA)
1000
SUPPLY VOLTAGE (V)
OUTPUT ACCURACY vs. LOAD CURRENT (VOUT = 1.8V)
MAX1733/4-04
OUTPUT ACCURACY vs. LOAD CURRENT (VOUT = 1.5V)
MAX1733/4-05
SWITCHING FREQUENCY vs. SUPPLY VOLTAGE
ILOAD = 50mA TO 250mA
MAX1733/4-06
3.0 2.0 OUTPUT ACCURACY (%) 1.0 0 -1.0 -2.0 -3.0 0.1 1 10 100 VIN = 5.0V, TA = +25C VIN = 3.6V, TA = -40C
3.0 2.0 OUTPUT ACCURACY (%) 1.0 0 -1.0 -2.0 -3.0 VIN = 2.7V, TA = +25C VIN = 3.6V, TA = -40C VIN = 3.6V, TA = +85C VIN = 5.0V, TA = +25C
1.50
VIN = 3.6V, TA = +85C VIN = 2.7V, TA = +25C
SWITCHING FREQUENCY (MHz)
1.25
1.00 VOUT = 1.8V 0.75 VOUT = 1.5V
0.50 0.1 1 10 100 1000 2.7 3.0 3.3 3.6 3.9 4.2 LOAD CURRENT (mA) SUPPLY VOLTAGE (V)
1000
LOAD CURRENT (mA)
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1733/4-07
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1733/4-08
VIN = 3.6V, VOUT = 1.8V, ILOAD = 20mA VOUT AC-COUPLED 20mV/div
VIN = 3.6V, VOUT = 1.8V, ILOAD = 200mA VOUT AC-COUPLED 20mV/div
VLX 2V/div
VLX 2V/div
400ns/div
400ns/div
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Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
Typical Operating Characteristics (continued)
(CIN = 2.2F ceramic, COUT = 22F tantalum, L = 10H, unless otherwise noted.)
SOFT-START AND SHUTDOWN RESPONSE
MAX1733/4-09
LOAD-TRANSIENT RESPONSE
MAX1733/4-12
VIN = 3.6V, VOUT = 1.8V, RLOAD = 7 VOUT 1V/div IIN 100mA/div
VIN = 3.6V, VOUT = 1.8V, ILOAD = 20mA TO 200mA
VOUT AC-COUPLED 50mV/div
VSHDN 5V/div
ILOAD 100mA/div
200s/div
4s/div
LIGHT-LOAD LINE-TRANSIENT RESPONSE
MAX1733/4-10
HEAVY-LOAD LINE-TRANSIENT RESPONSE
MAX1733/4-11
VIN = 3.4V TO 3.8V, VOUT = 1.8V, ILOAD = 20mA VOUT AC-COUPLED 50mV/div
VIN = 3.4V TO 3.8V, VOUT = 1.8V, ILOAD = 200mA VOUT AC-COUPLED 50mV/div
VIN AC-COUPLED 200mV/div
VIN AC-COUPLED 200mV/div
4s/div
4s/div
Pin Description
PIN 1 2 3 NAME IN GND SHDN FB 4 OUT 5 LX FUNCTION Supply Voltage Input. Input range from +2.7V to +5.5V. Bypass with a 2.2F ceramic capacitor to GND. Ground Active-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes high impedance and quiescent current drops to 0.01A. MAX1733 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external voltage-divider between the output voltage and GND. MAX1734 Voltage Sense Input. OUT is connected to an internal voltage-divider. Inductor Connection
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Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
INPUT +2.7V TO +5.5V IN C1 2.2F LX L1 10H VOUT C2 22F IN
MAX1733
SHDN FB
R1
MAX1733 MAX1734
CURRENT LIMIT
R2 GND DIGITAL SOFT-START CONTROL LOGIC P LX N SHUTDOWN CONTROL OUT (FB) VREF
Figure 1. MAX1733 Typical Application Circuit
SHDN
Detailed Description
The MAX1733/MAX1734 step-down DC-DC converters deliver over 250mA to outputs as low as 1.25V. They use a unique proprietary current-limited control scheme that maintains extremely low quiescent supply current (40A), and their high 1.2MHz (max) operating frequency permits small, low-cost external components. Figure 2 is a simplified functional diagram.
( ) ARE FOR MAX1733 ONLY.
GND
Figure 2. Simplified Functional Diagram
Control Scheme
The MAX1733/MAX1734 use 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 400ns 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 400ns 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 highside 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 MAX1733/MAX1734 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 to reduce peak inductor current, resulting in increased efficiency and reduced output voltage ripple. When delivering medium and higher output currents, the MAX1733/MAX1734 extend either the on-time or the offtime, as necessary to maintain regulation, resulting in nearly constant frequency operation with high efficiency and low output voltage ripple.
Shutdown Mode
Connecting SHDN to GND places the MAX1733/ MAX1734 in shutdown mode and reduces supply current to 0.01A. In shutdown, the control circuitry, internal switching MOSFET, and synchronous rectifier turn off and LX goes high impedance. Connect SHDN to IN for normal operation.
Soft-Start
The MAX1733/MAX1734 have internal soft-start circuitry that limits current draw at startup, reducing transients on the input source. Soft-start is particularly useful for higher impedance input sources, such as Li+ and alkaline cells. Soft-start is implemented by starting with the current limit at 25% of its full current value and gradually increasing it in 25% steps until the full current limit is reached. See Soft-Start and Shutdown Response in the Typical Operating Characteristics section.
Design Information
Setting the Output Voltage (MAX1733)
Select an output voltage for the MAX1733 by connecting FB to a resistive divider between the output and
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Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
Table 1. Suggested Inductors
INDUCTOR VALUE (H) 10 10 MANUFACTURER Sumida Coilcraft PART NUMBER CR43-100 CDRH4D18-100 DT1608C-103
Table 3. Component Suppliers
COMPANY AVX Coilcraft Kemet Murata Sumida USA Japan PHONE 843-946-0238 847-639-6400 408-986-0424 814-237-1431 847-956-0666 81-3-3607-5111 408-573-4150 FAX 843-626-3123 847-639-1469 408-986-1442 814-238-0490 847-956-0702 81-3-3607-5144 408-573-4159
Table 2. Suggested Capacitors
CAPACITOR TYPE Tantalum (22F) Ceramic (2.2F) MANUFACTURER Taiyo Yuden AVX PART NUMBER LMK212BJ225MG TAJA226M006R
Taiyo Yuden
IOUT VOUT VIN - OUT 1 / 2 IRMS = VIN Tables 2 and 3 list some suggested capacitors and suppliers.
(
)
GND (Figure 1). Choose R2 to be less than 50k: V R1 = R2 x OUT - 1 VREF where VREF = 1.25V.
Using Ceramic COUT with MAX1733
The circuit of Figure 3 is designed to allow the use of ceramic output capacitors with the MAX1733. Feedback is derived from the LX pin instead of the output to remove the effects of phase lag in the feedback loop. Compared to the standard applications circuit, there are three benefits: 1) availability of ceramic vs. tantalum; 2) size of 2.2F 0805 vs. 22F A-case; 3) output ripple less than 10mVp-p vs. greater than 30mVp-p. Increase the output capacitance to 4.7F to further reduce the output ripple. Note that this circuit exhibits load regulation equal to the series resistance of the inductor multiplied by the load current. This small amount of load regulation is helpful in reducing overshoot of the output voltage during load transients.
Inductor Selection
The MAX1733/MAX1734 are optimized to use a 10H inductor over the entire operating range. A 300mA rated inductor is enough to prevent saturation for output currents up to 250mA. Saturation occurs when the inductor's magnetic flux density reaches the maximum level the core can support and inductance falls. Choose a low DC-resistance inductor to improve efficiency. Tables 1 and 3 list some suggested inductors and suppliers.
10H IN Li+ 2.7V TO 4.2V 2.2F X7R LX
OUTPUT 1.8V at 250mA 2.2F X7R
Capacitor Selection
The MAX1733/MAX1734 require output voltage ripple (approximately 30mVp-p) for stable switching behavior. Use a 10F to 47F tantalum output capacitor with about 200m to 300m ESR to provide stable switching while minimizing output ripple. Choose input and output capacitors to filter inductor currents for acceptable voltage ripple. The input capacitor reduces peak currents and noise at the voltage source. Input capacitors must meet the input ripple requirements and voltage rating. Use the following equation to calculate the maximum RMS input current:
MAX1733
GND
30k
1000pF ON OFF SHDN FB
68k
Figure 3. Using a Ceramic Output Capacitor with the MAX1733 _______________________________________________________________________________________ 7
Low-Voltage, Step-Down DC-DC Converters in SOT23 MAX1733/MAX1734
Layout Considerations
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 inductor, input filter capacitor, and output filter capacitor as close to the device as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2 inches (5mm). Keep noisy traces, such as the LX trace, away from the voltage-feedback network; also keep them separate, using grounded copper. The MAX1733/MAX1734 evaluation kit data sheet includes a proper PC board layout and routing scheme.
Chip Information
TRANSISTOR COUNT: 1190 PROCESS: BiCMOS
Package Information
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.
8 _____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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