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19-2296; Rev 3; 8/05
Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
General Description
The MAX1920/MAX1921 step-down converters deliver over 400mA 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 (50A), and their high 1.2MHz (max) operating frequency permits small, low-cost external components. This combination makes the MAX1920/MAX1921 excellent highefficiency alternatives to linear regulators in spaceconstrained 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 MAX1920 provides an adjustable output voltage (1.25V to 4V). The MAX1921 provides factory-preset output voltages (see the Selector Guide). Both are available in space-saving 6-pin SOT23 packages. The MAX1920 is also available in a 6-pin TDFN package. 400mA Guaranteed Output Current Internal Synchronous Rectifier for >90% Efficiency Tiny 6-Pin SOT23 Package Available in 6-Pin TDFN Package (MAX1920) Up to 1.2MHz Switching Frequency for Small External Components 50A Quiescent Supply Current 0.1A Logic-Controlled Shutdown 2V to 5.5V Input Range Fixed 1.5V, 1.8V, 2.5V, 3V, and 3.3V Output Voltages (MAX1921) Adjustable Output Voltage (MAX1920) 1.5% Initial Accuracy Soft-Start Limits Startup Current
Features
MAX1920/MAX1921
Ordering Information
PART TEMP RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 6 SOT23-6 6 SOT23-6 6 TDFN 6 TDFN 6 SOT23-6 6 SOT23-6 MAX1920EUT-T MAX1920EUT+T MAX1920ETT-T MAX1920ETT+T MAX1921EUT_ _-T MAX1921EUT_ _+T
Applications
Next-Generation Wireless Handsets PDAs, Palmtops, and Handy-Terminals Battery-Powered Equipment CDMA Power Amplifier Supply
Note: The MAX1921 offers five preset output voltage options. See the Selector Guide, and then insert the proper designator into the blanks above to complete the part number. +Denotes lead-free package.
Typical Operating Circuit
INPUT 2V TO 5.5V IN CIN LX 4.75k 5600pF 4.7F 4.7H OUTPUT 1.5V UP TO 400mA
Pin Configuration
FB 5 LX 4 3 PGND
TOP VIEW
IN 1
6 LX
MAX1921
AGND PGND
AGND 2
MAX1920 MAX1921
5 PGND
AGND 6 4 OUT (FB) 1
MAX1920
ON SHDN OFF OUT
SHDN 3
2 IN
SOT23-6
SHDN
TDFN
( ) ARE FOR MAX1920 ONLY A "+" sign will replace the first pin indicator on lead-free packages.
________________________________________________________________ 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.
Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 MAX1920/MAX1921
ABSOLUTE MAXIMUM RATINGS
IN, FB, SHDN to AGND . . . . . . . . . . . . . . . . . . . . .-0.3V to +6V OUT to AGND, LX to PGND . . . . . . . . . . . .-0.3V to (IN + 0.3V) AGND to PGND . . . . . . . . . . . . . . . . . . . . . . . . . .-0.3V to +0.3V OUT Short Circuit to GND . . . . . . . . . . . . . . . . . . . . . . . . . . .10s Continuous Power Dissipation (TA = +70C) 6-Pin SOT23-6 (derate 8.7mW/C above +70C) . . . .695mW 6-Pin TDFN (derate 18.2mW/C above +70C) . . .1454.5mW Operating Temperature Range . . . . . . . . . . . . . .-40C to +85C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . .+150C Storage Temperature . . . . . . . . . . . . . . . . . . . .-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 = 3.6V, SHDN = IN, TA = 0C to +85C. Typical parameters are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Range Startup Voltage UVLO Threshold UVLO Hysteresis Quiescent Supply Current Quiescent Supply Current Dropout Shutdown Supply Current Output Voltage Accuracy (MAX1921) IOUT IIN IIN ISHDN No switching, no load SHDN = IN, OUT/FB = 0 SHDN = GND IOUT = 0, TA = +25C IOUT = 0 to 400mA, TA = -40C to +85C IOUT = 0 to 200mA, TA = -40C to +85C OUT BIAS Current Output Voltage Range (MAX1920) FB Feedback Threshold (MAX1920) FB Feedback Hysteresis (MAX1920) FB Bias Current (MAX1920) Load Regulation Line Regulation SHDN Input Voltage High SHDN Input Voltage Low SHDN Leakage Current High-Side Current Limit VIH VIL ISHDN ILIMP SHDN = GND or IN 525 0.001 730 SHDN = 0 OUT at regulation voltage Figure 4, IN = 4.5V TA= 25C VFB TA = -40C to +85C VHYS IFB FB = 1.5V IOUT = 0 to 400mA VIN = 2.5V to 5.5V 1.6 0.4 1.000 950 1.25 1.231 1.220 1.210 5 0.01 0.005 0.2 0.20 1.25 1.25 8 UVLO VIN rising VIN falling 1.50 1.85 1.65 200 50 220 0.1 -1.5 -3 -3 70 300 4.0 +1.5 +3 +3 1 16 4.00 1.269 1.280 1.280 mV A %/mA %/V V V A mA V A V % SYMBOL I(LX) < 400mA VIN I(LX) < 200mA (MAX1921EUT15, MAX1921EUT18) CONDITIONS MIN 2.5 2.0 TYP MAX 5.5 2.5 2.0 1.95 V V V mV A A A UNITS
2
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, SHDN = IN, TA = 0C to +85C. Typical parameters are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER 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 SYMBOL ILIMN RONHS RONSR ILXOFF ILXLEAK ILXLKR tON(MIN) tOFF(MIN) IN = SHDN = 5.5V, LX = 0 to IN IN unconnected, VLX = 5.5V, SHDN = GND 0.28 0.28 ILX = -40mA, VIN = 3V ILX = 40mA, VIN = 3V CONDITIONS MIN 350 TYP 550 0.6 0.5 60 0.1 0.1 0.4 0.4 5.0 5.0 0.5 0.5 MAX 800 1.1 0.9 UNITS mA mA A A s s
MAX1920/MAX1921
Note 1: All devices are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design.
Typical Operating Characteristics
(CIN = 2.2F ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1920 toc01
EFFICIENCY vs. LOAD CURRENT (VOUT = 2.5V)
MAX1920 toc02
EFFICIENCY vs. LOAD CURRENT (VOUT = 1.5V)
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 VIN = 5V VIN = 3.3V VIN = 2.5V
MAX1920 toc03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1
VIN = 3.6V VIN = 5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VIN = 2.7V
100
VIN = 4.2V
VIN = 3.3V
VIN = 5V
1
10
100
1000
0.1
1
10
100
1000
0.1
1
10
100
1000
LOAD CURRENT ( mA)
LOAD CURRENT ( mA)
LOAD CURRENT ( mA)
OUTPUT VOLTAGE ACCURACY vs. LOAD (VOUT = 3.3V)
MAX1920 toc04
OUTPUT VOLTAGE ACCURACY vs. LOAD (VOUT = 2.5V)
MAX1920 toc05
OUTPUT VOLTAGE ACCURACY vs. LOAD (VOUT = 1.5V)
MAX1920 toc06
3.399 3.366 OUTPUT VOLTAGE 3.333 3.300 VIN = 4.2V 3.267 VIN = 3.6V 3.234 3.201 0 50
2.575 2.550 OUTPUT VOLTAGE 2.525 2.500 VIN = 3V 2.475 2.450 2.425
1.545 1.530 OUTPUT VOLTAGE 1.515 1.500 1.485 1.470 1.455 VIN = 5V
VIN = 5V
VIN = 5V
VIN = 3.3V VIN = 2.5V
100 150 200 250 300 350 400 LOAD (mA)
0
50
100 150 200 250 300 350 400 LOAD (mA)
0
50
100 150 200 250 300 350 400 LOAD (mA)
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 MAX1920/MAX1921
Typical Operating Characteristics (continued)
(CIN = 2.2F ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
SWITCHING FREQUENCY vs. LOAD (VOUT = 1.8V)
MAX1920 toc07
SWITCHING FREQUENCY vs. LOAD (VOUT = 1.5V)
MAX1920 toc08
NO LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1920 toc09
10,000
10,000
10,000 NO-LOAD SUPPLY CURRENT (A) VOUT = 2.5V 1000 VOUT = 3.3V
SWITCHING FREQUENCY (kHz)
1000
SWITCHING FREQUENCY (kHz)
1000
100
100
100 VOUT = 1.5V 10
10 VIN = 3.3 1 0.1 1 10 LOAD (mA) 100 1000
10 VIN = 3.3 1 0.1 1 10 LOAD (mA) 100 1000
1 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5..0 5.5 SUPPLY VOLTAGE (V)
LIGHT-LOAD SWITCHING WAVEFORM
MAX1920 toc10
MEDIUM-LOAD SWITCHING WAVEFORM
MAX1920 toc11
SOFT-START AND SHUTDOWN RESPONSE
MAX1920 toc12
VOUT AC-COUPLED 5mV/div
VOUT AC-COUPLED 5mV/div
VOUT 1V/div
IIN 100mA/div VLX 2V/div VLX 2V/div VSHDN 5V/div VIN = 3.3V, VOUT = 1.5V, ILOAD = 40mA 1s/div VIN = 3.3V, VOUT = 1.5V, ILOAD = 250mA 1s/div VIN = 3.3V, VOUT = 1.5V, RLOAD = 6 200s/div
MEDIUM-LOAD LINE-TRANSIENT RESPONSE
MAX1920 toc13
LIGHT-LOAD LINE-TRANSIENT RESPONSE
MAX1920 toc14
LOAD-TRANSIENT RESPONSE
MAX1920 toc15
VIN = 3.3V, VOUT = 1.5V, ILOAD = 20mA TO 320mA VIN AC-COUPLED 200mV/div VIN AC-COUPLED 200mV/div
VOUT AC-COUPLED 100mV/div IL 200mA/div
VOUT AC-COUPLED 5mV/div VIN = 3.8V to 4.2V, VOUT = 1.5V, ILOAD = 250mA 4s/div VIN = 3.8V to 4.2V, VOUT = 1.5V, ILOAD = 20mA 4s/div
VOUT AC-COUPLED 5mV/div
ILOAD 200mA/div
40s/div
4
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
Pin Description
PIN TDFN* 2 6 1 -- 5 3 4 SO 1 2 3 4 4 5 6 NAME FUNCTION Supply voltage input for MAX1921EUT15 and MAX1921EUT18 is 2V to 5.5V. Supply voltage input for MAX1920 and other voltage versions of MAX1921 is 2.5V to 5.5V. Bypass IN to GND with a 2.2F ceramic capacitor as close to IN as possible. Analog Ground. Connect to PGND. Active-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes high-impedance and quiescent current drops to 0.1A. MAX1921 Voltage Sense Input. OUT is connected to an internal voltage-divider. MAX1920 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external resistive voltage-divider between the output voltage and GND. Power Ground. Connect to AGND. Inductor Connection
MAX1920/MAX1921
IN AGND SHDN OUT FB PGND LX
*MAX1920 only.
Detailed Description
The MAX1920/MAX1921 step-down DC-DC converters deliver over 400mA to outputs as low as 1.25V. They use a unique proprietary current-limited control scheme that maintains extremely low quiescent supply current (50A), and their high 1.2MHz (max) operating frequency permits small, low-cost external components.
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 MAX1920/MAX1921 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 MAX1920/MAX1921 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.
Control Scheme
The MAX1920/MAX1921 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 on-time 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
INPUT 2V TO 5.5V CIN 2 OUTPUT UP TO 400mA
Shutdown Mode
Connecting SHDN to GND places the MAX1920/ MAX1921 in shutdown mode and reduces supply current to 0.1A. In shutdown, the control circuitry, internal switching MOSFET, and synchronous rectifier turn off and LX becomes high impedance. Connect SHDN to IN for normal operation.
1
IN
LX
6 R1
L
Soft-Start
The MAX1920/MAX1921 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.
5
COUT CFF
MAX1921
AGND PGND
5
ON OFF
3
SHDN
OUT
4
Figure 1. Typical Output Application Circuit (MAX1921)
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 MAX1920/MAX1921
Design Procedure
The MAX1920/MAX1921 are optimized for small external components and fast transient response. There are several application circuits (Figures 1 through 4) to allow the choice between ceramic or tantalum output capacitor and internally or externally set output voltages. The use of a small ceramic output capacitor is preferred for higher reliability, improved voltage-positioning transient response, reduced output ripple, and the smaller size and greater availability of ceramic versus tantalum capacitors. In order to calculate the smallest inductor, several calculations are needed. First, calculate the maximum duty cycle of the application as: DutyCycle(MAX) = VOUT x 100% VIN (MIN)
Second, calculate the critical voltage across the inductor as: if DutyCycle(MAX) < 50%, then VCRITICAL = (VIN(MIN) - VOUT), else VCRITICAL = VOUT Last, calculate the minimum inductor value as: L(MIN) = 2.5 x 10 -6 x VCRITICAL Select the next standard value larger than L(MIN). The L(MIN) calculation already includes a margin for inductance tolerance. Although values much larger than L(MIN) work, transient performance, efficiency, and inductor size suffer. A 550mA rated inductor is enough to prevent saturation for output currents up to 400mA. 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 2 and 3 list some suggested inductors and suppliers.
Voltage Positioning
Figures 1 and 2 are the application circuits that utilize small ceramic output capacitors. For stability, the circuit obtains feedback from the LX node through R1, while load transients are fed-forward through CFF. Because there is no D.C. feedback from the output, the output voltage exhibits load regulation that is equal to the output load current multiplied by the inductor's series resistance. This small amount of load regulation is similar to voltage positioning as used by high-powered microprocessor supplies intended for personal computers. For the MAX1920/MAX1921, voltage positioning eliminates or greatly reduces undershoot and overshoot during load transients (see the Typical Operating Characteristics), which effectively halves the peak-to-peak output voltage excursions compared to traditional step-down converters. For convenience, Table 1 lists the recommended external component values for use with the MAX1921 application circuit of Figure 1 with various input and output voltages.
Capacitor Selection
For nearly all applications, the input capacitor, CIN, may be as small as 2.2F ceramic with X5R or X7R
Table 2. Suggested Inductors Table 1. MAX1921 Suggested Components for Figure 1
INPUT SOURCE OUTPUT 3.3V 3.0V 2.5V 5V 3.3V, 1 Li+, 3 x AA 2.5V, 2 x AA PART NUMBER Coilcraft LPO1704 L (H) 4.7 6.8 10 4.7 Sumida CDRH3D16 Sumida CDRH2D18 Toko D312F Toko D412F Toko D52LC 6.8 10 N/A 4.7 6.8 4.7 10 4.7 10 4.7 6.8 10 RL Isat (A) (ohms max) 0.200 0.320 0.410 0.080 0.095 0.160 0.081 0.108 0.38 0.79 0.230 0.490 0.087 0.105 0.150 1.10 0.90 0.80 0.90 0.73 0.55 0.63 0.57 0.74 0.50 0.84 0.55 1.14 0.95 0.76 3.2 x 3.2 x 2.0 = 20.5mm3 3.6 x 3.6 x 1.2 = 15.6mm3 4.6 x 4.6 x 1.2 = 25.4mm3 5.0 x 5.0 x 2.0 = 50.0mm3 3.8 x 3.8 x 1.8 = 26.0mm3 SIZE 6.6 x 5.5 x 1.0 = 36.3mm3
L = 10H, COUT = 10F, R1 = 8.25k, CFF = 3300pF L = 6.8H, COUT = 6.8F, R1 = 5.62k, CFF = 4700pF L = 10H, COUT = 10F, R1 = 8.25k, CFF = 3300pF
1.8V 1.5V
L = 4.7H, COUT = 4.7F, R1 = 4.75k, CFF = 5600pF
Inductor Selection
6
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
dielectric. The input capacitor filters peak currents and noise at the voltage source and, therefore, must meet the input ripple requirements and voltage rating. Calculate the maximum RMS input current as: IIN (RMS) = IOUT (MAX) x VOUT (VIN - VOUT ) VIN Tantalum Output Capacitor For tantalum COUT, use the application circuit of Figure 3 or Figure 4. With tantalum COUT, the equivalent series resistance (ESR) of COUT must be large enough for stability. Generally, 25mV of ESR-ripple at the feedback node is sufficient. The simplified calculation is: ESRCOUT (MIN) = 8.0 x 10 -2 x VOUT Because tantalum capacitors rarely specify minimum ESR, choose a capacitor with typical ESR that is about twice as much as ESR COUT (MIN). Although ESRs greater than this work, output ripple becomes larger. For tantalum C OUT , calculate the minimum output capacitance as: COUT (MIN) = 1.25 x L x IOUT (MAX) ESRCOUT (MIN) x VCRITICAL
MAX1920/MAX1921
The output capacitor, COUT, may be either ceramic or tantalum depending upon the chosen application circuit (see Figures 1 through 4). Table 3 lists some suggested capacitor suppliers. Ceramic Output Capacitor For ceramic COUT, use the application circuit of Figure 1 or Figure 2. Calculate the minimum capacitor value as: COUT (MIN) = 2.5 x 10 -6 x VCRITICAL Select the next standard value larger than COUT(MIN). The COUT(MIN) calculation already includes a margin for capacitor tolerance. Values much larger than COUT(MIN) always improve transient performance and stability, but capacitor size and cost increase.
The 1.25 multiplier is for capacitor tolerance. Select any standard value larger than COUT(MIN).
Feedback and Compensation
The MAX1921 has factory preset output voltages of 1.5V, 1.8V, 2.5V, 3V, and 3.3V, while the MAX1920 is externally adjusted by connecting FB to a resistive voltage-divider. When using a ceramic output capacitor, the feedback network must include a compensation feed-forward capacitor, CFF.
INPUT 2V TO 5.5V CIN
1
IN
LX
6 R1
L
OUTPUT UP TO 400mA
INPUT 2V TO 5.5V CIN
1
IN
LX
6
L
OUTPUT UP TO 400mA
COUT CFF
COUT 2
2
MAX1920
AGND PGND
5
MAX1921
AGND PGND
5
ON OFF
3
SHDN
FB
4
ON OFF R2
3
SHDN
OUT
4
Figure 2. Typical Application Circuit (MAX1920)
Figure 3. MAX1921 Application Circuit Using Tantalum Output Capacitor
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 MAX1920/MAX1921
Table 3. Component Suppliers
SUPPLIER Coilcraft Kemet Murata Sumida Taiyo Yuden Toko USA Japan USA Japan USA Japan PHONE 847-639-6400 408-986-0424 814-237-1431 847-956-0666 81-3-3607-5111 408-573-4150 81-3-3833-5441 847-297-0070 81-3-3727-1161 WEBSITE www.coilcraft.com www.kemet.com www.murata.com www.sumida.com www.T-Yuden.com www.yuden.co.jp www.tokoam.com www.toko.co.jp
MAX1920 Using Ceramic COUT When using the application circuit of Figure 2, the inductor's series resistance causes a small amount of load regulation, as desired for a voltage-positioning load transient response. Choose R1 and R2 such that VOUT is high at no load by about half of this load regulation: V + RL x IOUT (MAX) / 2 R1 = R2 x OUT - 1 VREF where R2 is chosen in the 50k to 500k range, VREF = 1.25V and RL is the typical series resistance of the inductor. Use 1% or better resistors. Next, calculate the equivalent resistance at the FB node as: R1 x R2 R1 + R2 Then, calculate CFF for 25mV ripple at FB. The simplified calculation is: Re q = R1 || R2 = CFF = 2.5 x 10 -5 Re q Select a standard capacitor value that is within 20% of the calculated CFF. MAX1920 Using Tantalum COUT When using the application circuit of Figure 4, choose R1 and R2 such as to obtain the desired VOUT: V R1 = R2 x OUT - 1 VREF where R2 is chosen to be less than 50k and VREF = 1.25V. Use 1% or better resistors.
MAX1921 Using Ceramic COUT
When using the application circuit of Figure 1, the inductor's series resistance causes a small amount of load regulation, as desired for a voltage-positioning load transient response. Choose R1 such that VOUT is high at no load by about half of this load regulation. The simplified calculation is: R1 = 5 x 104 x RL (MAX) where RL(MAX) is the maximum series resistance of the inductor. Select a standard resistor value that is within 20% of this calculation. Next, calculate CFF for 25mV ripple at the internal feedback node. The simplified calculation is: CFF = 2.5 x 10
-5
R1
where R1 is the standard resistor value that is used. Select a standard capacitor value that is within 20% of the calculated CFF.
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 0.2in (5mm). Keep noisy traces, such as the LX trace, away from the voltagefeedback network; also keep them separate, using grounded copper. The MAX1920/MAX1921 evaluation kit data sheet includes a proper PC board layout and routing scheme.
INPUT 2V TO 5.5V CIN
1
IN
LX
6
L
OUTPUT UP TO 400mA
COUT 2
MAX1920
AGND PGND
5
R1
ON OFF
3
SHDN
FB
4
R2
Figure 4. MAX1920 Application Circuit Using Tantalum Output Capacitor 8
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
Selector Guide
PART MAX1920EUT MAX1920ETT MAX1921EUT33 MAX1921EUT30 MAX1921EUT25 MAX1921EUT18 MAX1921EUT15 VOUT (V) Adjustable Adjustable 3.3 3.0 2.5 1.8 1.5 TOP MARK ABCO ADR ABCJ ABCK ABCL ABCM ABCN
MAX1920/MAX1921
Chip Information
TRANSISTOR COUNT: 1467
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.)
D2 D A2
N
PIN 1 ID
0.35x0.35 b
PIN 1 INDEX AREA
E DETAIL A
E2 e
[(N/2)-1] x e REF.
A1
k
C L
C L
A
L e e
L
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
1
2
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6, 8, &10L, DFN THIN.EPS
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 MAX1920/MAX1921
Package Information (continued)
(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.)
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 MAX. 0.80 3.10 3.10 0.05
0.20 0.40 0.25 MIN. 0.20 REF.
PACKAGE VARIATIONS PKG. CODE T633-1 T633-2 T833-1 T833-2 T833-3 T1033-1 T1433-1 T1433-2 N 6 6 8 8 8 10 14 14 D2 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.700.10 1.700.10 E2 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 e 0.95 BSC 0.95 BSC 0.65 BSC 0.65 BSC 0.65 BSC 0.50 BSC 0.40 BSC 0.40 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEA MO229 / WEEC MO229 / WEEC MO229 / WEEC MO229 / WEED-3 ------b 0.400.05 0.400.05 0.300.05 0.300.05 0.300.05 0.250.05 0.200.05 0.200.05 [(N/2)-1] x e 1.90 REF 1.90 REF 1.95 REF 1.95 REF 1.95 REF 2.00 REF 2.40 REF 2.40 REF
DOWNBONDS ALLOWED
NO NO NO NO YES NO YES NO
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
2
2
10
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Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23
Package Information (continued)
(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.)
6LSOT.EPS
PACKAGE OUTLINE, SOT 6L BODY
MAX1920/MAX1921
21-0058
G
1
1
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) 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

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