General Description
The MAX1920/MAX1921 step-down converters deliver
over 400mA to outputs as low as 1.25V. These convert-
ers use a unique proprietary current-limited control
scheme that achieves over 90% efficiency. These
devices maintain extremely low quiescent supply current
(50µA), and their high 1.2MHz (max) operating frequency
permits small, low-cost external components. This combi-
nation makes the MAX1920/MAX1921 excellent high-
efficiency alternatives to linear regulators in space-
constrained applications.
Internal synchronous rectification greatly improves effi-
ciency 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.
Applications
Next-Generation Wireless Handsets
PDAs, Palmtops, and Handy-Terminals
Battery-Powered Equipment
CDMA Power Amplifier Supply
Features
♦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
♦50µA Quiescent Supply Current
♦0.1µA 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
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.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
________________________________________________________________ Maxim Integrated Products 1
19-2296; Rev 3; 8/05
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.
MAX1921
IN
AGND
SHDN
LX
PGND
OUT
OFF
ON
4.75kΩ4.7µF
4.7µH
5600pF
OUTPUT
1.5V UP TO 400mA
INPUT
2V TO 5.5V
CIN
Typical Operating Circuit
PART
TEMP RANGE
PIN-PACKAGE
MAX1920EUT-T -40°C to +85°C 6 SOT23-6
MAX1920EUT+T -40°C to +85°C 6 SOT23-6
MAX1920ETT-T -40°C to +85°C 6 TDFN
MAX1920ETT+T -40°C to +85°C 6 TDFN
MAX1921EUT_ _-T -40°C to +85°C 6 SOT23-6
MAX1921EUT_ _+T
-40°C to +85°C 6 SOT23-6
Ordering Information
AGND
AGND
OUT (FB)SHDN
16LX
LX
FB
5PGND
PGND
IN
IN
MAX1920
MAX1921 MAX1920
SOT23-6
TOP VIEW
2
34
( ) ARE FOR MAX1920 ONLY
A "+" sign will replace the first pin indicator on lead-free packages.
SHDN
123
654
TDFN
Pin Configuration
MAX1920/MAX1921
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 = +70°C)
6-Pin SOT23-6 (derate 8.7mW/°C above +70°C) . . . .695mW
6-Pin TDFN (derate 18.2mW/°C above +70°C) . . .1454.5mW
Operating Temperature Range . . . . . . . . . . . . . .-40°C to +85°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . .+150°C
Storage Temperature . . . . . . . . . . . . . . . . . . . .-65°C to +150°C
Lead Temperature (soldering 10s) . . . . . . . . . . . . . . . . .+300°C
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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= 0°C to +85°C. Typical parameters are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
I(LX) < 400mA 2.5 5.5
Input Voltage Range VIN I(LX) < 200mA
(MAX1921EUT15, MAX1921EUT18) 2.0 2.5 V
Startup Voltage 2.0 V
VIN rising
1.85 1.95
UVLO Threshold UVLO VIN falling
1.50 1.65
V
UVLO Hysteresis
200
mV
Quiescent Supply Current IIN No switching, no load 50 70 µA
Quiescent Supply Current
Dropout IIN SHDN = IN, OUT/FB = 0
220
300 µA
Shutdown Supply Current ISHDN SHDN = GND 0.1 4.0 µA
IOUT = 0, TA = +25°C
-1.5 +1.5
IOUT = 0 to 400mA, TA = -40°C to +85°C -3 +3
Output Voltage Accuracy
(MAX1921)
IOUT = 0 to 200mA, TA = -40°C to +85°C -3 +3
%
SHDN = 0 1
OUT BIAS Current IOUT OUT at regulation voltage 8 16 µA
Output Voltage Range (MAX1920)
Figure 4, IN = 4.5V
1.25 4.00
V
TA= 25°C
1.231 1.25 1.269
1.220 1.25 1.280
FB Feedback Threshold
(MAX1920) VFB
TA = -40°C to +85°C
1.210 1.280
V
FB Feedback Hysteresis
(MAX1920) VHYS 5mV
FB Bias Current (MAX1920) IFB FB = 1.5V
0.01 0.20
µA
Load Regulation IOUT = 0 to 400mA
0.005
%/mA
Line Regulation VIN = 2.5V to 5.5V 0.2
%/V
SHDN Input Voltage High VIH 1.6 V
SHDN Input Voltage Low VIL 0.4 V
SHDN Leakage Current ISHDN SHDN = GND or IN
0.001 1.000
µA
High-Side Current Limit ILIMP
525 730
950 mA
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
_______________________________________________________________________________________ 3
100
0
0.1 1 10 100 1000
EFFICIENCY vs. LOAD CURRENT
(VOUT = 2.5V)
20
MAX1920 toc02
LOAD CURRENT ( mA)
EFFICIENCY (%)
40
60
80
70
50
30
10
90
VIN = 2.7V
VIN = 3.3V VIN = 5V
100
0
0.1 1 10 100 1000
EFFICIENCY vs. LOAD CURRENT
(VOUT = 1.5V)
20
MAX1920 toc03
LOAD CURRENT ( mA)
EFFICIENCY (%)
40
60
80
70
50
30
10
90 VIN = 2.5V
VIN = 3.3V
VIN = 5V
3.201
3.234
3.267
3.300
3.333
3.366
3.399
0 10050 150 200 250 300 350 400
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 3.3V)
MAX1920 toc04
LOAD (mA)
OUTPUT VOLTAGE
VIN = 3.6V
VIN = 5V
VIN = 4.2V
2.425
2.450
2.475
2.500
2.525
2.550
2.575
0 10050 150 200 250 300 350 400
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 2.5V)
MAX1920 toc05
LOAD (mA)
OUTPUT VOLTAGE
VIN = 3V
VIN = 5V
1.455
1.470
1.485
1.500
1.515
1.530
1.545
0 10050 150 200 250 300 350 400
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 1.5V)
MAX1920 toc06
LOAD (mA)
OUTPUT VOLTAGE
VIN = 5V
VIN = 3.3V
VIN = 2.5V
Typical Operating Characteristics
(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 3.6V, SHDN = IN, TA= 0°C to +85°C. Typical parameters are at TA= +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN TYP MAX
UNITS
Low-Side Current Limit ILIMN
350 550
800 mA
High-Side On-Resistance
RONHS
ILX = -40mA, VIN = 3V 0.6 1.1 Ω
Rectifier On-Resistance
RONSR
ILX = 40mA, VIN = 3V 0.5 0.9 Ω
Rectifier Off-Current Threshold
ILXOFF
60 mA
LX Leakage Current
ILXLEAK
IN = SHDN = 5.5V, LX = 0 to IN 0.1 5.0 µA
LX Reverse Leakage Current
ILXLKR
IN unconnected, VLX = 5.5V, SHDN = GND 0.1 5.0 µA
Minimum On-Time
tON(MIN) 0.28
0.4 0.5 µs
Minimum Off-Time
tOFF(MIN) 0.28
0.4 0.5 µs
Note 1: All devices are 100% production tested at TA= +25°C. Limits over the operating temperature range are guaranteed
by design.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
4_______________________________________________________________________________________
Typical Operating Characteristics (continued)
(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
10,000
1
0.1 1 100 1000
SWITCHING FREQUENCY vs. LOAD
(VOUT = 1.8V)
10
100
1000
MAX1920 toc07
LOAD (mA)
SWITCHING FREQUENCY (kHz)
10
VIN = 3.3
10,000
1
0.1 1 100 1000
SWITCHING FREQUENCY vs. LOAD
(VOUT = 1.5V)
10
100
1000
MAX1920 toc08
LOAD (mA)
SWITCHING FREQUENCY (kHz)
10
VIN = 3.3
NO LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX1920 toc09
SUPPLY VOLTAGE (V)
NO-LOAD SUPPLY CURRENT (µA)
5..04.54.03.53.02.52.0
10
100
1000
10,000
1
1.5 5.5
VOUT = 3.3V
VOUT = 2.5V
VOUT = 1.5V
LIGHT-LOAD SWITCHING WAVEFORM
MAX1920 toc10
VOUT
AC-COUPLED
5mV/div
VLX
2V/div
1µs/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 40mA
MEDIUM-LOAD SWITCHING WAVEFORM
MAX1920 toc11
VOUT
AC-COUPLED
5mV/div
VLX
2V/div
1µs/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 250mA
SOFT-START AND SHUTDOWN RESPONSE
MAX1920 toc12
VOUT
1V/div
IIN
100mA/div
VSHDN
5V/div
200µs/div
VIN = 3.3V, VOUT = 1.5V,
RLOAD = 6Ω
MEDIUM-LOAD
LINE-TRANSIENT RESPONSE
MAX1920 toc13
VIN
AC-COUPLED
200mV/div
VOUT
AC-COUPLED
5mV/div
4µs/div
VIN = 3.8V to 4.2V,
VOUT = 1.5V, ILOAD = 250mA
LIGHT-LOAD
LINE-TRANSIENT RESPONSE
MAX1920 toc14
VIN
AC-COUPLED
200mV/div
VOUT
AC-COUPLED
5mV/div
4µs/div
VIN = 3.8V to 4.2V,
VOUT = 1.5V, ILOAD = 20mA
LOAD-TRANSIENT RESPONSE
MAX1920 toc15
VOUT
AC-COUPLED
100mV/div
ILOAD
200mA/div
IL
200mA/div
40µs/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 20mA TO 320mA
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 (50µA),
and their high 1.2MHz (max) operating frequency permits
small, low-cost external components.
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
remains on until either the high-side switch turns on again
or the inductor current approaches zero. The internal syn-
chronous 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 effi-
ciency and reduced output voltage ripple. When deliver-
ing medium and higher output currents, the
MAX1920/MAX1921 extend 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.
Shutdown Mode
Connecting SHDN to GND places the MAX1920/
MAX1921 in shutdown mode and reduces supply cur-
rent to 0.1µA. 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.
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 alka-
line cells. Soft-start is implemented by starting with the
current limit at 25% of its full current value and gradual-
ly increasing it in 25% steps until the full current limit is
reached. See Soft-Start and Shutdown Response in the
Typical Operating Characteristics.
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
_______________________________________________________________________________________ 5
Pin Description
PIN
T D F N *
SO
NAME
FUNCTION
21IN
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 2.2µF
ceramic capacitor as close to IN as possible.
62
AGND
Analog Ground. Connect to PGND.
13
SHDN
Active-Low Shutdown Input. Connect SHDN to IN for normal operation. In shutdown, LX becomes
high-impedance and quiescent current drops to 0.1µA.
—4OUT MAX1921 Voltage Sense Input. OUT is connected to an internal voltage-divider.
54FB
MAX1920 Voltage Feedback Input. FB regulates to 1.25V nominal. Connect FB to an external
resistive voltage-divider between the output voltage and GND.
35
PGND
Power Ground. Connect to AGND.
46LX Inductor Connection
MAX1921
IN
AGND
SHDN
LX
PGND
OUT
OFF
ON
R1 COUT
L
CFF
OUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
CIN
1
2
34
5
6
Figure 1. Typical Output Application Circuit (MAX1921)
*MAX1920 only.
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 volt-
ages. The use of a small ceramic output capacitor is
preferred for higher reliability, improved voltage-posi-
tioning transient response, reduced output ripple, and
the smaller size and greater availability of ceramic versus
tantalum capacitors.
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 volt-
age 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.
Inductor Selection
In order to calculate the smallest inductor, several cal-
culations are needed. First, calculate the maximum
duty cycle of the application as:
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:
Select the next standard value larger than L(MIN). The
L(MIN) calculation already includes a margin for induc-
tance 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.
Capacitor Selection
For nearly all applications, the input capacitor, CIN,
may be as small as 2.2µF ceramic with X5R or X7R
L MIN VCRITICAL
().=× ×
−
25 10 6
DutyCycle MAX V
V MIN
OUT
IN
() () %=×100
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
6_______________________________________________________________________________________
Table 1. MAX1921 Suggested
Components for Figure 1
INPUT SOURCE
OUTPUT
5V
3.3V, 1 Li+,
3 x AA
2.5V, 2 x AA
3.3V
3.0V
L = 10µH, COUT = 10µF,
R1 = 8.25kΩ, CFF = 3300pF
2.5V L = 6.8µH, COUT = 6.8µF,
R1 = 5.62kΩ, CFF = 4700pF
N/A
1.8V
1.5V
L = 10µH,
COUT = 10µF,
R1 = 8.25kΩ,
CFF = 3300pF
L = 4.7µH, COUT = 4.7µF,
R1 = 4.75kΩ, CFF = 5600pF
Table 2. Suggested Inductors
PART
NUMBER
L
(µH)
RL
(ohms max)
Isat (A)
SIZE
4.7 0.200
1.10
6.8 0.320
0.90
Coilcraft
LPO1704
10 0.410
0.80
6.6 x 5.5 x 1.0
= 36.3mm3
4.7 0.080
0.90
6.8 0.095
0.73
Sumida
CDRH3D16
10 0.160
0.55
3.8 x 3.8 x 1.8
= 26.0mm3
4.7 0.081
0.63
Sumida
CDRH2D18
6.8 0.108
0.57
3.2 x 3.2 x 2.0
= 20.5mm3
4.7 0.38
0.74
Toko
D312F 10 0.79
0.50
3.6 x 3.6 x 1.2
= 15.6mm3
4.7 0.230
0.84
Toko
D412F 10 0.490
0.55
4.6 x 4.6 x 1.2
= 25.4mm3
4.7 0.087
1.14
6.8 0.105
0.95
Toko
D52LC
10 0.150
0.76
5.0 x 5.0 x 2.0
= 50.0mm3
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:
The output capacitor, COUT, may be either ceramic or
tantalum depending upon the chosen application cir-
cuit (see Figures 1 through 4). Table 3 lists some sug-
gested capacitor suppliers.
Ceramic Output Capacitor
For ceramic COUT, use the application circuit of Figure 1
or Figure 2. Calculate the minimum capacitor value as:
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.
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 sta-
bility. Generally, 25mV of ESR-ripple at the feedback
node is sufficient. The simplified calculation is:
Because tantalum capacitors rarely specify minimum
ESR, choose a capacitor with typical ESR that is about
twice as much as ESRCOUT(MIN). Although ESRs
greater than this work, output ripple becomes larger.
For tantalum COUT, calculate the minimum output
capacitance as:
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 volt-
age-divider. When using a ceramic output capacitor,
the feedback network must include a compensation
feed-forward capacitor, CFF.
C MIN LI MAX
ESR MIN V
OUT OUT
COUT CRITICAL
(). ()
()
=× ×
×
125
ESR MIN V
COUT OUT
().=× ×
−
80 10 2
C MIN V
OUT CRITICAL
().=× ×
−
25 10 6
I RMS I MAX VVV
V
IN OUT OUT IN OUT
IN
() () ()
=× −
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
_______________________________________________________________________________________ 7
MAX1920
IN
AGND
SHDN
LX
PGND
FB
OFF
ON
R1 COUT
L
CFF
OUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
CIN
R2
2
1
3
5
6
4
Figure 2. Typical Application Circuit (MAX1920)
MAX1921
IN
AGND
SHDN
LX
PGND
OUT
OFF
ON
COUT
LOUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
CIN
2
1
3
5
6
4
Figure 3. MAX1921 Application Circuit Using Tantalum Output
Capacitor
MAX1920/MAX1921
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:
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 feed-
back node. The simplified calculation is:
where R1 is the standard resistor value that is used.
Select a standard capacitor value that is within 20% of
the calculated CFF.
MAX1920 Using Ceramic COUT
When using the application circuit of Figure 2, the induc-
tor’s series resistance causes a small amount of load
regulation, as desired for a voltage-positioning load tran-
sient response. Choose R1 and R2 such that VOUT is
high at no load by about half of this load regulation:
where R2 is chosen in the 50kΩto 500kΩrange, VREF
= 1.25V and RLis the typical series resistance of the
inductor. Use 1% or better resistors.
Next, calculate the equivalent resistance at the FB node as:
Then, calculate CFF for 25mV ripple at FB. The simpli-
fied calculation is:
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:
where R2 is chosen to be less than 50kΩand VREF =
1.25V. Use 1% or better resistors.
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 gra-
dients 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 voltage-
feedback network; also keep them separate, using
grounded copper. The MAX1920/MAX1921 evaluation
kit data sheet includes a proper PC board layout and
routing scheme.
RR V
V
OUT
REF
12 1=× −
Cq
FF =×
−
25 10 5
. Re
Re ||
qRR RR
RR
==
×
+
12 12
12
RR VRIMAX
V
OUT L OUT
REF
12 21=× +× −
()/
CR
FF =×
−
25 10 1
5
.
RRMAX
L
15 10
4
=× × ()
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
8_______________________________________________________________________________________
Table 3. Component Suppliers
SUPPLIER PHONE WEBSITE
Coilcraft 847-639-6400
www.coilcraft.com
Kemet 408-986-0424 www.kemet.com
Murata 814-237-1431
www.murata.com
USA
847-956-0666
Sumida
Japan
81-3-3607-5111
www.sumida.com
USA
408-573-4150
www.T-Yuden.com
Taiyo
Yuden Japan
81-3-3833-5441
www.yuden.co.jp
USA
847-297-0070
www.tokoam.com
Toko Japan
81-3-3727-1161
www.toko.co.jp
MAX1920
IN
AGND
SHDN
LX
PGND
FB
OFF
ON
R1
COUT
LOUTPUT
UP TO 400mA
INPUT
2V TO 5.5V
CIN
R2
2
1
3
5
6
4
Figure 4. MAX1920 Application Circuit Using Tantalum Output
Capacitor
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
_______________________________________________________________________________________ 9
Chip Information
TRANSISTOR COUNT: 1467
PART VOUT (V) TOP MARK
MAX1920EUT Adjustable ABCO
MAX1920ETT Adjustable ADR
MAX1921EUT33 3.3 ABCJ
MAX1921EUT30 3.0 ABCK
MAX1921EUT25 2.5 ABCL
MAX1921EUT18 1.8 ABCM
MAX1921EUT15 1.5 ABCN
Selector Guide
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.)
6, 8, &10L, DFN THIN.EPS
L
CL
C
PIN 1
INDEX
AREA
D
E
L
e
L
A
e
E2
N
G
1
2
21-0137
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
k
e
[(N/2)-1] x e
REF.
PIN 1 ID
0.35x0.35
DETAIL A
b
D2
A2
A1
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.)
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
10 ______________________________________________________________________________________
COMMON DIMENSIONS
SYMBOL MIN. MAX.
A0.70 0.80
D2.90 3.10
E2.90 3.10
A1 0.00 0.05
L0.20 0.40
PKG. CODE ND2 E2 eJEDEC SPEC b[(N/2)-1] x e
PACKAGE VARIATIONS
0.25 MIN.k
A2 0.20 REF.
2.30±0.101.50±0.106T633-1 0.95 BSC MO229 / WEEA 1.90 REF0.40±0.05
1.95 REF0.30±0.05
0.65 BSC
2.30±0.108T833-1
2.00 REF0.25±0.05
0.50 BSC
2.30±0.1010T1033-1
2.40 REF0.20±0.05- - - -
0.40 BSC
1.70±0.10 2.30±0.1014T1433-1
1.50±0.10
1.50±0.10
MO229 / WEEC
MO229 / WEED-3
0.40 BSC - - - - 0.20±0.05 2.40 REFT1433-2 14 2.30±0.101.70±0.10
T633-2 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF
T833-2 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF
T833-3 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF
-DRAWING NOT TO SCALE-
G2
2
21-0137
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
DOWNBONDS
ALLOWED
NO
NO
NO
NO
YES
NO
YES
NO
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
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
©2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.
6LSOT.EPS
PACKAGE OUTLINE, SOT 6L BODY
21-0058 1
1
G
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.)