Maxim MAX1920EUT Low-voltage, 400ma step-down dc-dc converters in sot23 Datasheet

19-2296; Rev 3; 8/05
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
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
(50µA), 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.
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
Ordering Information
PART
Applications
Next-Generation Wireless Handsets
PDAs, Palmtops, and Handy-Terminals
Battery-Powered Equipment
CDMA Power Amplifier Supply
MAX1920EUT-T
TEMP RANGE
PIN-PACKAGE
-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
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
TOP VIEW
LX
4.75kΩ
5600pF
CIN
MAX1921
AGND
PGND
4.7µF
IN 1
AGND 2
LX
IN
OUTPUT
1.5V UP TO 400mA
FB
4.7µH
AGND
INPUT
2V TO 5.5V
Pin Configuration
6
5
4
6 LX
MAX1920
MAX1921
5 PGND
MAX1920
ON
4 OUT (FB)
1
SOT23-6
2
3
PGND
SHDN 3
IN
OUT
SHDN
SHDN
OFF
TDFN
( ) ARE FOR MAX1920 ONLY
A "+" sign will replace the first pin indicator on lead-free packages.
________________________________________________________________ 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
MAX1920/MAX1921
General Description
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
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 = +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
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
Input Voltage Range
SYMBOL
VIN
CONDITIONS
MIN
TYP
I(LX) < 400mA
2.5
5.5
I(LX) < 200mA
(MAX1921EUT15, MAX1921EUT18)
2.0
2.5
Startup Voltage
UVLO Threshold
MAX
2.0
UVLO
VIN rising
VIN falling
1.85
1.50
UVLO Hysteresis
1.95
1.65
200
UNITS
V
V
V
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
IOUT = 0, TA = +25°C
0.1
4.0
µA
Output Voltage Accuracy
(MAX1921)
OUT BIAS Current
IOUT
Output Voltage Range (MAX1920)
-1.5
+1.5
IOUT = 0 to 400mA, TA = -40°C to +85°C
-3
+3
IOUT = 0 to 200mA, TA = -40°C to +85°C
-3
+3
OUT at regulation voltage
Figure 4, IN = 4.5V
TA= 25°C
FB Feedback Threshold
(MAX1920)
VFB
TA = -40°C to +85°C
FB Feedback Hysteresis
(MAX1920)
FB Bias Current (MAX1920)
1
SHDN = 0
8
1.25
4.00
1.231
1.25
1.269
1.220
1.25
1.280
1.210
VHYS
IFB
16
%
µA
V
V
1.280
5
mV
FB = 1.5V
0.01
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
SHDN Input Voltage Low
VIL
SHDN Leakage Current
High-Side Current Limit
ISHDN
2
ILIMP
0.20
1.6
SHDN = GND or IN
525
µA
V
0.4
V
0.001
1.000
µA
730
950
mA
_______________________________________________________________________________________
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
(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
350
550
800
mA
Ω
ILIMN
Low-Side Current Limit
High-Side On-Resistance
RONHS
ILX = -40mA, VIN = 3V
0.6
1.1
Rectifier On-Resistance
RONSR
ILX = 40mA, VIN = 3V
0.5
0.9
Rectifier Off-Current Threshold
ILXOFF
LX Leakage Current
ILXLEAK
IN = SHDN = 5.5V, LX = 0 to IN
ILXLKR
IN unconnected, VLX = 5.5V, SHDN = GND
Ω
60
mA
0.1
5.0
µA
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
LX Reverse Leakage Current
Note 1: All devices are 100% production tested at TA = +25°C. Limits over the operating temperature range are guaranteed
by design.
Typical Operating Characteristics
(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
50
40
50
40
70
50
30
20
20
10
10
10
30
0
1
10
100
1000
VIN = 5V
40
20
0.1
VIN = 3.3V
60
30
0
0
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)
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 2.5V)
OUTPUT VOLTAGE ACCURACY vs. LOAD
(VOUT = 1.5V)
VIN = 5V
3.300
VIN = 4.2V
3.267
VIN = 5V
1.530
OUTPUT VOLTAGE
3.333
2.550
OUTPUT VOLTAGE
3.366
1.545
MAX1920 toc05
2.575
MAX1920 toc04
3.399
OUTPUT VOLTAGE
80
VIN = 5V
60
VIN = 2.5V
90
2.525
2.500
VIN = 3V
2.475
VIN = 5V
1.515
1.500
VIN = 3.3V
1.485
VIN = 2.5V
VIN = 3.6V
3.234
2.450
3.201
1.470
2.425
0
50
100 150 200 250 300 350 400
LOAD (mA)
MAX1920 toc06
60
VIN = 3.3V
70
100
MAX1920 toc03
80
VIN = 5V
VIN = 4.2V
70
VIN = 2.7V
90
EFFICIENCY (%)
EFFICIENCY (%)
80
EFFICIENCY vs. LOAD CURRENT
(VOUT = 1.5V)
EFFICIENCY (%)
VIN = 3.6V
90
100
MAX1920 toc01
100
EFFICIENCY vs. LOAD CURRENT
(VOUT = 2.5V)
MAX1920 toc02
EFFICIENCY vs. LOAD CURRENT
(VOUT = 3.3V)
1.455
0
50
100 150 200 250 300 350 400
LOAD (mA)
0
50
100 150 200 250 300 350 400
LOAD (mA)
_______________________________________________________________________________________
3
MAX1920/MAX1921
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics (continued)
(CIN = 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
SWITCHING FREQUENCY vs. LOAD
(VOUT = 1.5V)
1000
100
10
1000
100
10
10,000
1
10
100
VOUT = 3.3V
100
VOUT = 1.5V
10
1
1
1
VOUT = 2.5V
1000
VIN = 3.3
VIN = 3.3
0.1
MAX1920 toc09
MAX1920 toc08
10,000
SWITCHING FREQUENCY (kHz)
MAX1920 toc07
10,000
NO LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE
NO-LOAD SUPPLY CURRENT (µA)
SWITCHING FREQUENCY vs. LOAD
(VOUT = 1.8V)
SWITCHING FREQUENCY (kHz)
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
1000
LOAD (mA)
0.1
1
10
100
1000
LOAD (mA)
LIGHT-LOAD SWITCHING WAVEFORM
2.0
2.5
3.0
3.5
4.0
4.5
5..0
5.5
SUPPLY VOLTAGE (V)
MEDIUM-LOAD SWITCHING WAVEFORM
MAX1920 toc10
1.5
SOFT-START AND SHUTDOWN RESPONSE
MAX1920 toc11
MAX1920 toc12
VOUT
1V/div
VOUT
AC-COUPLED
5mV/div
VOUT
AC-COUPLED
5mV/div
IIN
100mA/div
VLX
2V/div
VLX
2V/div
VSHDN
5V/div
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 40mA
VIN = 3.3V, VOUT = 1.5V,
RLOAD = 6Ω
VIN = 3.3V, VOUT = 1.5V,
ILOAD = 250mA
1µs/div
1µs/div
MEDIUM-LOAD
LINE-TRANSIENT RESPONSE
LIGHT-LOAD
LINE-TRANSIENT RESPONSE
MAX1920 toc13
200µs/div
LOAD-TRANSIENT RESPONSE
MAX1920 toc15
MAX1920 toc14
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
4
VOUT
AC-COUPLED
5mV/div
VIN = 3.8V to 4.2V,
VOUT = 1.5V, ILOAD = 250mA
VIN = 3.8V to 4.2V,
VOUT = 1.5V, ILOAD = 20mA
4µs/div
4µs/div
ILOAD
200mA/div
40µs/div
_______________________________________________________________________________________
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
PIN
NAME
FUNCTION
1
IN
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.
6
2
AGND
Analog Ground. Connect to PGND.
1
3
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.
—
4
OUT
5
4
FB
3
5
PGND
4
6
LX
TDFN*
SO
2
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 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 (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
INPUT
2V TO 5.5V
1
IN
LX
L
6
OUTPUT
UP TO 400mA
R1
CIN
2
ON
3
MAX1921
AGND
PGND
SHDN
OUT
5
COUT
CFF
4
OFF
Figure 1. Typical Output Application Circuit (MAX1921)
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.
Shutdown Mode
Connecting SHDN to GND places the MAX1920/
MAX1921 in shutdown mode and reduces supply current 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 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
MAX1920/MAX1921
Pin Description
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
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) =
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:
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.
VOUT
× 100%
VIN (MIN)
L(MIN) = 2.5 × 10 −6 × 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.
Capacitor Selection
For nearly all applications, the input capacitor, CIN,
may be as small as 2.2µF ceramic with X5R or X7R
Table 2. Suggested Inductors
PART
NUMBER
Table 1. MAX1921 Suggested
Components for Figure 1
Coilcraft
LPO1704
INPUT SOURCE
OUTPUT
5V
3.3V, 1 Li+,
3 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
1.8V
1.5V
L = 10µH,
COUT = 10µF,
R1 = 8.25kΩ,
CFF = 3300pF
2.5V, 2 x AA
N/A
L = 4.7µH, COUT = 4.7µF,
R1 = 4.75kΩ, CFF = 5600pF
Inductor Selection
6
Sumida
CDRH3D16
L
(µH)
RL
Isat (A)
(ohms max)
4.7
0.200
1.10
6.8
0.320
0.90
10
0.410
0.80
4.7
0.080
0.90
6.8
0.095
0.73
10
0.160
0.55
Sumida
CDRH2D18
4.7
0.081
0.63
6.8
0.108
0.57
Toko
D312F
4.7
0.38
0.74
10
0.79
0.50
Toko
D412F
4.7
0.230
0.84
10
0.490
0.55
4.7
0.087
1.14
6.8
0.105
0.95
10
0.150
0.76
Toko
D52LC
_______________________________________________________________________________________
SIZE
6.6 x 5.5 x 1.0
= 36.3mm3
3.8 x 3.8 x 1.8
= 26.0mm3
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
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
IIN (RMS) = IOUT (MAX) ×
VOUT (VIN − VOUT )
VIN
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 × 10 −6 × VCRITICAL
1
IN
LX
L
6
R1
CIN
2
ON
3
MAX1920
AGND
PGND
SHDN
FB
ESRCOUT (MIN) = 8.0 × 10 −2 × 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 ×
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.
INPUT
2V TO 5.5V
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:
OUTPUT
UP TO 400mA
COUT
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
OFF
IN
LX
6
L
OUTPUT
UP TO 400mA
COUT
2
ON
4
1
CIN
CFF
5
L × IOUT (MAX)
ESRCOUT (MIN) × VCRITICAL
3
MAX1921
AGND
PGND
SHDN
OUT
5
4
OFF
R2
Figure 2. Typical Application Circuit (MAX1920)
Figure 3. MAX1921 Application Circuit Using Tantalum Output
Capacitor
_______________________________________________________________________________________
7
MAX1920/MAX1921
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:
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Table 3. Component Suppliers
SUPPLIER
Coilcraft
PHONE
WEBSITE
847-639-6400
www.coilcraft.com
Kemet
408-986-0424
www.kemet.com
Murata
814-237-1431
www.murata.com
Sumida
Taiyo
Yuden
USA
847-956-0666
Japan
81-3-3607-5111
USA
Japan
USA
Toko
Japan
www.sumida.com
408-573-4150
www.T-Yuden.com
81-3-3833-5441
www.yuden.co.jp
847-297-0070
www.tokoam.com
81-3-3727-1161
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 × IOUT (MAX) / 2 
R1 = R2 ×  OUT
− 1
VREF


where R2 is chosen in the 50kΩ to 500kΩ 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:
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 × 104 × 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 × 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.
INPUT
2V TO 5.5V
1
IN
LX
6
OUTPUT
UP TO 400mA
L
CIN
COUT
2
ON
3
MAX1920
AGND
PGND
SHDN
FB
5
R1
4
OFF
R2
Figure 4. MAX1920 Application Circuit Using Tantalum Output
Capacitor
8
R1 × R2
R1 + R2
Then, calculate CFF for 25mV ripple at FB. The simplified calculation is:
Re q = R1 || R2 =
CFF = 2.5 × 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 ×  OUT − 1
V
 REF

where R2 is chosen to be less than 50kΩ and 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 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.
_______________________________________________________________________________________
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
VOUT (V)
TOP MARK
MAX1920EUT
Adjustable
ABCO
MAX1920ETT
Adjustable
ADR
PART
MAX1921EUT33
3.3
ABCJ
MAX1921EUT30
3.0
ABCK
MAX1921EUT25
2.5
ABCL
MAX1921EUT18
1.8
ABCM
MAX1921EUT15
1.5
ABCN
Chip Information
TRANSISTOR COUNT: 1467
Package Information
6, 8, &10L, DFN THIN.EPS
(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
PIN 1 ID
N
0.35x0.35
b
PIN 1
INDEX
AREA
E
[(N/2)-1] x e
REF.
E2
DETAIL A
e
k
A1
CL
CL
A
L
L
e
e
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
G
1
2
_______________________________________________________________________________________
9
MAX1920/MAX1921
Selector Guide
MAX1920/MAX1921
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.)
COMMON DIMENSIONS
MIN.
MAX.
D
0.70
2.90
0.80
3.10
E
A1
2.90
0.00
3.10
0.05
L
k
0.20
0.40
0.25 MIN.
A2
0.20 REF.
SYMBOL
A
PACKAGE VARIATIONS
PKG. CODE
N
D2
E2
e
JEDEC SPEC
b
[(N/2)-1] x e
DOWNBONDS
ALLOWED
T633-1
6
1.50±0.10
2.30±0.10
0.95 BSC
MO229 / WEEA
0.40±0.05
1.90 REF
NO
T633-2
6
1.50±0.10
2.30±0.10
0.95 BSC
MO229 / WEEA
0.40±0.05
1.90 REF
NO
T833-1
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
NO
T833-2
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
NO
T833-3
8
1.50±0.10
2.30±0.10
0.65 BSC
MO229 / WEEC
0.30±0.05
1.95 REF
YES
T1033-1
10
1.50±0.10
2.30±0.10
0.50 BSC
MO229 / WEED-3
0.25±0.05
2.00 REF
NO
T1433-1
14
1.70±0.10
2.30±0.10
0.40 BSC
----
0.20±0.05
2.40 REF
YES
T1433-2
14
1.70±0.10
2.30±0.10
0.40 BSC
----
0.20±0.05
2.40 REF
NO
PACKAGE OUTLINE, 6,8,10 & 14L,
TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
10
21-0137
______________________________________________________________________________________
G
2
2
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
6LSOT.EPS
PACKAGE OUTLINE, SOT 6L BODY
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
© 2005 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.
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.)
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