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.
Ordering Information
PART
MAX1920EUT-T
MAX1920EUT+T
MAX1920ETT-T
MAX1920ETT+T
MAX1921EUT_ _-T
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
6 SOT23-6
6 SOT23-6
6 TDFN
6 TDFN
6 SOT23-6
Applications
●
●
●
●
Next-Generation Wireless Handsets
PDAs, Palmtops, and Handy-Terminals
Battery-Powered Equipment
CDMA Power Amplifier Supply
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
a lead-free package.
Typical Operating Circuit
INPUT
2V TO 5.5V
C
IN
4.7µH
4.75kΩ
5600pF
OUTPUT
1.5V UP TO 400mA
4.7µF
Pin Configuration
AGND
FB
5
2
IN
IN
LX
IN 1
•
6
6 LX
MAX1921
AGND
PGND
AGND 2
MAX1920
MAX1921
5 PGND
MAX1920
SHDN 3
4 OUT (FB)
SHDN
OFF
SHDN
OUT
SOT23-6
( ) ARE FOR MAX1920 ONLY
A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES.
TDFN
19-2296; Rev 3; 8/05
PGND
ON
•
1
LX
4
3
TOP VIEW
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 (T
A
= +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
(V
IN
= 3.6V,
SHDN
= IN, T
A
= 0°C to +85°C. Typical parameters are at T
A
= +25°C, 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)
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
V
IH
V
IL
I
SHDN
I
LIMP
SHDN
= GND or IN
525
0.001
730
V
FB
T
A
= -40°C to +85°C
V
HYS
I
FB
FB = 1.5V
I
OUT
= 0 to 400mA
V
IN
= 2.5V to 5.5V
1.6
0.4
1.000
950
I
OUT
I
IN
I
IN
I
SHDN
No switching, no load
SHDN
= IN, OUT/FB = 0
SHDN
= GND
I
OUT
= 0, T
A
= +25°C
I
OUT
= 0 to 400mA, T
A
= -40°C to +85°C
I
OUT
= 0 to 200mA, T
A
= -40°C to +85°C
SHDN
= 0
OUT at regulation voltage
Figure 4, IN = 4.5V
T
A
= +25°C
1.25
1.231
1.220
1.210
5
0.01
0.005
0.2
0.20
1.25
1.25
8
-1.5
-3
-3
UVLO
V
IN
rising
V
IN
falling
1.50
1.85
1.65
200
50
220
0.1
70
300
4.0
+1.5
+3
+3
1
16
4.00
1.269
1.280
1.280
mV
µA
%/mA
%/V
V
V
µA
mA
V
µA
V
%
SYMBOL
I(LX) < 400mA
V
IN
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
µA
µA
µA
UNITS
www.maximintegrated.com
Maxim Integrated
│
2
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Electrical Characteristics (continued)
(V
IN
= 3.6V,
SHDN
= IN, T
A
= 0°C to +85°C. Typical parameters are at T
A
= +25°C, 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
I
LIMN
R
ONHS
R
ONSR
I
LXOFF
I
LXLEAK
I
LXLKR
t
ON(MIN)
t
OFF(MIN)
IN =
SHDN
= 5.5V, LX = 0 to IN
IN unconnected, V
LX
= 5.5V,
SHDN
= GND
0.28
0.28
I
LX
= -40mA, V
IN
= 3V
I
LX
= 40mA, V
IN
= 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
Ω
Ω
mA
µA
µA
µs
µs
Note 1:
All devices are 100% production tested at T
A
= +25°C. Limits over the operating temperature range are guaranteed by
design.
Typical Operating Characteristics
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 3.3V)
MAX1920 toc01
(C
IN
= 2.2μF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 2.5V)
MAX1920 toc02
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.1
V
IN
= 3.6V
V
IN
= 5V
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
V
IN
= 2.7V
V
IN
= 3.3V
V
IN
= 5V
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
V
IN
= 2.5V
V
IN
= 3.3V
V
IN
= 5V
V
IN
= 4.2V
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)
MAX1920 toc04
MAX1920 toc05
3.366
OUTPUT VOLTAGE
3.333
3.300
3.267
3.234
3.201
V
IN
= 4.2V
V
IN
= 3.6V
V
IN
= 5V
2.550
OUTPUT VOLTAGE
2.525
2.500
2.475
2.450
2.425
V
IN
= 5V
1.530
OUTPUT VOLTAGE
1.515
1.500
1.485
1.470
1.455
V
IN
= 3.3V
V
IN
= 5V
V
IN
= 3V
V
IN
= 2.5V
0
50
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)
www.maximintegrated.com
Maxim Integrated
│
3
MAX1920 toc06
3.399
OUTPUT VOLTAGE ACCURACY vs. LOAD
(V
OUT
= 3.3V)
2.575
OUTPUT VOLTAGE ACCURACY vs. LOAD
(V
OUT
= 2.5V)
1.545
OUTPUT VOLTAGE ACCURACY vs. LOAD
(V
OUT
= 1.5V)
MAX1920 toc03
100
100
100
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 1.5V)
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Typical Operating Characteristics (continued)
(C
IN
= 2.2μF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
MAX1920 toc07
MAX1920 toc08
NO-LOAD SUPPLY CURRENT (µA)
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY (kHz)
V
OUT
= 2.5V
1000
V
OUT
= 3.3V
1000
1000
100
100
100
10
V
IN
= 3.3
1
0.1
1
10
LOAD (mA)
100
1000
10
1
10
V
OUT
= 1.5V
V
IN
= 3.3
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
V
OUT
AC-COUPLED
5mV/div
V
OUT
AC-COUPLED
5mV/div
V
LX
2V/div
V
IN
= 3.3V, V
OUT
= 1.5V,
I
LOAD
= 40mA
1µs/div
V
IN
= 3.3V, V
OUT
= 1.5V,
I
LOAD
= 250mA
1µs/div
V
LX
2V/div
V
IN
= 3.3V, V
OUT
= 1.5V,
R
LOAD
= 6Ω
200µs/div
MEDIUM-LOAD
LINE-TRANSIENT RESPONSE
MAX1920 toc13
LIGHT-LOAD
LINE-TRANSIENT RESPONSE
MAX1920 toc14
LOAD-TRANSIENT RESPONSE
V
IN
= 3.3V, V
OUT
= 1.5V,
I
LOAD
= 20mA TO 320mA
MAX1920 toc15
V
IN
AC-COUPLED
200mV/div
V
IN
AC-COUPLED
200mV/div
V
IN
= 3.8V to 4.2V,
V
OUT
= 1.5V, I
LOAD
= 250mA
4µs/div
V
OUT
AC-COUPLED
5mV/div
V
IN
= 3.8V to 4.2V,
V
OUT
= 1.5V, I
LOAD
= 20mA
4µs/div
V
OUT
AC-COUPLED
5mV/div
40µs/div
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Maxim Integrated
│
4
MAX1920 toc09
10,000
SWITCHING FREQUENCY vs. LOAD
(V
OUT
= 1.8V)
10,000
SWITCHING FREQUENCY vs. LOAD
(V
OUT
= 1.5V)
10,000
NO LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE
V
OUT
1V/div
I
IN
100mA/div
V
SHDN
5V/div
V
OUT
AC-COUPLED
100mV/div
I
L
200mA/div
I
LOAD
200mA/div
MAX1920/MAX1921
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Pin Description
PIN
SOT
1
2
3
4
4
5
6
*MAX1920
only.
TDFN*
2
6
1
—
5
3
4
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 2.2µF ceramic capacitor as close as possible to IN.
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 0.1µA.
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
IN
AGND
SHDN
OUT
FB
PGND
LX
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.
either the high-side switch turns on again or the inductor
current approaches zero. The internal synchronous recti-
fier 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 medi-
um 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.
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
INPUT
2V TO 5.5V
C
IN
2
1
6
R1
5
C
FF
L
OUTPUT
UP TO 400mA
C
OUT
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
IN
LX
MAX1921
AGND
PGND
ON
OFF
3
SHDN
OUT
4
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 increas-
ing it in 25% steps until the full current limit is reached.
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