19-2296; Rev 1; 7/02
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 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.
o
400mA Guaranteed Output Current
o
Internal Synchronous Rectifier for >90%
Efficiency
o
Tiny 6-Pin SOT23 Package
o
Up to 1.2MHz Switching Frequency for Small
External Components
o
50µA Quiescent Supply Current
o
0.1µA Logic-Controlled Shutdown
o
2V to 5.5V Input Range
o
Fixed 1.5V, 1.8V, 2.5V, 3V, and 3.3V Output
Voltages (MAX1921)
o
Adjustable Output Voltage (MAX1920)
o
±1.5% Initial Accuracy
o
Soft-Start Limits Startup Current
Features
MAX1920/MAX1921
Ordering Information
PART
MAX1920EUT-T
MAX1921EUT_
_-T
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
6 SOT23-6
6 SOT23-6
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.
Typical Operating Circuit
INPUT
2V TO 5.5V
IN
C
IN
LX
4.75kΩ
5600pF
4.7µF
4.7µH
OUTPUT
1.5V UP TO 400mA
Pin Configuration
TOP VIEW
IN
1
6
LX
MAX1921
AGND
PGND
AGND
2
ON
SHDN
OFF
OUT
MAX1920
MAX1921
5
PGND
SHDN
3
4
OUT (FB)
SOT23-6
( ) ARE FOR MAX1920 ONLY
________________________________________________________________
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 = +70°C)
6-Pin SOT23-6 (derate 8.7mW/°C above +70°C) . . . .695mW
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)
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
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
SHDN
= 0
OUT at regulation voltage
Figure 4, IN = 4.5V
T
A
= 25°C
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
1.25
1.231
1.220
1.210
5
0.01
0.005
0.2
0.20
1.25
1.25
8
UVLO
V
IN
rising
V
IN
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
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
2
_______________________________________________________________________________________
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
MAX1920/MAX1921
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
(C
IN
= 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 3.3V)
MAX1920 toc01
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 2.5V)
MAX1920 toc02
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 1.5V)
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 2.5V
MAX1920 toc03
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.1
V
IN
= 3.6V
V
IN
= 5V
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
V
IN
= 2.7V
100
V
IN
= 4.2V
V
IN
= 3.3V
V
IN
= 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
(V
OUT
= 3.3V)
MAX1920 toc04
OUTPUT VOLTAGE ACCURACY vs. LOAD
(V
OUT
= 2.5V)
MAX1920 toc05
OUTPUT VOLTAGE ACCURACY vs. LOAD
(V
OUT
= 1.5V)
MAX1920 toc06
3.399
3.366
OUTPUT VOLTAGE
3.333
3.300
V
IN
= 4.2V
3.267
V
IN
= 3.6V
3.234
3.201
0
50
2.575
2.550
OUTPUT VOLTAGE
2.525
2.500
V
IN
= 3V
2.475
2.450
2.425
1.545
1.530
OUTPUT VOLTAGE
1.515
1.500
1.485
1.470
1.455
V
IN
= 5V
V
IN
= 5V
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 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)
_______________________________________________________________________________________
3
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
MAX1920/MAX1921
Typical Operating Characteristics (continued)
(C
IN
= 2.2µF ceramic, Circuit of Figure 1, components of Table 1, unless otherwise noted.)
SWITCHING FREQUENCY vs. LOAD
(V
OUT
= 1.8V)
MAX1920 toc07
SWITCHING FREQUENCY vs. LOAD
(V
OUT
= 1.5V)
MAX1920 toc08
NO LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX1920 toc09
10,000
10,000
10,000
NO-LOAD SUPPLY CURRENT (µA)
V
OUT
= 2.5V
1000
V
OUT
= 3.3V
SWITCHING FREQUENCY (kHz)
1000
SWITCHING FREQUENCY (kHz)
1000
100
100
100
V
OUT
= 1.5V
10
10
V
IN
= 3.3
1
0.1
1
10
LOAD (mA)
100
1000
10
V
IN
= 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
V
OUT
AC-COUPLED
5mV/div
V
OUT
AC-COUPLED
5mV/div
V
OUT
1V/div
I
IN
100mA/div
V
LX
2V/div
V
LX
2V/div
V
SHDN
5V/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
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
MAX1920 toc15
V
IN
= 3.3V, V
OUT
= 1.5V,
I
LOAD
= 20mA TO 320mA
V
IN
AC-COUPLED
200mV/div
V
IN
AC-COUPLED
200mV/div
V
OUT
AC-COUPLED
100mV/div
I
L
200mA/div
V
OUT
AC-COUPLED
5mV/div
V
IN
= 3.8V to 4.2V,
V
OUT
= 1.5V, I
LOAD
= 250mA
4µs/div
V
IN
= 3.8V to 4.2V,
V
OUT
= 1.5V, I
LOAD
= 20mA
4µs/div
V
OUT
AC-COUPLED
5mV/div
I
LOAD
200mA/div
40µs/div
4
_______________________________________________________________________________________
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Pin Description
PIN
1
2
3
NAME
IN
AGND
SHDN
OUT
4
5
6
FB
PGND
LX
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 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 0.1µA.
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
MAX1920/MAX1921
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.
INPUT
2V TO 5.5V
C
IN
1
IN
LX
6
R1
L
OUTPUT
UP TO 400mA
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.
C
OUT
C
FF
2
MAX1921
AGND
PGND
5
ON
OFF
3
SHDN
OUT
4
Figure 1. Typical Output Application Circuit (MAX1921)
_______________________________________________________________________________________
5
Integrated technical exchanges
京公网安备 11010802033920号
EEWORLD
