General Description
The MAX1644 constant-off-time, PWM step-down DC-
DC converter is ideal for use in applications such as PC
cards, CPU daughter cards, and desktop computer
bus-termination boards. The device features internal
synchronous rectification for high efficiency and
reduced component count. It requires no external
Schottky diode. The internal 0.10
PMOS power switch
and 0.10
NMOS synchronous-rectifier switch easily
deliver continuous load currents up to 2A. The
MAX1644 produces a preset +3.3V or +2.5V output
voltage or an adjustable output from +1.1V to V
IN
. It
achieves efficiencies as high as 95%.
The MAX1644 uses a unique current-mode, constant-
off-time, PWM control scheme, which includes an Idle
ModeTM to maintain high efficiency during light-load
operation. The programmable constant-off-time archi-
tecture sets switching frequencies up to 350kHz, allow-
ing the user to optimize performance trade-offs
between efficiency, output switching noise, component
size, and cost. The device also features an adjustable
soft-start to limit surge currents during start-up, a 100%
duty cycle mode for low-dropout operation, and a low-
power shutdown mode that disconnects the input from
the output and reduces supply current below 1µA. The
MAX1644 is available in a 16-pin SSOP package.
Applications
+5V to +3.3V/+2.5V Conversion
CPU I/O Supply
+3.3V PC Card and CardBus Applications
Notebook and Subnotebook Computers
Desktop Bus-Termination Boards
CPU Daughter Card Supply
Features
o ±1% Output Accuracy
o 95% Efficiency
o Internal PMOS and NMOS Switches
70m
On-Resistance at V
IN
= +4.5V
100m
On-Resistance at V
IN
= +3V
o Output Voltage
+3.3V or +2.5V Pin-Selectable
+1.1V to V
IN
Adjustable
o +3V to +5.5V Input Voltage Range
o 360µA (max) Operating Supply Current
o < 1µA Shutdown Supply Current
o Programmable Constant-Off-Time Operation
o 350kHz (max) Switching Frequency
o Idle Mode Operation at Light Loads
o Thermal Shutdown
o Adjustable Soft-Start Inrush Current Limiting
o 100% Duty Cycle During Low-Dropout Operation
o Output Short-Circuit Protection
o 16-Pin SSOP Package
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
SHDN
LX
PGND
LX
PGND
V
CC
FBSEL
REF
GND
TOP VIEW
MAX1644
SSOP
IN
LX
COMP
IN
SS
TOFF
FB
19-1457; Rev 2; 3/02
PART
MAX1644EAE
-40°C to +85°C
TEMP RANGE
PIN-PACKAGE
16 SSOP
Pin Configuration
Ordering Information
Idle Mode is a trademark of Maxim Integrated Products, Inc.
Typical Operating Circuit
TOFF
COMP
V
CC
FBSEL
SHDN
IN
PGND
GND
REF
SS
LX
FB
MAX1644
R
TOFF
OUTPUT
+1.1V TO
V
IN
INPUT
+3V TO
+5.5V
________________________________________________________________ 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.
EVALUATION KIT
AVAILABLE
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
2
_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V
IN
= V
CC
= +3.3V, FBSEL = GND, T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at T
A
= +25°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.
V
CC
, IN to GND ........................................................-0.3V to +6V
IN to V
CC
.............................................................................±0.3V
GND to PGND.....................................................................±0.3V
All Other Pins to GND.................................-0.3V to (V
CC
+ 0.3V)
LX Current (Note 1)...........................................................±3.75A
REF Short Circuit to GND Duration ............................Continuous
ESD Protection .....................................................................±2kV
Continuous Power Dissipation (T
A
= +70°C)
SSOP (derate 16.7mW/°C above +70°C;
part mounted on 1 in.
2
of 1oz. copper) ............................1.2W
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10sec) ............................ +300°C
Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward bias these diodes should take care not to exceed
the IC's package power dissipation limits.
Hysteresis = 15°C
SHDN = GND
SHDN = GND
I
LOAD
= 0 to
2A,
V
FB
= V
OUT
V
FB
= 1.2V
(Note 2)
V
IN
= V
CC
= 3V, I
LOAD
= 1A, FBSEL = V
CC
I
LX
= 0.5A
FBSEL = REF, V
CC
, or unconnected
V
IN
= V
CC
= 3V to 5.5V,
I
LOAD
= 0, FBSEL = GND or REF
FBSEL = GND
I
REF
= -1µA to +10µA
CONDITIONS
°C
150
T
SHDN
Thermal Shutdown
Threshold
µA
15
I
IN
PMOS Switch Off-Leakage
Current
µA
<1
3
I
CC(SHDN)
Shutdown Supply Current
µA
240
360
I
IN
+ I
CC
No-Load Supply Current
kHz
350
f
Switching Frequency
A
0.25
0.45
0.65
I
IM
Idle Mode Current
Threshold
A
2.5
2.9
3.3
I
LIMIT
Current-Limit Threshold
m
100
200
R
ON, P
PMOS Switch
On-Resistance
70
150
mV
0.5
1
V
REF
Reference Load Regulation
2.500
2.525
2.550
V
3.300
3.333
3.366
V
3.0
5.5
V
IN
, V
CC
Input Voltage
V
1.089
1.100
1.111
V
REF
Reference Voltage
mV
200
V
DO
Dropout Voltage
%
0.4
1.089
1.100
1.111
V
OUT
Preset Output Voltage
Adjustable Output Voltage
Range
V
V
REF
V
IN
0.2
DC Load Regulation Error
UNITS
MIN
TYP
MAX
SYMBOL
PARAMETER
V
IN
= V
CC
= 4V to 5.5V,
FBSEL = unconnected
V
IN
= V
CC
= 3V to 5.5V,
FBSEL = V
CC
V
IN
= V
CC
= 3V to 5.5V,
FBSEL = REF
V
IN
= 4.5V
V
IN
= 3V
I
LX
= 0.5A
m
100
200
R
ON, N
NMOS Switch
On-Resistance
70
150
V
IN
= 4.5V
V
IN
= 3V
FBSEL = GND
FBSEL = REF, V
CC
, or unconnected
1
AC Load Regulation Error
%
2
A
2.5
RMS LX Output Current
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________
3
ELECTRICAL CHARACTERISTICS (continued)
(V
IN
= V
CC
= +3.3V, FBSEL = GND, T
A
= 0°C to +85°C, unless otherwise noted. Typical values are at T
A
= +25°C.)
ELECTRICAL CHARACTERISTICS
(V
IN
= V
CC
= +3.3V, FBSEL = GND, T
A
= -40°C to +85°C, unless otherwise noted. Typical values are at T
A
= +25°C.) (Note 3)
Note 2: Recommended operating frequency, not production tested.
Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested.
R
TOFF
= 150k
V
FB
= 1.2V
V
FB
= 1.2V
I
LX
= 0.5A
V
IN
= 3.0V to 5.5V, I
LOAD
= 0,
FBSEL = GND or REF
I
LX
= 0.5A
CONDITIONS
µs
1.03
1.63
t
OFF
Off-Time Default Period
nA
0
250
I
FB
FB Input Bias Current
µA
360
I
IN
+ I
CC
No-Load Supply Current
A
0.2
0.7
I
IM
Idle Mode Current
Threshold
A
2.3
3.5
I
LIMIT
Current-Limit Threshold
m
200
R
ON, P
PMOS Switch
On-Resistance
m
150
200
R
ON, N
2.48
2.57
3.276
3.390
V
3.0
5.5
V
IN
Input Voltage
V
1.08
1.12
V
REF
Reference Voltage
V
V
REF
V
IN
Adjustable Output Voltage
1.08
1.12
UNITS
MIN
TYP
MAX
SYMBOL
PARAMETER
V
IN
= V
CC
= 4V to 5.5V,
FBSEL = unconnected
V
IN
= 3V to 5.5V, FBSEL = V
CC
V
IN
= 3V to 5.5V, FBSEL = REF
V
IN
= 4.5V
V
IN
= 3V
NMOS Switch
On-Resistance
150
V
IN
= 4.5V
V
IN
= 3V
V
I
LOAD
= 0 to 2A,
V
FB
= V
OUT
V
OUT
Preset Output Voltage
Maximum Output RMS
Current
A
RMS
SS Sink Current
I
SS
100
µA
SHDN Input Current
I
SHDN
-0.5
0.5
µA
SS Source Current
I
SS
3.5
5
6.5
µA
SHDN Input Low Threshold
V
IL
0.8
V
SHDN Input High Threshold
V
IH
2.0
V
FBSEL Input Current
-5
+5
µA
0.9
1.3
5.8
V
SS
= 1V
V
SHDN
= 0 to V
CC
FBSEL = REF
PARAMETER
SYMBOL
MIN
TYP
MAX
UNITS
Undervoltage Lockout
Threshold
V
UVLO
2.5
2.6
2.7
V
FB Input Bias Current
I
FB
0
80
200
nA
t
OFF
1.13
1.33
1.53
0.20
0.33
Off-Time Start-Up Period
t
OFF
4
·
t
OFF
µs
On-Time Period
t
ON
0.4
µs
CONDITIONS
V
FB
= 1.2V
R
TOFF
= 150k
R
TOFF
= 30.1k
V
IN
falling, hysteresis = 40mV
FB = GND
Off-Time Default Period
4.3
5.6
µs
R
TOFF
= 499k
FBSEL Logic Thresholds
0.2
V
FBSEL = GND
0.7
·
V
CC
0.7
·
V
CC
- 0.2
+ 0.2
FBSEL = unconnected
V
CC
- 0.2
FBSEL = V
CC
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
4
_______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
1
10
0
10
30
20
40
50
70
60
80
90
100
0.001
0.01
0.1
EFFICIENCY vs. OUTPUT CURRENT
MAX1644-01
OUTPUT CURRENT (A)
EFFICIENCY (%)
V
IN
= 5V, V
OUT
= 3.3V,
L = 6.0
µH, R
TOFF
= 120k
100
1
10
0
10
30
20
40
50
70
60
80
90
0.001
0.01
0.1
EFFICIENCY vs. OUTPUT CURRENT
MAX1644-02
OUTPUT CURRENT (A)
EFFICIENCY (%)
V
IN
= 3.3V, V
OUT
= 1.5V,
L = 4.7
µH, R
TOFF
= 200k
V
IN
= 5V, V
OUT
= 1.5V,
L = 6.0
µH, R
TOFF
= 270k
1
10
-1.0
-0.9
-0.7
-0.8
-0.6
-0.5
-0.3
-0.4
-0.2
-0.1
0
0.0001
0.001
0.01
0.1
DC LOAD-REGULATION ERROR
vs. OUTPUT CURRENT
MAX1644-03
OUTPUT CURRENT (A)
DC LOAD-REGULATION ERROR (%)
A
B
E
C
D
A: V
IN
= 3.3V, V
OUT
= 1.5V, L = 4.7
µH,
R
TOFF
= 200k
, FBSEL = GND
B: V
IN
= 3.3V, V
OUT
= 1.5V, L = 4.7
µH,
R
TOFF
= 200k
, FBSEL = REF
C: V
IN
= 5V, V
OUT
= 3.3V, L = 6.0
µH,
R
TOFF
= 120k
, FBSEL = OPEN
D: V
IN
= 5V, V
OUT
= 1.5V, L = 6.0
µH,
R
TOFF
= 270k
, FBSEL = GND
E: V
IN
= 5V, V
OUT
= 1.5V, L = 6.0
µH,
R
TOFF
= 270k
, FBSEL = REF
0
100
50
250
200
150
300
350
0
1.0
0.5
1.5
2.0
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
MAX1644-04
OUTPUT CURRENT (A)
SWITCHING FREQUENCY (kHz)
V
IN
= 3.3V,
V
OUT
= 1.5V,
L = 4.7
µH,
R
TOFF
= 200k
V
IN
= 5V,
V
OUT
= 1.5V,
L = 6.0
µH,
R
TOFF
= 270k
V
IN
= 5V,
V
OUT
= 3.3V,
L = 6.0
µH,
R
TOFF
= 120k
0
150
100
50
200
250
300
350
400
450
500
0
2
1
3
4
5
6
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1644-07
SUPPLY VOLTAGE
SUPPLY CURRENT I
CC
(
µ
A)
0
0.03
0.02
0.01
0.04
0.05
0.06
0.07
0.08
0.09
0.10
SHUTDOWN SUPPLY CURRENT I
IN
+ I
CC
(
µ
A)
I
OUT
= 0
UNDERVOLTAGE
LOCKOUT
SHDN = V
IN
= V
CC
SHDN = GND
0
1.0
0.5
2.5
2.0
1.5
4.0
3.5
3.0
4.5
0
200
100
300
400
500
600
OFF-TIME vs. R
TOFF
MAX1644-06
R
TOFF
(k
)
t
OFF
(
µ
s)
V
SHDN
5V/div
I
IN
1A/div
0
0
V
OUT
2V/div
V
SS
1V/div
0
0
2ms/div
START-UP AND
SHUTDOWN TRANSIENTS
MAX1644-09
V
IN
= 5.0V, V
OUT
= 3.3V, I
OUT
= 2A
MAX1644
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
_______________________________________________________________________________________
5
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
NAME
FUNCTION
1
SHDN
Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal
MOSFETs. Drive high or connect to V
CC
for normal operation.
PIN
Pin Description
V
IN
V
OUT
20mV/div
4V
3V
20
µs/div
LINE-TRANSIENT RESPONSE
MAX1644-10
V
OUT
= 1.5V, I
OUT
= 2A
I
L
V
OUT
50mV/div
2A
0
20
µs/div
LOAD-TRANSIENT RESPONSE
(FBSEL = REF)
MAX1644-11
V
IN
= 3.3V, V
OUT
= 1.5V
2, 4
IN
Supply Voltage Input for the internal PMOS power switch
3, 14, 16
LX
Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch. Connect
the inductor from this node to output filter capacitor and load.
5
SS
Soft-Start. Connect a capacitor from SS to GND to limit inrush current during start-up.
6
COMP
Integrator Compensation. Connect a capacitor from COMP to V
CC
for integrator compensation. See the
Integrator Amplifier section.
7
TOFF
Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation. Connect a
resistor from TOFF to GND to adjust the PMOS switch off-time.
8
FB
Feedback Input for both preset-output and adjustable-output operating modes. Connect directly to
output for fixed-voltage operation or to a resistor-divider for adjustable operating modes.
9
GND
Analog Ground
10
REF
Reference Output. Bypass REF to GND with a 1µF capacitor.
11
FBSEL
Feedback Select Input. Selects AC load-regulation error and output voltage. See Table 2 for program-
ming instructions.
12
V
CC
Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass V
CC
with a 10
and 2.2µF low-
pass filter. See Figure 1.
13, 15
PGND
Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
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