19-1457; Rev 3; 8/05
KIT
ATION
EVALU
BLE
AVAILA
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
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 0.10Ω PMOS power switch
and 0.10Ω 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
Mode™ 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.
♦
±1% Output Accuracy
♦
95% Efficiency
♦
Internal PMOS and NMOS Switches
70mΩ On-Resistance at V
IN
= +4.5V
100mΩ On-Resistance at V
IN
= +3V
♦
Output Voltage
+3.3V or +2.5V Pin-Selectable
+1.1V to V
IN
Adjustable
♦
+3V to +5.5V Input Voltage Range
♦
360µA (max) Operating Supply Current
♦
< 1µA Shutdown Supply Current
♦
Programmable Constant-Off-Time Operation
♦
350kHz (max) Switching Frequency
♦
Idle Mode Operation at Light Loads
♦
Thermal Shutdown
♦
Adjustable Soft-Start Inrush Current Limiting
♦
100% Duty Cycle During Low-Dropout Operation
♦
Output Short-Circuit Protection
♦
16-Pin SSOP Package
Features
MAX1644
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
Ordering Information
PART
MAX1644EAE
MAX1644EAE+
TEMP RANGE
40°C to +85°C
40°C to +85°C
PIN-PACKAGE
16 SSOP
16 SSOP
+Denotes
lead-free package.
Pin Configuration
TOP VIEW
OUTPUT
+1.1V TO
V
IN
SHDN 1
IN 2
LX 3
IN 4
SS 5
16 LX
15 PGND
14 LX
Typical Operating Circuit
INPUT
+3V TO
+5.5V
IN
LX
MAX1644
FB
V
CC
FBSEL
SHDN
COMP
TOFF
R
TOFF
GND
REF
SS
PGND
MAX1644
13 PGND
12 V
CC
11 FBSEL
10 REF
9
GND
COMP 6
TOFF 7
FB 8
SSOP
Idle Mode is a trademark of Maxim Integrated Products, Inc.
A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES.
________________________________________________________________
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.
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
MAX1644
ABSOLUTE MAXIMUM RATINGS
V
CC
, IN to GND ........................................................-0.3V to +6V
Continuous Power Dissipation (T
A
= +70°C)
IN to V
CC
.............................................................................±0.3V
SSOP (derate 16.7mW/°C above +70°C;
GND to PGND.....................................................................±0.3V
part mounted on 1 in.
2
of 1oz. copper) ............................1.2W
Operating Temperature Range ...........................-40°C to +85°C
All Other Pins to GND.................................-0.3V to (V
CC
+ 0.3V)
LX Current (Note 1)...........................................................±3.75A
Storage Temperature Range .............................-65°C to +150°C
REF Short Circuit to GND Duration ............................Continuous
Lead Temperature (soldering, 10s) ................................ +300°C
ESD Protection .....................................................................±2kV
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.
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
= V
CC
= +3.3V, FBSEL = GND,
T
A
= 0°C to +85°C,
unless otherwise noted. Typical values are at T
A
= +25°C.)
PARAMETER
Input Voltage
SYMBOL
V
IN
, V
CC
V
IN
= V
CC
= 4V to 5.5V,
FBSEL = unconnected
Preset Output Voltage
V
OUT
I
LOAD
= 0 to
2A,
V
FB
= V
OUT
V
IN
= V
CC
= 3V to 5.5V,
FBSEL = V
CC
V
IN
= V
CC
= 3V to 5.5V,
FBSEL = REF
Adjustable Output Voltage
Range
AC Load Regulation Error
DC Load Regulation Error
Dropout Voltage
Reference Voltage
Reference Load Regulation
PMOS Switch
On-Resistance
NMOS Switch
On-Resistance
Current-Limit Threshold
RMS LX Output Current
Idle Mode Current
Threshold
Switching Frequency
No-Load Supply Current
Shutdown Supply Current
PMOS Switch Off-Leakage
Current
Thermal Shutdown
Threshold
2
I
IM
f
I
IN
+ I
CC
I
CC(SHDN)
I
IN
T
SHDN
(Note 2)
V
FB
= 1.2V
SHDN
= GND
SHDN
= GND
Hysteresis = 15°C
150
240
<1
0.25
0.45
V
DO
V
REF
ΔV
REF
R
ON, P
R
ON, N
I
LIMIT
I
REF
= -1µA to +10µA
I
LX
= 0.5A
I
LX
= 0.5A
V
IN
= 4.5V
V
IN
= 3V
V
IN
= 4.5V
V
IN
= 3V
2.5
V
IN
= V
CC
= 3V to 5.5V,
I
LOAD
= 0, FBSEL = GND or REF
FBSEL = GND
FBSEL = REF, V
CC
, or unconnected
FBSEL = GND
FBSEL = REF, V
CC
, or unconnected
V
IN
= V
CC
= 3V, I
LOAD
= 1A, FBSEL = V
CC
1.089
1.100
0.5
70
100
70
100
2.9
CONDITIONS
MIN
3.0
3.300
2.500
1.089
V
REF
1
2
0.2
0.4
200
1.111
1
150
200
150
200
3.3
2.5
0.65
350
360
3
15
3.333
2.525
1.100
TYP
MAX
5.5
3.366
2.550
1.111
V
IN
V
%
%
mV
V
mV
mΩ
mΩ
A
A
A
kHz
µA
µA
µA
°C
V
UNITS
V
_______________________________________________________________________________________
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
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.)
PARAMETER
Undervoltage Lockout
Threshold
FB Input Bias Current
Off-Time Default Period
Off-Time Start-Up Period
On-Time Period
SS Source Current
SS Sink Current
SHDN
Input Current
SHDN
Input Low Threshold
SHDN
Input High Threshold
FBSEL Input Current
SYMBOL
V
UVLO
I
FB
t
OFF
t
OFF
t
ON
I
SS
I
SS
I
SHDN
V
IL
V
IH
FBSEL = GND
FBSEL = REF
FBSEL Logic Thresholds
FBSEL = unconnected
FBSEL = V
CC
Maximum Output RMS
Current
2.0
-5
0.9
0.7
·
V
CC
- 0.2
V
CC
- 0.2
+5
0.2
1.3
0.7
·
V
CC
+ 0.2
CONDITIONS
V
IN
falling, hysteresis = 40mV
V
FB
= 1.2V
R
TOFF
= 150kΩ
R
TOFF
= 30.1kΩ
R
TOFF
= 499kΩ
FB = GND
0.4
3.5
100
-0.5
MIN
2.5
0
1.13
0.20
TYP
2.6
80
1.33
0.33
4.3
4
·
t
OFF
5
6.5
0.5
0.8
MAX
2.7
200
1.53
5.6
µs
µs
µA
µA
µA
V
V
µA
UNITS
V
nA
µs
MAX1644
V
SS
= 1V
V
SHDN
= 0 to V
CC
V
5.8
A
RMS
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)
PARAMETER
Input Voltage
SYMBOL
V
IN
V
IN
= V
CC
= 4V to 5.5V,
I
LOAD
= 0 to 2A, FBSEL = unconnected
V
IN
= 3V to 5.5V, FBSEL = V
CC
V
FB
= V
OUT
V
IN
= 3V to 5.5V, FBSEL = REF
V
IN
= 3.0V to 5.5V, I
LOAD
= 0,
FBSEL = GND or REF
V
REF
R
ON, P
R
ON, N
I
LIMIT
I
IM
I
IN
+ I
CC
I
FB
t
OFF
V
FB
= 1.2V
V
FB
= 1.2V
R
TOFF
= 150kΩ
I
LX
= 0.5A
I
LX
= 0.5A
V
IN
= 4.5V
V
IN
= 3V
V
IN
= 4.5V
V
IN
= 3V
2.3
0.2
CONDITIONS
MIN
3.0
3.276
2.48
1.08
V
REF
1.08
TYP
MAX
5.5
3.390
2.57
1.12
V
IN
1.12
150
200
150
200
3.5
0.7
360
250
1.63
V
UNITS
V
Preset Output Voltage
V
OUT
Adjustable Output Voltage
Reference Voltage
PMOS Switch
On-Resistance
NMOS Switch
On-Resistance
Current-Limit Threshold
Idle Mode Current
Threshold
No-Load Supply Current
FB Input Bias Current
Off-Time Default Period
V
V
mΩ
mΩ
A
A
µA
nA
µs
0
1.03
Note 2:
Recommended operating frequency, not production tested.
Note 3:
Specifications from 0°C to -40°C are guaranteed by design, not production tested.
_______________________________________________________________________________________
3
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
MAX1644
Typical Operating Characteristics
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
DC LOAD-REGULATION ERROR
vs. OUTPUT CURRENT
MAX1644-02
EFFICIENCY vs. OUTPUT CURRENT
MAX1644-01
EFFICIENCY vs. OUTPUT CURRENT
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
V
IN
= 5V, V
OUT
= 1.5V,
L = 6.0μH, R
TOFF
= 270kΩ
V
IN
= 3.3V, V
OUT
= 1.5V,
L = 4.7μH, R
TOFF
= 200kΩ
0
DC LOAD-REGULATION ERROR (%)
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
0.01
0.1
1
10
-1.0
0.0001
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.001
0.01
0.1
1
V
IN
= 5V, V
OUT
= 3.3V,
L = 6.0μH, R
TOFF
= 120kΩ
A
B
D
E
C
10
0
0.001
0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
A: V
IN
= 3.3V, V
OUT
= 1.5V, L = 4.7μH,
R
TOFF
= 200kΩ, FBSEL = GND
B: V
IN
= 3.3V, V
OUT
= 1.5V, L = 4.7μH,
R
TOFF
= 200kΩ, FBSEL = REF
C: V
IN
= 5V, V
OUT
= 3.3V, L = 6.0μH,
R
TOFF
= 120kΩ, FBSEL = OPEN
D: V
IN
= 5V, V
OUT
= 1.5V, L = 6.0μH,
R
TOFF
= 270kΩ, FBSEL = GND
E: V
IN
= 5V, V
OUT
= 1.5V, L = 6.0μH,
R
TOFF
= 270kΩ, FBSEL = REF
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
MAX1644-04
OFF-TIME vs. R
TOFF
4.0
3.5
MAX1644-06
350
300
SWITCHING FREQUENCY (kHz)
250
200
150
100
50
0
0
0.5
1.0
1.5
V
IN
= 5V,
V
OUT
= 3.3V,
L = 6.0μH,
R
TOFF
= 120kΩ
V
IN
= 5V,
V
OUT
= 1.5V,
L = 6.0μH,
R
TOFF
= 270kΩ
4.5
V
IN
= 3.3V,
V
OUT
= 1.5V,
L = 4.7μH,
R
TOFF
= 200kΩ
3.0
t
OFF
(μs)
2.5
2.0
1.5
1.0
0.5
0
2.0
0
100
200
300
R
TOFF
(kΩ)
400
500
600
OUTPUT CURRENT (A)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
500
450
SUPPLY CURRENT I
CC
(μA)
400
350
300
250
200
150
100
50
0
0
1
2
3
4
5
6
SUPPLY VOLTAGE
UNDERVOLTAGE
LOCKOUT
SHDN = GND
SHDN = V
IN
= V
CC
I
OUT
= 0
MAX1644-07
STARTUP AND
SHUTDOWN TRANSIENTS
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
SHUTDOWN SUPPLY CURRENT I
IN
+ I
CC
(μA)
0
MAX1644-09
0.10
V
SHDN
5V/div
I
IN
1A/div
V
OUT
2V/div
0
0
V
SS
1V/div
0
V
IN
= 5.0V, V
OUT
= 3.3V, I
OUT
= 2A
2ms/div
4
_______________________________________________________________________________________
MAX1644-03
100
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
MAX1644
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
(FBSEL = REF)
MAX1644-10
LINE-TRANSIENT RESPONSE
4V
V
IN
3V
V
OUT
= 1.5V, I
OUT
= 2A
V
OUT
50mV/div
2A
I
L
0
V
IN
= 3.3V, V
OUT
= 1.5V
V
OUT
20mV/div
20μs/div
20μs/div
Pin Description
PIN
1
2, 4
3, 14, 16
5
6
7
8
9
10
11
12
13, 15
NAME
SHDN
IN
LX
SS
COMP
TOFF
FB
GND
REF
FBSEL
V
CC
PGND
FUNCTION
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.
Supply Voltage Input for the internal PMOS power switch
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.
Soft-Start. Connect a capacitor from SS to GND to limit inrush current during start-up.
Integrator Compensation. Connect a capacitor from COMP to V
CC
for integrator compensation. See the
Integrator Amplifier
section.
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.
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.
Analog Ground
Reference Output. Bypass REF to GND with a 1µF capacitor.
Feedback Select Input. Selects AC load-regulation error and output voltage. See Table 2 for program-
ming instructions.
Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass V
CC
with a 10Ω and 2.2µF low-
pass filter. See Figure 1.
Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
_______________________________________________________________________________________
MAX1644-11
5
