MAX1644.pdf

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19-1457; Rev 2; 3/02
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 0.10
Features
±1% Output Accuracy
95% Efficiency
Ω
On-Resistance at V IN = +4.5V
100m
Ω
On-Resistance at V IN = +3V
Ω
PMOS power switch
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.
Ω
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
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
Applications
16-Pin SSOP Package
+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
TEMP RANGE
PIN-PACKAGE
16 SSOP
-40°C to +85°C
Pin Configuration
Typical Operating Circuit
TOP VIEW
INPUT
+3V TO
+5.5V
IN
LX
OUTPUT
+1.1V TO
V IN
SHDN
1
16
LX
MAX1644
FB
IN
LX
2
3
15
14
PGND
LX
V CC
FBSEL
SHDN
PGND
IN
SS
4
5
MAX1644
13
12
PGND
V CC
FBSEL
REF
GND
GND
REF
SS
COMP
COMP
6
7
8
11
10
9
TOFF
TOFF
FB
R TOFF
Idle Mode is a trademark of Maxim Integrated Products, Inc.
SSOP
________________________________________________________________ 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.
Internal PMOS and NMOS Switches
70m
and 0.10
< 1µA Shutdown Supply Current
Output Short-Circuit Protection
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2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
ABSOLUTE MAXIMUM RATINGS
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.
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
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Input Voltage
V IN , V CC
3.0
5.5
V
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
3.300
3.333
3.366
Preset Output Voltage
V OUT
I LOAD = 0 to
2A,
V FB = V OUT
2.500
2.525
2.550
V
1.089
1.100
1.111
Adjustable Output Voltage
Range
V IN = V CC = 3V to 5.5V,
I LOAD = 0, FBSEL = GND or REF
V REF
V IN
V
AC Load Regulation Error
FBSEL = GND
FBSEL = REF, V CC , or unconnected
1
%
2
DC Load Regulation Error
FBSEL = GND
0.2
%
FBSEL = REF, V CC , or unconnected
0.4
Dropout Voltage
V DO
V IN = V CC = 3V, I LOAD = 1A, FBSEL = V CC
200
mV
Reference Voltage
V REF
1.089
1.100
1.111
V
Reference Load Regulation
∆ V REF
I REF = -1µA to +10µA
0.5
1
mV
PMOS Switch
On-Resistance
R ON, P
I LX = 0.5A
V IN = 4.5V
V IN = 3V
70
150
m Ω
100
200
NMOS Switch
On-Resistance
R ON, N
I LX = 0.5A
V IN = 4.5V
V IN = 3V
70
150
m
Ω
100
200
Current-Limit Threshold
I LIMIT
2.5
2.9
3.3
A
RMS LX Output Current
2.5
A
Idle Mode Current
Threshold
I IM
0.25
0.45
0.65
A
Switching Frequency
f
(Note 2)
350
kHz
No-Load Supply Current
I IN + I CC
V FB = 1.2V
240
360
µA
Shutdown Supply Current
I CC(SHDN)
SHDN = GND
<1
3
µA
PMOS Switch Off-Leakage
Current
I IN
SHDN = GND
15
µA
Thermal Shutdown
Threshold
T SHDN
Hysteresis = 15°C
150
°C
2 _______________________________________________________________________________________
323064253.022.png
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
SYMBOL
CONDITIONS
MIN
TYP
MAX UNITS
Undervoltage Lockout
Threshold
V UVLO
V IN falling, hysteresis = 40mV
2.5
2.6
2.7
V
FB Input Bias Current
I FB
V FB = 1.2V
R TOFF = 150k
0
80
200
nA
Ω
R TOFF = 30.1k
1.13
1.33
1.53
Off-Time Default Period
t OFF
Ω
0.20
0.33
µs
R TOFF = 499k
Ω
4.3
5.6
Off-Time Start-Up Period
t OFF
FB = GND
4 · t OFF
µs
On-Time Period
t ON
0.4
µs
SS Source Current
I SS
3.5
5
6.5
µA
SS Sink Current
I SS
V SS = 1V
V SHDN = 0 to V CC
100
µA
SHDN Input Current
I SHDN
-0.5
0.5
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
FBSEL = GND
0.2
FBSEL = REF
0.9
1.3
FBSEL Logic Thresholds
FBSEL = unconnected
0.7 · V CC
0.7 · V CC
V
- 0.2
+ 0.2
FBSEL = V CC
V CC - 0.2
Maximum Output RMS
Current
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
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Input Voltage
V IN
3.0
5.5
V
I LOAD = 0 to 2A,
V FB = V OUT
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
3.276
3.390
Preset Output Voltage
V OUT
2.48
2.57
V
1.08
1.12
Adjustable Output Voltage
V IN = 3.0V to 5.5V, I LOAD = 0,
FBSEL = GND or REF
V REF
V IN
V
Reference Voltage
V REF
1.08
1.12
V
PMOS Switch
On-Resistance
R ON, P
I LX = 0.5A
V IN = 4.5V
V IN = 3V
150
m
Ω
200
NMOS Switch
On-Resistance
R ON, N
I LX = 0.5A
V IN = 4.5V
V IN = 3V
150
m
Ω
200
Current-Limit Threshold
I LIMIT
2.3
3.5
A
Idle Mode Current
Threshold
I IM
0.2
0.7
A
No-Load Supply Current
I IN + I CC
V FB = 1.2V
360
µA
FB Input Bias Current
I FB
V FB = 1.2V
0
250
nA
Off-Time Default Period
t OFF
R TOFF = 150k
Ω
1.03
1.63
µs
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
323064253.023.png
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
Typical Operating Characteristics
(Circuit of Figure 1, T A = +25°C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
EFFICIENCY vs. OUTPUT CURRENT
DC LOAD-REGULATION ERROR
vs. OUTPUT CURRENT
100
100
0
A
90
90
-0.1
B
80
V IN = 5V, V OUT = 3.3V,
L = 6.0 µ H, R TOFF = 120k Ω
80
-0.2
70
60
70
60
-0.3
-0.4
D
C
V IN = 5V, V OUT = 1.5V,
L = 6.0 µ H, R TOFF = 270k Ω
E
50
50
-0.5
40
40
V IN = 3.3V, V OUT = 1.5V,
L = 4.7 µ H, R TOFF = 200k Ω
-0.6
30
20
30
20
-0.7
-0.8
10
10
-0.9
0
0
-1.0
0.001
0.01
0.1
1
10
0.001
0.01
0.1
1
10
0.0001 0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
OFF-TIME vs. R TOFF
A: V IN = 3.3V, V OUT = 1.5V, L = 4.7 µ H,
R TOFF = 200k Ω , FBSEL = GND
B: V IN = 3.3V, V OUT = 1.5V, L = 4.7 µ H,
R TOFF = 200k Ω , FBSEL = REF
C: V IN = 5V, V OUT = 3.3V, L = 6.0
µ
H,
4.5
350
, FBSEL = OPEN
D: V IN = 5V, V OUT = 1.5V, L = 6.0
Ω
4.0
3.5
3.0
µ
H,
300
, FBSEL = GND
E: V IN = 5V, V OUT = 1.5V, L = 6.0 µ H,
R TOFF = 270k Ω , FBSEL = REF
Ω
V IN = 5V,
V OUT = 3.3V,
L = 6.0
250
H,
R TOFF = 120k Ω
µ
V IN = 3.3V,
V OUT = 1.5V,
L = 4.7 µ H,
R TOFF = 200k Ω
200
2.5
2.0
1.5
V IN = 5V,
V OUT = 1.5V,
L = 6.0 µ H,
R TOFF = 270k
150
Ω
100
1.0
0.5
50
0
0
0
0.5
1.0
1.5
2.0
0
100
200
300
400
500
600
OUTPUT CURRENT (A)
R TOFF (k
Ω
)
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1644-07
START-UP AND
SHUTDOWN TRANSIENTS
500
0.10
V SHDN
5V/div
450
I OUT = 0
0.09
0
400
0.08
350
0.07
I IN
1A/div
0
300
SHDN = V IN = V CC
0.06
250
0.05
V OUT
2V/div
200
UNDERVOLTAGE
LOCKOUT
0.04
0
150
100
50
0.03
0.02
0.01
SHDN = GND
V SS
1V/div
0
V IN = 5.0V, V OUT = 3.3V, I OUT = 2A
0
0
0
1
2
3
4
5
6
2ms/div
SUPPLY VOLTAGE
4 _______________________________________________________________________________________
R TOFF = 120k
R TOFF = 270k
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2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T A = +25°C, unless otherwise noted.)
LINE-TRANSIENT RESPONSE
LOAD-TRANSIENT RESPONSE
(FBSEL = REF)
4V
2A
V IN
3V
I L
0
V OUT = 1.5V, I OUT = 2A
V IN = 3.3V, V OUT = 1.5V
V OUT
20mV/div
V OUT
50mV/div
20 µ s/div
20
µ
s/div
Pin Description
PIN
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.
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
Ω
and 2.2µF low-
pass filter. See Figure 1.
13, 15
PGND
Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
_______________________________________________________________________________________ 5
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