19-1986; Rev 1; 3/02
KIT
EVALUATION
LE
AVAILAB
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
General Description
The MAX1843 constant-off-time, pulse-width modulated
(PWM) step-down DC-DC converter is ideal for use in 5V
and 3.3V to low-voltage conversion necessary in note-
book and subnotebook computers. This device features
an internal PMOS power switch and internal synchronous
rectification for high efficiency and reduced component
count. An external Schottky diode is not required. The
internal 90mΩ power switch and 70mΩ NMOS synchro-
nous-rectifier switch easily deliver continuous load cur-
rents up to 2.7A. The MAX1843 produces a preset +2.5V,
+1.8V, or +1.5V output voltage or an adjustable output
from +1.1V to V
IN
. It achieves efficiencies as high as
95%.
The MAX1843 uses a unique current-mode, constant-off-
time, PWM control scheme, which includes Idle Mode™
to maintain high efficiency during light-load operation.
The programmable constant-off-time architecture sets
switching frequencies up to 1MHz, allowing the user to
optimize performance trade-offs between efficiency, out-
put switching noise, component size, and cost. The
MAX1843 features an adjustable soft-start to limit surge
currents during startup, a 100% duty-cycle mode for low
dropout operation, and a low-power shutdown mode that
disconnects the input from the output and reduces sup-
ply current below 1µA. The MAX1843 is available in a 28-
pin QFN package with an exposed backside pad.
o
±1% Output Accuracy
o
Up to 1MHz Switching Frequency
o
95% Efficiency
o
Internal PMOS/NMOS Switches
90mΩ/70mΩ On-Resistance at V
IN
= +4.5V
110mΩ/80mΩ On-Resistance at V
IN
= +3V
o
Output Voltage
+2.5V, +1.8V, or +1.5V Pin Selectable
+1.1V to V
IN
Adjustable
o
+3V to +5.5V Input Voltage Range
o
350µA Operating Supply Current
o
<1µA Shutdown Supply Current
o
Programmable Constant-Off-Time Operation
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
28-Pin QFN Package
Features
MAX1843
Applications
5V or 3.3V to Low-Voltage Conversion
CPU I/O Ring
Chipset Supplies
Notebook and Subnotebook Computers
PART
MAX1843EGI
Ordering Information
TEMP RANGE
-40°C to +85°C
PIN-PACKAGE
28 QFN
Pin Configuration
N.C.
N.C.
N.C.
28
27
26
25
24
23
22
21 PGND
20 PGND
19 LX
N.C.
18 LX
17 PGND
16 V
CC
15 FBSEL
8
TOFF
9
FB
10
N.C.
11
N.C.
12
N.C.
13
GND
14
REF
LX
LX
TOP VIEW
INPUT
+3V TO
+5.5V
IN
10Ω
V
CC
LX
MAX1843
FB
PGND
GND
OUTPUT
+1.1V TO
V
IN
N.C. 1
IN 2
LX 3
IN 4
N.C. 5
2.2µF
470pF
SHDN
COMP
TOFF
SHDN
Typical Configuration
FBSEL
REF
SS
0.01µF
COMP 7
MAX1843
1µF
SS 6
Idle Mode is a trademark of Maxim Integrated Products.
QFN
________________________________________________________________
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.
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
MAX1843
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
28-Pin QFN (derate 20mW/°C above +70°C, part mounted
GND to PGND.....................................................................±0.3V
on 1in
2
of 1oz copper)......................................................1.6W
Operating Temperature Range ...........................-40°C to +85°C
All Other Pins to GND.................................-0.3V to (V
CC
+ 0.3V)
LX Current (Note 1).............................................................±4.7A
Storage Temperature Range .............................-65°C to +150°C
REF Short Circuit to GND Duration ............................Continuous
Junction Temperature ......................................................+150°C
ESD Protection .....................................................................±2kV
Lead Temperature (soldering, 10s) ................................ +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
Input Voltage
SYMBOL
V
IN
, V
CC
T
A
= +25°C
to +85°C
FBSEL = V
CC
V
IN
=
+3V to
+5.5V
FBSEL =
unconnected
Preset Output Voltage
V
OUT
I
LOAD
=
0 to 2.5A
FBSEL = REF
T
A
= 0°C to
+85°C
V
FB
=
V
OUT
FBSEL = GND
T
A
= 0°C to
+85°C
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
V
DO
V
REF
∆V
REF
R
ON,P
R
ON,N
I
LIMIT
V
IN
= V
CC
= +3V, I
LOAD
= 1A
T
A
= +25°C to +85°C
T
A
= 0°C to +85°C
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
3.1
1.089
1.084
1.100
1.100
0.5
90
110
70
80
3.6
V
IN
= V
CC
= +3V to +5.5V, FBSEL = GND
1.084
V
REF
2
0.4
250
1.111
1.117
2
200
250
150
200
4.1
3.1
1.100
1.117
V
IN
V
%
%
mV
V
mV
mΩ
mΩ
A
A
T
A
= +25°C
to +85°C
1.791
1.089
1.818
1.100
1.845
1.111
T
A
= 0°C to
+85°C
T
A
= +25°C
to +85°C
T
A
= 0°C to
+85°C
T
A
= +25°C
to +85°C
CONDITIONS
MIN
3.0
2.500
2.487
1.500
1.492
1.800
2.525
2.525
1.515
1.515
1.818
TYP
MAX
5.5
2.550
2.563
1.530
1.538
V
1.836
UNITS
V
2
_______________________________________________________________________________________
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
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
Idle-Mode Current Threshold
Switching Frequency
No-Load Supply Current
Shutdown Supply Current
Thermal Shutdown Threshold
Undervoltage Lockout
FB Input Bias Current
Off-Time
Off-Time Startup Period
On-Time
SS Source Current
SS Sink Current
SHDN
Input Current
SHDN
Logic Input Low Voltage
SHDN
Logic Input High Voltage
FBSEL Input Current
t
ON
I
SS
I
SS
I
SHDN
V
IL
V
IH
I
FB
2.0
V
FBSEL
= 0 to V
CC
FBSEL = GND
FBSEL = REF
FBSEL Logic Thresholds
FBSEL = unconnected
FBSEL = V
CC
Maximum Output RMS Current
0.9
0.7V
CC
- 0.2
V
CC
- 0.2
3.1
A
RMS
-4
4
0.2
1.3
0.7V
CC
+ 0.2
V
V
SS
= 1V
V
SHDN
= 0 to V
CC
t
OFF
f
I
IN
+ I
CC
I
IN
+ I
CC
T
SHDN
V
UVLO
(Note 2)
V
FB
= 1.2V
SHDN
= GND, includes PMOS leakage
Hysteresis = 15°C
V
IN
falling, hysteresis = 90mV
V
FB
= 1.2V
R
TOFF
= 110kΩ
R
TOFF
= 30.1kΩ
R
TOFF
= 499kΩ
FB = GND
(Note 2)
0.4
4
100
-1
1
0.8
5
6
2.5
0
0.9
0.24
3.8
350
<1
160
2.6
60
1.00
0.30
4.5
4 t
OFF
2.7
250
1.1
0.37
5.2
µs
µs
µA
µA
µA
V
V
µA
µs
SYMBOL
CONDITIONS
MIN
0.3
TYP
0.6
MAX
0.9
1
600
15
UNITS
A
MHz
µA
µA
°C
V
nA
MAX1843
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
CC
CONDITIONS
FBSEL = V
CC
MIN
3.0
2.475
1.485
1.782
1.078
V
REF
1.078
MAX
5.5
2.756
1.545
1.854
1.122
V
IN
1.122
200
250
UNITS
V
Preset Output Voltage
V
OUT
V
IN
= +3V to +5.5V,
I
LOAD
= 0 to 2.5A,
V
FB
= V
OUT
FBSEL = unconnected
FBSEL = REF
FBSEL = GND
V
Adjustable Output Voltage
Range
Reference Voltage
PMOS Switch On-Resistance
V
REF
R
ON,P
V
IN
= V
CC
= +3V to +5.5V, FBSEL = GND
V
V
mΩ
I
LX
= 0.5A
V
IN
= +4.5V
V
IN
= +3V
_______________________________________________________________________________________
3
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
MAX1843
ELECTRICAL CHARACTERISTICS (continued)
(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
NMOS Switch On-Resistance
Current-Limit Threshold
Idle-Mode Current Threshold
No-Load Supply Current
FB Input Bias Current
Off-Time
SYMBOL
R
ON,N
I
LIMIT
I
IN
+ I
CC
I
FB
t
OFF
V
FB
= 1.2V
V
FB
= 1.2V
I
LX
= 0.5A
CONDITIONS
V
IN
= +4.5V
V
IN
= +3V
MIN
MAX
150
200
UNITS
mΩ
A
A
µA
nA
µs
2.9
0.2
0
0.85
4.3
1.0
600
300
1.15
R
TOFF
= 110kΩ
Note 2:
Recommended operating frequency, not production tested.
Note 3:
Specifications from 0°C to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
(V
IN
= +5.0V, L = 2.5µH)
MAX1843 toc03
EFFICIENCY vs. OUTPUT CURRENT
(V
IN
= +3.3V, L = 1.5µH)
95
90
EFFICIENCY (%)
85
80
75
70
65
60
55
10
50
0.001
V
OUT
= +1.8V, R
TOFF
= 43kΩ,
f
PWM
= 1050kHz
V
OUT
= +2.5V, R
TOFF
= 56kΩ,
f
PWM
= 1000kHz
0.01
0.1
1
10
V
OUT
= +2.5V, R
TOFF
= 39kΩ,
f
PWM
= 610kHz
MAX1843 toc04
100
95
90
EFFICIENCY (%)
85
80
75
70
65
60
55
50
0.001
V
OUT
= +1.8V, R
TOFF
= 75kΩ,
f
PWM
= 910kHz
V
OUT
= +1.5V, R
TOFF
= 1OOkΩ,
f
PWM
= 770kHz
0.01
0.1
1
V
OUT
= +2.5V, R
TOFF
= 47kΩ,
f
PWM
= 1070kHz
100
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
EFFICIENCY vs. OUTPUT CURRENT
(f
PWM
= 270kHz)
95
90
EFFICIENCY (%)
85
80
75
70
65
60
55
50
0.001
0.01
0.1
I
OUT
(A)
1
10
-0.4
0.001
V
IN
= +3.3V, V
OUT
= +1.8V,
L = 4.7µH, R
TOFF
= 160kΩ
V
IN
= +5V, V
OUT
= +1.8V, L = 5.6µH,
R
TOFF
= 240kΩ
MAX1843 toc05
NORMALIZED OUTPUT ERROR
vs. OUTPUT CURRENT
V
IN
= +3.3V, V
OUT
= +1.5V, L = 1.5µH
NORMALIZED OUTPUT ERROR (%)
0
MAX1843 toc06
100
0.1
-0.1
-0.2
V
IN
= +5V, V
OUT
= +1.5V, L = 2.5µH
-0.3
0.01
0.1
1
10
OUTPUT CURRENT (A)
4
_______________________________________________________________________________________
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
MAX1843
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
500
NO-LOAD SUPPLY CURRENT, I
IN
+ I
CC
(µA)
450
400
350
300
250
200
150
100
50
0
0
1
2
3
V
IN
(V)
4
5
6
SHUTDOWN
NO LOAD
MAX1843 toc01
OFF-TIME vs. R
TOFF
SHUTDOWN SUPPLY CURRENT, I
IN
+ I
CC
(nA)
90
80
70
60
50
40
30
20
10
0
4.5
4.0
3.5
t
OFF
(µs)
3.0
2.5
2.0
1.5
1.0
0.5
0
0
50 100 150 200 250 300 350 400 450 500
R
TOFF
(kΩ)
MAX1843 toc02
100
5.0
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
V
IN
= +5V, V
OUT
= +2.5V, L = 2.5µH
1000
FREQUENCY (kHz)
800
600
400
V
IN
= +5V, V
OUT
= +1.5V, L = 2.5µH
200
0
0
0.5
1.0
1.5
2.0
2.5
3.0
OUTPUT CURRENT (A)
MAX1843 toc07
STARTUP AND SHUTDOWN
MAX1843 toc08
1200
0
I
INPUT
1A/div
V
IN
= +3.3V, V
OUT
= +1.5V, L = 1.5µH
0
V
SHDN
5V/div
V
OUTPUT
1V/div
V
SS
2V/div
0
0
1ms/div
R
OUT
= 0.5Ω, R
TOFF
= 56kΩ
V
IN
= +3.3V, V
OUT
= +1.5V
LOAD-TRANSIENT RESPONSE
MAX1843 toc09
LINE-TRANSIENT RESPONSE
MAX1843 toc10
V
INPUT
2V/div
V
OUTPUT
50mV/div
0
I
L
2A/div
V
OUTPUT
20mV/div
AC-COUPLED
10µs/div
20µs/div
I
OUT
= 2.5A, V
OUT
= +1.5V
R
TOFF
= 100kΩ, L = 2.2µH
_______________________________________________________________________________________
5
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