MAX1742_05 [MAXIM]
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches;
| 型号： | MAX1742_05 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches |
| 文件： | 总16页 (文件大小：287K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-1760; Rev 2; 9/05
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
General Description
Features
The MAX1742/MAX1842 constant-off-time, pulse-width-
modulated (PWM) step-down DC-DC converters are ideal
for use in 5V and 3.3V to low-voltage conversion neces-
sary in notebook and subnotebook computers. These
devices feature internal synchronous rectification for high
efficiency and reduced component count. They require
no external Schottky diode. The internal 90mΩ PMOS
power switch and 70mΩ NMOS synchronous-rectifier
switch easily deliver continuous load currents up to 1A.
The MAX1742/MAX1842 produce a preset 2.5V, 1.8V, or
1.5V output voltage or an adjustable output from 1.1V to
♦
1% Output Accuracy
♦ 95% Efficiency
♦ Internal PMOS and NMOS Switches
90mΩ On-Resistance at V = 4.5V
IN
110mΩ On-Resistance at V = 3V
IN
♦ Output Voltage
2.5V, 1.8V, or 1.5V Pin Selectable
1.1V to V Adjustable
IN
♦ 3V to 5.5V Input Voltage Range
V . They achieve efficiencies as high as 95%.
IN
♦ 600μA (max) Operating Supply Current
♦ <1μA Shutdown Supply Current
♦ Programmable Constant-Off-Time Operation
♦ 1MHz (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
The MAX1742/MAX1842 use 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 architec-
ture sets switching frequencies up to 1MHz, allowing the
user to optimize performance trade-offs between effi-
ciency, output switching noise, component size, and
cost. Both devices are designed for continuous output
currents up to 1A. The MAX1742 uses a peak current
limit of 1.3A (min) and is suitable for applications requir-
ing small external component size and high efficiency.
The MAX1842 has a higher current limit of 3.1A (min)
and is intended for applications requiring an occasional
burst of output current up to 2.7A. Both devices also fea-
ture an adjustable soft-start to limit surge currents during
startup, a 100% duty cycle mode for low-dropout opera-
tion, and a low-power shutdown mode that disconnects
the input from the output and reduces supply current
below 1µA. The MAX1742/MAX1842 are available in 16-
pin QSOP packages.
♦ 16-Pin QSOP Package
Ordering Information
PART
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
MAX1742EEE
MAX1742EEE+
MAX1842EEE
MAX1842EEE+
16 QSOP
16 QSOP
16 QSOP
16 QSOP
For similar devices that provide continuous output cur-
rents up to 2A and 3A, refer to the MAX1644 and
MAX1623 data sheets.
+ Denotes lead-free package.
Applications
5V or 3.3V to Low-Voltage Conversion
CPU I/O Ring
Typical Configuration
OUTPUT
INPUT
3V TO
5.5V
1.1V TO
IN
LX
FB
Chipset Supplies
V
IN
10Ω
MAX1742
MAX1842
Notebook and Subnotebook Computers
V
PGND
GND
FBSEL
REF
CC
470pF
2.2μF
SHDN
COMP
TOFF
Pin Configuration appears at end of data sheet.
1μF
SS
Idle Mode is a trademark of Maxim Integrated Products.
0.01μF
________________________________________________________________ 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.
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
ABSOLUTE MAXIMUM RATINGS
V
CC
, IN to GND ........................................................-0.3V to +6V
Continuous Power Dissipation (T = +70°C)
A
IN to V ............................................................................. 0.3V
GND to PGND..................................................................... 0.3V
SSOP (derate 16.7mW/°C above +70°C;
CC
part mounted on 1 in.2 of 1oz. copper)...............................1W
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................ +300°C
All Other Pins to GND.................................-0.3V to (V
+ 0.3V)
CC
LX Current (Note 1)............................................................. 4.7A
REF Short Circuit to GND Duration ............................Continuous
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 = V
IN
= 3.3V, FBSEL = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
A A
CC
PARAMETER
SYMBOL
, V
CONDITIONS
MIN
TYP
MAX
UNITS
Input Voltage
V
3.0
5.5
V
IN CC
T
= +25°C
A
2.500
2.525
2.525
1.515
1.515
1.818
1.818
1.100
1.100
2.550
2.563
1.530
1.538
1.836
1.845
1.111
1.117
to +85°C
FBSEL =
V
CC
T
A
= +0°C
2.487
1.500
1.492
1.800
1.791
1.089
1.084
to +85°C
T
A
= +25°C
V
= 3V to
IN
to +85°C
5.5V,
FBSEL =
I
= 0
unconnected
LOAD
T
A
= +0°C
to 1A for
to +85°C
Preset Output Voltage
V
MAX1742,
V
OUT
T
A
= +25°C
I
= 0
LOAD
to +85°C
to 2.5A for
MAX1842,
FBSEL =
REF
T
A
= +0°C
V
V
= V
OUT
FB
to +85°C
T
A
= +25°C
to +85°C
FBSEL =
GND
T
A
= +0°C
to +85°C
Adjustable Output Voltage
Range
= V
= 3V to 5.5V, I
CC LOAD
= 0,
IN
V
V
V
REF
IN
FBSEL = GND
AC Load Regulation Error
DC Load Regulation Error
Dropout Voltage
2
%
%
0.4
V
V
= V
= 3V, I = 1A
LOAD
250
1.111
1.117
2
mV
DO
IN
CC
T
T
= +25°C to +85°C
= +0°C to +85°C
1.089
1.084
1.100
1.100
0.5
A
Reference Voltage
V
V
REF
A
Reference Load Regulation
ΔV
I
= -1µA to +10µA
mV
REF
REF
V
= 4.5V
= 3V
90
200
250
150
200
PMOS Switch
On-Resistance
IN
R
I
= 0.5A
= 0.5A
ON, P
LX
LX
V
V
V
110
70
IN
IN
IN
mΩ
= 4.5V
= 3V
NMOS Switch
On-Resistance
R
ON, N
I
80
2
_______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
ELECTRICAL CHARACTERISTICS (continued)
(V = V
IN
= 3.3V, FBSEL = GND, T = 0°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.)
A A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
1.5
MAX
1.7
4.1
3.1
0.5
0.9
1
UNITS
MAX1742
MAX1842
1.3
3.1
Current-Limit Threshold
RMS LX Output Current
Idle Mode Current Threshold
I
A
A
A
LIMIT
3.6
MAX1742
MAX1842
(Note 2)
0.1
0.3
0.3
0.6
I
IM
Switching Frequency
f
MHz
µA
No-Load Supply Current
Shutdown Supply Current
I
IN
+ I
V = 1.2V
FB
350
<1
600
5
CC
I
(SHDN) SHDN = GND
CC
µA
PMOS Switch Off-Leakage
Current
I
IN
SHDN = GND
15
µA
Thermal Shutdown Threshold
Undervoltage Lockout Threshold
FB Input Bias Current
T
Hysteresis = 15°C
160
2.6
60
°C
V
SHDN
V
V
V
falling, hysteresis = 90mV
= 1.2V
2.5
0
2.7
250
1.1
UVLO
IN
I
FB
nA
FB
R
TOFF
R
TOFF
R
TOFF
= 110kΩ
= 30.1kΩ
= 499kΩ
0.9
0.24
3.8
1.00
0.30
4.5
✕
Off-Time Default Period
t
µs
0.37
5.2
OFF
OFF
Off-Time Startup Period
On-Time Period
t
FB = GND
(Note 2)
4
t
µs
µs
µA
µA
µA
V
OFF
t
0.4
4
ON
SS Source Current
I
SS
5
6
SS Sink Current
I
SS
V
V
= 1V
100
-1
SS
SHDN Input Current
SHDN Input Low Threshold
SHDN Input High Threshold
FBSEL Input Current
I
= 0 to V
1
SHDN
SHDN
CC
V
0.8
IL
V
2.0
-4
V
IH
+4
0.2
1.3
µA
FBSEL = GND
FBSEL = REF
0.9
✕
✕
0.7 VCC
- 0.2
0.7 VCC
+0.2
FBSEL Logic Thresholds
FBSEL = unconnected
V
V
- 0.2
CC
FBSEL = V
CC
_______________________________________________________________________________________
3
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
ELECTRICAL CHARACTERISTICS
(V = V
IN
= 3.3V, FBSEL = GND, T = -40°C to +85°C, unless otherwise noted. Typical values are at T = +25°C.) (Note 3)
A A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
MAX
UNITS
Input Voltage
V
3.0
5.5
V
IN
V
= 3V to 5.5V,
IN
I
= 0 to 1A
LOAD
2.475
2.575
FBSEL = V
CC
for MAX1742,
= 0 to 2.5A
Preset Output Voltage
V
I
FBSEL = unconnected
FBSEL = REF
1.485
1.782
1.078
1.545
1.854
1.122
V
OUT
LOAD
for MAX1842,
V
= V
OUT
FB
FBSEL = GND
Adjustable Output Voltage
Range
V
= V
= 3V to 5.5V, I
= 0,
IN
CC
LOAD
V
V
V
V
REF
IN
FBSEL = GND
Reference Voltage
V
1.078
1.122
200
250
150
200
1.8
REF
V
V
V
V
= 4.5V
= 3V
PMOS Switch
On-Resistance
IN
IN
IN
IN
R
I
I
= 0.5A
= 0.5A
ON, P
ON, N
LIMIT
LX
mΩ
= 4.5V
= 3V
NMOS Switch
On-Resistance
R
LX
MAX1742
MAX1842
MAX1742
MAX1842
1.2
2.9
Current-Limit Threshold
I
4.3
A
0.05
0.2
0.55
1.0
Idle Mode Current Threshold
I
IM
No-Load Supply Current
FB Input Bias Current
Off-Time Default Period
I
+ I
V
V
= 1.2V
= 1.2V
600
300
1.15
µA
nA
µs
IN
CC
FB
FB
I
0
FB
t
R
= 110kΩ
0.85
OFF
TOFF
Note 2: Recommended operating frequency, not production tested.
Note 3: Specifications from 0°C to -40°C are guaranteed by design, not production tested.
4
_______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
Typical Operating Characteristics
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
MAX1742
EFFICIENCY vs. OUTPUT CURRENT
MAX1742
EFFICIENCY vs. OUTPUT CURRENT
MAX1742
EFFICIENCY vs.OUTPUT CURRENT
(V = 5.0V, L = 6.0μH)
(V = 3.3V, L = 3.9μH)
(f
= 270kHz)
IN
IN
PWM
100
95
90
85
80
75
70
65
60
55
50
100
95
90
85
80
75
70
65
60
55
50
100
95
90
85
80
75
70
65
60
55
50
V
= 5V, V
= 1.8V,
OUT
IN
V
= 2.5V, R
= 47kΩ, f = 926kHz
V
= 2.5V, R
= 36kΩ, f = 456kHz
TOFF
OUT
TOFF
OUT
L = 15μH, R
= 240kΩ
TOFF
V
= 1.8V, R
f = 833kHz
= 75kΩ,
OUT
TOFF
V
= 1.8V, R
= 43kΩ, f = 869kHz
OUT
TOFF
V
= 3.3V, V
= 1.8V,
IN
OUT
L = 10μH, R
= 160kΩ
TOFF
V
= 1.5V, R
= 56kΩ, f = 833kHz
TOFF
V
= 1.5V, R
= 100kΩ, f = 692kHz
TOFF
OUT
OUT
0.001
0.01
0.1
1
0.001
0.01
0.1
1
0.001
0.01
0.1
1
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
MAX1742
NORMALIZED OUTPUT ERROR
vs. OUTPUT CURRENT
MAX1742
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
0.5
0.4
1100
1000
900
800
700
600
500
400
300
200
100
0
V
= 5V, V
= 2.5V, L = 6μH
IN
OUT
0.3
V
= 5V, V
= 1.5V, L = 6μH
IN
OUT
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
V
= 5V, V
= 1.5V, L = 6μH
IN
OUT
V
= 3.3V, V
= 1.5V
IN
OUT
V
= 3.3V, V
= 1.5V, L = 3.9μH
OUT
IN
0.001
0.01
0.1
1
0
0.2
0.4
0.6
0.8
1.0
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
_______________________________________________________________________________________
5
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
MAX1742
STARTUP AND SHUTDOWN
MAX1742
LOAD-TRANSIENT RESPONSE
MAX1742 toc07
MAX1742 toc06
I
INPUT
0A 1A/div
V
OUTPUT
AC-COUPLED,
50mV/div
V
SHDN
0V
5V/div
V
OUTPUT
I
L
0V 1V/div
0.5A/div
0V
V
0V
SS
2V/div
10μs/div
1ms/div
MAX1742
LINE-TRANSIENT RESPONSE
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX1742 toc08
MAX1742 toc09
500
450
400
350
300
250
200
150
100
50
100
90
80
70
60
50
40
30
20
10
0
V
INPUT
NO LOAD
2V/div
0V
V
OUTPUT
20mV/div
AC-COUPLED
SHUTDOWN
0
20μs/div
= 1.5V, R = 100kΩ, L = 6μH
TOFF
0
1
2
3
4
5
6
I
= 1A, V
OUT
OUT
V
(V)
IN
OFF-TIME vs. R
TOFF
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
50 100 150 200 250 300 350 400 450 500
(kΩ)
R
TOFF
6
_______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
MAX1842
MAX1842
EFFICIENCY vs. OUTPUT CURRENT
EFFICIENCY vs. OUTPUT CURRENT
(V = 5.0V, L = 2.5μH)
IN
(V = 3.3V, L = 1.5μH)
IN
100
100
95
90
85
80
75
70
65
60
55
50
V
f
= 2.5V, R
= 1070kHz
= 47kΩ,
TOFF
V
f
= 2.5V, R
= 610kHz
= 39kΩ,
TOFF
OUT
PWM
OUT
PWM
95
90
85
80
75
70
65
60
55
50
V
= 1.8V, R
= 75kΩ,
TOFF
V
= 1.8V, R
= 43kΩ,
OUT
OUT
TOFF
f
= 910kHz
f
= 1050kHz
PWM
PWM
V
f
= 1.5V, R
= 770kHz
= 1OOkΩ,
TOFF
V
f
= 2.5V, R
= 1000kHz
= 56kΩ,
TOFF
OUT
PWM
OUT
PWM
0.001
0.01
0.1
1
10
0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
MAX1842
MAX1842
NORMALIZED OUTPUT ERROR
vs. OUTPUT CURRENT
EFFICIENCY vs. OUTPUT CURRENT
(f
= 270kHz)
PWM
0.1
0
100
95
90
85
80
75
70
65
60
55
50
V
R
= 5V, V
= 1.8V, L = 5.6μH,
OUT
IN
V
= 3.3V, V
= 1.5V, L = 1.5μH
OUT
IN
= 240kΩ
TOFF
-0.1
-0.2
-0.3
-0.4
V
= 3.3V, V
= 1.8V,
OUT
IN
L = 4.7μH, R
= 160kΩ
TOFF
V
= 5V, V
0.01
= 1.5V, L = 2.5μH
OUT
IN
0.001
0.1
1
10
0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
I
(A)
OUT
MAX1842
SWITCHING FREQUENCY
vs. OUTPUT CURRENT
MAX1842
STARTUP AND SHUTDOWN
MAX1842 toc16
1200
1000
800
600
400
200
0
V
= 5V, V
= 2.5V, L = 2.5μH
IN
OUT
I
INPUT
0
1A/div
V
SHDN
0
5V/div
V
V
= 3.3V, V
= 1.5V, L = 1.5μH
OUT
IN
V
OUTPUT
= 5V, V
= 1.5V, L = 2.5μH
0
0
1V/div
IN
OUT
V
SS
2V/div
1ms/div
= 0.5Ω, R
0
0.5
1.0
1.5
2.0
2.5
3.0
R
= 56kΩ
TOFF
OUT
OUTPUT CURRENT (A)
V
= 3.3V, V
= 1.5V
OUT
IN
_______________________________________________________________________________________
7
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)
A
MAX1842
LINE-TRANSIENT RESPONSE
MAX1842
LOAD-TRANSIENT RESPONSE
MAX1842 toc18
MAX1842 toc17
V
INPUT
2V/div
V
OUTPUT
0
50mV/div
V
OUTPUT
20mV/div
AC-COUPLED
I
L
2A/div
20μs/div
= 1.5V, R = 100kΩ, L = 2.2μH
TOFF
10μs/div
I
= 2.5A, V
OUT
OUT
Pin Description
PIN
NAME
FUNCTION
Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal
1
SHDN
IN
MOSFETs. Drive high or connect to V for normal operation.
CC
2, 4
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 the output filter capacitor and load.
3, 14, 16
LX
5
6
SS
Soft-Start. Connect a capacitor from SS to GND to limit inrush current during startup.
Integrator Compensation. Connect a capacitor from COMP to V
for integrator compensation. See
CC
COMP
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.
7
8
TOFF
FB
Feedback Input—for both preset-output and adjustable-output operating modes. Connect directly to
output for fixed-voltage operation or to a resistive divider for adjustable operating modes.
9
GND
REF
Analog Ground
10
11
Reference Output. Bypass REF to GND with a 1µF capacitor.
Feedback Select Input. Selects output voltage. See Table 3 for programming instructions.
FBSEL
Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass V
pass filter. See Figure 1.
with a 10Ω and 2.2µF low-
CC
12
V
CC
13, 15
PGND
Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
8
_______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
output is in regulation or the current limit is reached.
_______________Detailed Description
When the PMOS switch turns off, it remains off for the
The MAX1742/MAX1842 synchronous, current-mode,
programmed off-time (t
). To control the current
OFF
constant-off-time, PWM DC-DC converters step down
input voltages of 3V to 5.5V to a preset output voltage of
2.5V, 1.8V, or 1.5V, or to an adjustable output voltage
under short-circuit conditions, the PMOS switch
remains off for approximately 4 x t when V
<
OUT
OFF
V
/ 4.
OUT(NOM)
from 1.1V to V . Both devices deliver up to 1A of contin-
IN
uous output current; the MAX1842 delivers bursts of out-
put current up to 2.7A (see the Extended Current Limit
section). Internal switches composed of a 0.09Ω PMOS
power switch and a 0.07Ω NMOS synchronous rectifier
switch improve efficiency, reduce component count, and
eliminate the need for an external Schottky diode.
Idle Mode
Under light loads, the devices improve efficiency by
switching to a pulse-skipping Idle Mode. Idle Mode
operation occurs when the current through the PMOS
switch is less than the Idle Mode threshold current. Idle
Mode forces the PMOS to remain on until the current
through the switch reaches the Idle Mode threshold,
thus minimizing the unnecessary switching that
degrades efficiency under light loads. In Idle Mode, the
device operates in discontinuous conduction. Current-
sense circuitry monitors the current through the NMOS
synchronous switch, turning it off before the current
reverses. This prevents current from being pulled from
the output filter through the inductor and NMOS switch to
ground. As the device switches between operating
modes, no major shift in circuit behavior occurs.
The MAX1742/MAX1842 optimize efficiency by operat-
ing in constant-off-time mode under heavy loads and in
Maxim’s proprietary Idle Mode under light loads. A sin-
gle resistor-programmable constant-off-time control
sets switching frequencies up to 1MHz, allowing the
user to optimize performance trade-offs in efficiency,
switching noise, component size, and cost. Under low-
dropout conditions, the device operates in a 100%
duty-cycle mode, where the PMOS switch remains con-
tinuously on. Idle Mode enhances light-load efficiency
by skipping cycles, thus reducing transition and gate-
charge losses.
100% Duty-Cycle Operation
When the input voltage drops near the output voltage,
the duty cycle increases until the PMOS MOSFET is on
continuously. The dropout voltage in 100% duty cycle
is the output current multiplied by the on-resistance of
the internal PMOS switch and parasitic resistance in the
inductor. The PMOS switch remains on continuously as
long as the current limit is not reached.
When power is drawn from a regulated supply, constant-
off-time PWM architecture essentially provides constant-
frequency operation. This architecture has the inherent
advantage of quick response to line and load transients.
The MAX1742/MAX1842s’ current-mode, constant-off-
time PWM architecture regulates the output voltage by
changing the PMOS switch on-time relative to the con-
stant off-time. Increasing the on-time increases the
peak inductor current and the amount of energy trans-
ferred to the load per pulse.
Shutdown
Drive SHDN to a logic-level low to place the
MAX1742/MAX1842 in low-power shutdown mode and
reduce supply current to less than 1µA. In shutdown, all
circuitry and internal MOSFETs turn off, and the LX
node becomes high impedance. Drive SHDN to a
Modes of Operation
The current through the PMOS switch determines the
mode of operation: constant-off-time mode (for load
currents greater than half the Idle Mode threshold), or
Idle Mode (for load currents less than half the Idle
Mode threshold). Current sense is achieved through a
proprietary architecture that eliminates current-sensing
I2R losses.
logic-level high or connect to V
for normal operation.
CC
Summing Comparator
Three signals are added together at the input of the
summing comparator (Figure 2): an output voltage error
signal relative to the reference voltage, an integrated
output voltage error correction signal, and the sensed
PMOS switch current. The integrated error signal is pro-
vided by a transconductance amplifier with an external
capacitor at COMP. This integrator provides high DC
accuracy without the need for a high-gain amplifier.
Connecting a capacitor at COMP modifies the overall
loop response (see the Integrator Amplifier section).
Constant-Off-Time Mode
Constant-off-time operation occurs when the current
through the PMOS switch is greater than the Idle Mode
threshold current (which corresponds to a load current
of half the Idle Mode threshold). In this mode, the regu-
lation comparator turns the PMOS switch on at the end
of each off-time, keeping the device in continuous-con-
duction mode. The PMOS switch remains on until the
_______________________________________________________________________________________
9
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
INPUT
C
IN
C
IN
= 10μF (MAX1742)
= 33μF (MAX1842)
L
OUTPUT
C
OUT
C
OUT
= 47μF (MAX1742)
= 150μF (MAX1842)
IN
LX
FB
10Ω
MAX1742
V
CC
PGND
2.2μF
470pF
GND
SHDN
COMP
FBSEL
V
OUT
V
OUT
V
OUT
= 2.5V, FBSEL = V
CC
= 1.8V, FBSEL = REF
= 1.5V, FBSEL = FLOATING
REF
SS
1μF
TOFF
0.01μF
R
TOFF
Figure 1. Typical Circuit
0.01μF
FBSEL
SS
FB
IN
FEEDBACK
SELECTION
V
IN
MAX1742
MAX1842
3.0V TO 5.5V
10μF
COMP
CURRENT
SENSE
REF
470pF
G
m
SKIP
10Ω
REF
PWM LOGIC
AND
V
CC
V
IN
DRIVERS
2.2μF
V
OUT
SUMMING
COMPARATOR
LX
C
OUT
SHDN
REF
CURRENT
SENSE
REF
TIMER
TOFF
1μF
PGND
GND
R
TOFF
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
Figure 2. Functional Diagram
chronous switch reduces conduction losses and
improves efficiency.
Synchronous Rectification
In a step-down regulator without synchronous rectifica-
tion, an external Schottky diode provides a path for cur-
rent to flow when the inductor is discharging. Replacing
the Schottky diode with a low-resistance NMOS syn-
The NMOS synchronous-rectifier switch turns on follow-
ing a short delay after the PMOS power switch turns off,
thus preventing cross conduction or “shoot through.” In
10 ______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
constant-off-time mode, the synchronous-rectifier
Table 1. MAX1742 Recommended
switch turns off just prior to the PMOS power switch
Component Values (I
= 1A)
OUT
turning on. While both switches are off, inductor current
flows through the internal body diode of the NMOS
switch. The internal body diode’s forward voltage is rel-
atively high.
V
V
f
L
R
IN
OUT
PWM
TOFF
(V)
5
(V)
3.3
2.5
1.8
1.5
2.5
1.8
1.5
(kHz)
850
(μH)
(kΩ)
39
5.6
5.6
5.6
5.6
3.9
3.9
3.9
Thermal Resistance
5
1070
910
47
Junction-to-ambient thermal resistance, θ , is highly
JA
5
75
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1742 evaluation kit
has 0.5in2 of copper area and a thermal resistance of
80°C/W with no forced airflow. Airflow over the board
significantly reduces the junction-to-ambient thermal
resistance. For heatsinking purposes, evenly distribute
the copper area connected at the IC among the high-
current pins.
5
770
100
39
3.3
3.3
3.3
610
1050
1000
43
56
Table 2. MAX1842 Recommended
Component Values (Continuous Output
Power Dissipation
Power dissipation in the MAX1742/MAX1842 is domi-
nated by conduction losses in the two internal power
switches. Power dissipation due to supply current in the
control section and average current used to charge
and discharge the gate capacitance of the internal
switches (i.e., switching losses) is approximately:
2
Current = 1A, Burst Output Current = 2.7A)
V
V
f
L
R
IN
OUT
PWM
TOFF
(V)
5
(V)
3.3
2.5
1.8
1.5
2.5
1.8
1.5
(kHz)
800
1180
850
715
570
985
940
(μH)
(kΩ)
39
2.2
2.2
2.2
2.2
1.5
1.5
1.5
5
47
P
= C x V
x f
DS
IN PWM
5
75
where C = 2.5nF and f
cy in PWM mode.
is the switching frequen-
PWM
5
100
39
3.3
3.3
3.3
This number is reduced when the switching frequency
43
decreases as the part enters Idle Mode. Combined con-
duction losses in the two power switches are approxi-
56
mated by:
2
P = I
x R
PMOS
D
OUT
MAXIMUM RECOMMENDED
OPERATING FREQUENCY vs. INPUT VOLTAGE
where R
is the on-resistance of the PMOS switch.
PMOS
1400
1200
1000
800
600
400
200
0
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
V
OUT
= 1.5V
θ
= (T
- T
) / P
A,MAX D(TOT)
JA
J,MAX
where: θ = junction-to-ambient thermal resistance
JA
V
OUT
= 1.8V
T
T
= maximum junction temperature
= maximum ambient temperature
= total losses
J,MAX
A,MAX
V
OUT
= 2.5V
P
D(TOT)
V
= 3.3V
5.1
OUT
__________________Design Procedure
For typical applications, use the recommended compo-
nent values in Tables 1 or 2. For other applications,
take the following steps:
2.6
3.1
3.6
4.1
(V)
4.6
5.6
V
IN
1) Select the desired PWM-mode switching frequency;
1MHz is a good starting point. See Figure 3 for maxi-
mum operating frequency.
Figure 3. Maximum Recommended Operating Frequency vs.
Input Voltage
______________________________________________________________________________________ 11
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
Programming the Switching Frequency
Table 3. Output Voltage Programming
and Off-Time
OUTPUT
VOLTAGE
(V)
The MAX1742/MAX1842 features a programmable
PWM mode switching frequency, which is set by the
PIN
FBSEL
FB
input and output voltage and the value of R
, con-
TOFF
sets the PMOS
nected from TOFF to GND. R
TOFF
V
Output voltage
2.5
CC
power switch off-time in PWM mode. Use the following
equation to select the off-time according to your
desired switching frequency in PWM mode:
Unconnected
REF
Output voltage
Output voltage
1.5
1.8
V
– V
− V
(
)
IN
OUT PMOS
t
=
OFF
Resistive
divider
GND
Adjustable
f
V
− V
+ V
PMOS NMOS
(
)
PWM IN
where:
t
= the programmed off-time
= the input voltage
OFF
V
V
V
IN
V
OUT
LX
= the output voltage
OUT
PMOS
= the voltage drop across the internal
PMOS power switch
MAX1742
MAX1842
R2
R1
V
= the voltage drop across the internal
NMOS synchronous-rectifier switch
NMOS
FB
f
= switching frequency in PWM mode
PWM
R1 = 50kΩ
Select R
according to the formula:
TOFF
R2 = R1(V / V - 1)
OUT REF
V
REF
= 1.1V
R
TOFF
= (t
- 0.07µs) (110kΩ / 1.00µs)
OFF
Recommended values for R
range from 36kΩ to
TOFF
Figure 4. Adjustable Output Voltage
430kΩ for off-times of 0.4µs to 4µs.
Inductor Selection
2) Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
The key inductor parameters must be specified: inductor
value (L) and peak current (I
). The following equa-
PEAK
3) Select R
as a function of off-time.
TOFF
tion includes a constant, denoted as LIR, which is the
ratio of peak-to-peak inductor AC current (ripple current)
to maximum DC load current. A higher value of LIR allows
smaller inductance but results in higher losses and ripple.
A good compromise between size and losses is found at
approximately a 25% ripple-current to load-current ratio
(LIR = 0.25), which corresponds to a peak inductor cur-
rent 1.125 times the DC load current:
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage
The output of the MAX1742/MAX1842 is selectable
between one of three preset output voltages: 2.5V,
1.8V, and 1.5V. For a preset output voltage, connect FB
to the output voltage and connect FBSEL as indicated
in Table 3. For an adjustable output voltage, connect
FBSEL to GND and connect FB to a resistive divider
between the output voltage and ground (Figure 4).
Regulation is maintained for adjustable output voltages
V
× t
OFF
OUT
L =
I
× LIR
OUT
where: I
= maximum DC load current
OUT
when V = V
FB
. Use 50kΩ for R1. R2 is given by the
REF
equation:
LIR = ratio of peak-to-peak AC inductor current
to DC load current, typically 0.25
⎛
⎞
V
OUT
R2 = R1
− 1
⎟
⎜
V
⎝
⎠
REF
where V
is typically 1.1V.
REF
12 ______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
The peak inductor current at full load is 1.125 x I
if
OUT
the above equation is used; otherwise, the peak current
is calculated by:
SHDN
V
× t
OFF
OUT
0
I
= I
+
PEAK
OUT
2 × L
1.8V
V
(V)
(A)
SS
Choose an inductor with a saturation current at least as
high as the peak inductor current. The inductor you
select should exhibit low losses at your chosen operat-
ing frequency.
0.7V
0
0
I
LIMIT
I
LIMIT
Capacitor Selection
The input filter capacitor reduces peak currents and
noise at the voltage source. Use a low-ESR and low-
ESL capacitor located no further than 5mm from IN.
Select the input capacitor according to the RMS input
ripple-current requirements and voltage rating:
t
Figure 5. Soft-Start Current Limit over Time
V
V
− V
OUT
(
)
OUT IN
decreases stability. Choose the capacitor values that
result in optimal performance.
I
= I
LOAD
RIPPLE
V
IN
Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit to reduce input surge currents at startup and
where I
= input RMS current ripple.
RIPPLE
The output filter capacitor affects the output voltage rip-
ple, output load-transient response, and feedback loop
stability. For stable operation, the MAX1742/MAX1842
at exit from shutdown. A timing capacitor, C , placed
SS
from SS to GND sets the rate at which the internal cur-
rent limit is changed. Upon power-up, when the device
comes out of undervoltage lockout (2.6V typ) or after
the SHDN pin is pulled high, a 4µA constant-current
source charges the soft-start capacitor and the voltage
on SS increases. When the voltage on SS is less than
approximately 0.7V, the current limit is set to zero. As
the voltage increases from 0.7V to approximately 1.8V,
the current limit is adjusted from 0 to the current-limit
threshold (see the Electrical Characteristics).The volt-
age across the soft-start capacitor changes with time
according to the equation:
requires a minimum output ripple voltage of V
≥
RIPPLE
1% x V
.
OUT
The minimum ESR of the output capacitor should be:
L
ESR > 1% ×
t
OFF
Stable operation requires the correct output filter capaci-
tor. When choosing the output capacitor, ensure that:
t
OFF
C
≥
≥
33μFV/μs for the MAX1742
79μFV/μs for the MAX1842
OUT
V
OUT
4μA × t
V
=
SS
C
SS
t
OFF
C
OUT
V
OUT
The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:
Integrator Amplifier
An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C , from COMP
V
− 0.7V
1.1V
SS
SSI
=
× I
LIMIT
LIMIT
COMP
to V
compensates the transconductance amplifier.
CC
For stability, choose C
= 470pF.
COMP
where I
is the current threshold from the Electrical
LIMIT
Characteristics.
A large capacitor value maintains a constant average
output voltage but slows the loop response to changes
in output voltage. A small capacitor value speeds up
the loop response to changes in output voltage but
______________________________________________________________________________________ 13
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
The constant-current source stops charging once the
MAX1842
voltage across the soft-start capacitor reaches 1.8V
MAXIMUM RECOMMENDED CONTINUOUS
(Figure 5).
OUTPUT CURRENT vs. TEMPERATURE
2.70
Extended Current Limit (MAX1842)
For applications requiring occasional short bursts of
2.65
high output current (up to 2.7A), the MAX1842 provides
2.60
a higher current-limit threshold. When using the
MAX1842, choose external components capable of
2.55
withstanding its higher peak current limit.
2.50
The MAX1842 is capable of delivering large output cur-
2.45
rents for limited durations, and its thermal characteris-
tics allow it to operate at continuously higher output
2.40
currents. Figure 6 shows its maximum recommended
2.35
continuous output current versus ambient temperature.
Figure 7 shows the maximum recommended burst cur-
2.30
rent versus the output current duty cycle at high tem-
25
35
45
55
65
75
85
peratures.
TEMPERATURE (°C)
Figure 7 assumes that the output current is a square
wave with a 100Hz frequency. The duty cycle is
defined as the duration of the burst current divided by
the period of the square wave. This figure shows the
limitations for continuous bursts of output current.
Figure 6. MAX1842 Maximum Recommended Continuous
Output Current vs. Temperature
MAXIMUM RECOMMENDED BURST CURRENT
Note that if the thermal limitations of the MAX1842 are
exceeded, it will enter thermal shutdown to prevent
destructive failure.
vs. BURST CURRENT DUTY CYCLE
2.7
T = +85°C
A
T = +55°C
A
2.6
2.5
2.4
2.3
2.2
Frequency Variation with Output Current
The operating frequency of the MAX1742/MAX1842 is
determined primarily by t
OUT
(set by R
), V , and
TOFF IN
OFF
V
as shown in the following formula:
f
V
= (V - V
NMOS
- V
) / [t
(V - V
IN
+
PMOS
PWM
IN
OUT
PMOS
OFF
)]
However, as the output current increases, the voltage
drop across the NMOS and PMOS switches increases
and the voltage across the inductor decreases. This
causes the frequency to drop. The change in frequency
can be approximated with the following formula:
I
IS A 100Hz SQUARE WAVE
OUT
FROM 1A TO THE BURST CURRENT
0
20
40
60
80
100
DUTY CYCLE (%)
Figure 7. MAX1842 Maximum Recommended Burst Current vs.
Burst Current Duty Cycle
Δf
= -I
x R
/ (V x t
)
PWM
OUT
PMOS
IN
OFF
where R
is the resistance of the internal MOSFETs
PMOS
(90mΩ typ).
1) Minimize switched-current and high-current ground
loops. Connect the input capacitor’s ground, the out-
put capacitor’s ground, and PGND. Connect the
resulting island to GND at only one point.
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1742/
MAX1842s’ intended output power level, high efficiency,
and low noise. Good layout includes the use of a ground
plane, careful component placement, and correct rout-
ing of traces using appropriate trace widths. The follow-
ing points are in order of decreasing importance:
2) Connect the input filter capacitor less than 5mm
away from IN. The connecting copper trace carries
large currents and must be at least 1mm wide,
preferably 2.5mm.
14 ______________________________________________________________________________________
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
3) Place the LX node components as close together
Pin Configuration
and as near to the device as possible. This reduces
resistive and switching losses as well as noise.
TOP VIEW
4) A ground plane is essential for optimum perfor-
SHDN
IN
1
2
3
4
5
6
7
8
16 LX
mance. In most applications, the circuit is located on
a multilayer board, and full use of the four or more
layers is recommended. Use the top and bottom lay-
ers for interconnections and the inner layers for an
uninterrupted ground plane. Avoid large AC currents
through the ground plane.
15 PGND
14 LX
LX
IN
MAX1742
MAX1842
13 PGND
SS
12 V
CC
COMP
TOFF
FB
11 FBSEL
10 REF
9
GND
Chip Information
TRANSISTOR COUNT: 3662
QSOP
A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES.
______________________________________________________________________________________ 15
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2005 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products, Inc.
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