Rainbow Electronics MAX1842 User Manual

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

________________________________________________________________ Maxim Integrated Products 1

19-1760; Rev 1; 2/02

General Description

The MAX1742/MAX1842 constant-off-time, pulse-width­modulated (PWM) step-down DC-DC converters are ideal
for use in 5V and 3.3V to low-voltage conversion neces­sary 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
VIN. They achieve efficiencies as high as 95%.

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 architec­ture sets switching frequencies up to 1MHz, allowing the
user to optimize performance trade-offs between effi­ciency, 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 requir­ing 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 fea­ture 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 supply current
below 1µA. The MAX1742/MAX1842 are available in 16­pin QSOP packages.

For similar devices that provide continuous output cur­rents up to 2A and 3A, refer to the MAX1644 and
MAX1623 data sheets.

Applications

5V or 3.3V to Low-Voltage Conversion

CPU I/O Ring

Chipset Supplies

Notebook and Subnotebook Computers

Features

♦ ±1% Output Accuracy

♦ 95% Efficiency

♦ Internal PMOS and NMOS Switches

90mΩ On-Resistance at V

IN

= 4.5V

110mΩ On-Resistance at VIN= 3V

♦ Output Voltage

2.5V, 1.8V, or 1.5V Pin Selectable

1.1V to V

IN

Adjustable

♦ 3V to 5.5V Input Voltage Range

♦ 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

♦ 16-Pin QSOP Package

PART

MAX1742EEE

-40°C to +85°C

TEMP RANGE PIN-PACKAGE

16 QSOP

Ordering Information

Idle Mode is a trademark of Maxim Integrated Products.

Typical Configuration

MAX1842EEE

-40°C to +85°C 16 QSOP

Pin Configuration appears at end of data sheet.

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

INPUT

3V TO

5.5V

2.2µF

10Ω

470pF

IN

V

CC

SHDN
COMP
TOFF

MAX1742
MAX1842

LX

FB

PGND

GND

FBSEL

REF

SS

0.01µF

OUTPUT

1.1V TO
V

IN

1µF

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VIN= VCC= 3.3V, FBSEL = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +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.

VCC, 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).............................................................±4.7A

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)...............................1W

Operating Temperature Range ...........................-40°C to +85°C

Storage Temperature Range .............................-65°C to +150°C

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.

Input Voltage V

Preset Output Voltage V

Adjustable Output Voltage
Range

AC Load Regulation Error 2%

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

IN

, V

OUT

3.0 5.5 V

CC

TA = +25°C

FBSEL =
V

CC

VIN = 3V to

5.5V,
I

= 0

LOAD

to 1A for
MAX1742,

= 0

I

LOAD

to 2.5A for
MAX1842,
V

= V

FB

VIN = VCC = 3V to 5.5V, I
FBSEL = GND

FBSEL =
unconnected

FBSEL =
REF

OUT

FBSEL =
GND

LOAD

to +85°C

TA = +0°C
to +85°C

TA = +25°C
to +85°C

= +0°C

T

A

to +85°C

TA = +25°C
to +85°C

T

= +0°C

A

to +85°C

TA = +25°C
to +85°C

T

= +0°C

A

to +85°C

= 0,

DC Load Regulation Error 0.4 %
Dropout Voltage V

Reference Voltage V

Reference Load Regulation ∆V
PMOS Switch

On-Resistance

NMOS Switch

On-Resistance

R

R

DO

REF

ON, P ILX

ON, N ILX

V

= VCC = 3V, I

IN

TA = +25°C to +85°C 1.089 1.100 1.111

TA = +0°C to +85°C 1.084 1.100 1.117

REF IREF

= -1µA to +10µA 0.5 2 mV

= 0.5A

= 0.5A

= 1A 250 mV

LOAD

V

= 4.5V 90 200

IN

V

= 3V 110 250

IN

V

= 4.5V 70 150

IN

V

= 3V 80 200

IN

2.500 2.525 2.550

2.487 2.525 2.563

1.500 1.515 1.530

1.492 1.515 1.538

1.800 1.818 1.836

1.791 1.818 1.845

1.089 1.100 1.111

1.084 1.100 1.117

V

REF

V

IN

V

V

V

mΩ

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VIN= VCC= 3.3V, FBSEL = GND, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER

SYMBOL

CONDITIONS

MIN

TYP

MAX

UNITS

MAX1742 1.3 1.5 1.7

Current-Limit Threshold I

LIMIT

MAX1842 3.1 3.6 4.1

A

RMS LX Output Current 3.1 A

MAX1742 0.1 0.3 0.5

Idle Mode Current Threshold I

IM

MAX1842 0.3 0.6 0.9

A

Switching Frequency f (Note 2) 1

MHz

No-Load Supply Current

VFB = 1.2V

µA

Shutdown Supply Current

SHDN = GND <1 5 µA

PMOS Switch Off-Leakage

Current

I

IN

SHDN = GND 15 µA

Thermal Shutdown Threshold

Hysteresis = 15°C

°C

Undervoltage Lockout Threshold

V

IN

falling, hysteresis = 90mV 2.5 2.6 2.7 V

FB Input Bias Current I

FB

V

FB

= 1.2V 0 60

nA

R

TOFF

= 110kΩ 0.9

1.1

R

TOFF

= 30.1kΩ

Off-Time Default Period t

OFF

R

TOFF

= 499kΩ 3.8 4.5 5.2

µs

Off-Time Startup Period t

OFF

FB = GND 4

✕

t

OFF

µs

On-Time Period t

ON

(Note 2) 0.4 µs

SS Source Current I

SS

4 5 6 µA

SS Sink Current I

SS

V

SS

= 1V

µA

SHDN Input Current

V

SHDN

= 0 to V

CC

-1 1 µA

SHDN Input Low Threshold V

IL

0.8 V

SHDN Input High Threshold V

IH

2.0 V

FBSEL Input Current -4 +4 µA

FBSEL = GND 0.2

FBSEL = REF 0.9 1.3

FBSEL = unconnected

- 0.2

+0.2

FBSEL Logic Thresholds

FBSEL = V

CC

V

CC

V

IIN + I

I

CC

T
V

CC

(SHDN)

SHDN

UVLO

350 600

160

1.00

0.24 0.30 0.37

I

SHDN

100

0.7 ✕ VCC

- 0.2

0.7 ✕ VCC

250

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(VIN= VCC= 3.3V, FBSEL = GND, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +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.

PARAMETER

CONDITIONS

UNITS

Input Voltage V

IN

3.0 5.5 V

VIN = 3V to 5.5V,
FBSEL = V

CC

FBSEL = REF

Preset Output Voltage V

OUT

I

LOAD

= 0 to 1A
for MAX1742,
I

LOAD

= 0 to 2.5A
for MAX1842,
V

FB

= V

OUT

FBSEL = GND

V

Adjustable Output Voltage
Range

VIN = VCC = 3V to 5.5V, I

LOAD

= 0,

FBSEL = GND

V

IN

V

Reference Voltage V

REF

V

V

IN

= 4.5V

PMOS Switch

On-Resistance

I

LX

= 0.5A

V

IN

= 3V

V

IN

= 4.5V

NMOS Switch

On-Resistance

I

LX

= 0.5A

V

IN

= 3V

mΩ

MAX1742 1.2 1.8

Current-Limit Threshold I

LIMIT

MAX1842 2.9 4.3
MAX1742

Idle Mode Current Threshold I

IM

MAX1842 0.2 1.0

A

No-Load Supply Current

VFB = 1.2V

µA

FB Input Bias Current I

FB

VFB = 1.2V 0

nA

Off-Time Default Period t

OFF

R

TOFF

= 110kΩ

µs

SYMBOL

MIN MAX

2.475 2.575

FBSEL = unconnected 1.485 1.545

1.782 1.854

1.078 1.122

V

REF

1.078

R

ON, P

R

ON, N

0.05

IIN + I

CC

0.85 1.15

1.122
200
250
150
200

0.55

600

300

Typical Operating Characteristics

(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

_______________________________________________________________________________________ 5

MAX1742

EFFICIENCY vs. OUTPUT CURRENT

= 5.0V, L = 6.0µH)

(V

100

95

90

85

80

75

70

EFFICIENCY (%)

65

60

55

50

0.001 0.01 1

V

V

= 1.8V, R

OUT

f = 833kHz

V

IN

= 2.5V, R

OUT

OUT

TOFF

= 75kΩ,

TOFF

= 1.5V, R

TOFF

OUTPUT CURRENT (A)

= 47kΩ, f = 926kHz

= 100kΩ, f = 692kHz

0.1

MAX1742 toc01

EFFICIENCY (%)

100

V

95

90

85

80

75

70

65

60

55

50

0.001 0.01 1

MAX1742

NORMALIZED OUTPUT ERROR

vs. OUTPUT CURRENT

0.5

0.4

0.3

VIN = 5V, V

0.2

0.1

0

-0.1

-0.2

-0.3

NORMALIZED OUTPUT ERROR (%)

-0.4

-0.5

0.001 0.01 1

= 1.5V, L = 6µH

OUT

VIN = 3.3V, V

OUTPUT CURRENT (A)

OUT

0.1

= 1.5V

MAX1742 toc04

EFFICIENCY vs. OUTPUT CURRENT

= 2.5V, R

OUT

V

OUT

MAX1742

= 3.3V, L = 3.9µH)

(V

IN

= 36kΩ, f = 456kHz

TOFF

V

= 1.8V, R

OUT

= 1.5V, R

TOFF

= 56kΩ, f = 833kHz

TOFF

OUTPUT CURRENT (A)

= 43kΩ, f = 869kHz

0.1

1100

1000

FREQUENCY (kHz)

EFFICIENCY vs.OUTPUT CURRENT

(f

PWM

VIN = 5V, V

µH, R

L = 15

OUTPUT CURRENT (A)

MAX1742 toc02

100

95

90

85

80

75

70

EFFICIENCY (%)

65

60

55

50

0.001 0.01 1

MAX1742

SWITCHING FREQUENCY

vs. OUTPUT CURRENT

VIN = 5V, V

900

800

700

600

500

400

300

200

100

0

0 0.2 0.4 0.6 0.8 1.0

= 2.5V, L = 6µH

OUT

VIN = 5V, V

OUT

VIN = 3.3V, V

OUTPUT CURRENT (A)

= 1.5V, L = 3.9µH

OUT

= 1.5V, L = 6µH

MAX1742

= 270kHz)

= 1.8V,

OUT

= 240kΩ

TOFF

VIN = 3.3V, V

µH, R

L = 10

OUT

TOFF

0.1

MAX1742 toc03

= 1.8V,
= 160kΩ

MAX1742 toc05

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)

0

1.0

0.5

2.0

1.5

3.0

2.5

3.5

4.5

4.0

5.0

0 100 150 20050 250 300 350 450400 500

OFF-TIME vs. R

TOFF

MAX1742 toc10

R

TOFF

(kΩ)

t

OFF

(µs)

MAX1742

STARTUP AND SHUTDOWN

1ms/div

MAX1742 toc06

MAX1742

LOAD-TRANSIENT RESPONSE

I

INPUT

0A

1A/div

V

SHDN

0V

5V/div

V

OUTPUT

0V

1V/div

V

0V

SS

2V/div

10µs/div

MAX1742 toc07

0V

MAX1742

LINE-TRANSIENT RESPONSE

OUT

20µs/div

= 1.5V, R

TOFF

I

OUT

= 1A, V

MAX1742 toc08

= 100kΩ, L = 6µH

V

INPUT

2V/div

0V

V

OUTPUT

20mV/div
AC-COUPLED

500

450

(µA)

CC

400

+ I

IN

350

300

250

200

150

100

NO-LOAD SUPPLY CURRENT, I

50

0

021 3456

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

NO LOAD

SHUTDOWN

VIN (V)

MAX1742 toc09

V

OUTPUT

AC-COUPLED,
50mV/div

I

L

0.5A/div

100

(nA)

90

CC

80

+ I

IN

70

60

50

40

30

20

10

SHUTDOWN SUPPLY CURRENT, I

0

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

_______________________________________________________________________________________ 7

Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)

0.001 0.01 0.1 1 10

MAX1842

EFFICIENCY vs. OUTPUT CURRENT

(V

IN

= 5.0V, L = 2.5µH)

MAX1842 toc11

OUTPUT CURRENT (A)

V

OUT

= 1.5V, R

TOFF

= 1OOkΩ,

f

PWM

= 770kHz

V

OUT

= 1.8V, R

TOFF

= 75kΩ,

f

PWM

= 910kHz

V

OUT

= 2.5V, R

TOFF

= 47kΩ,

f

PWM

= 1070kHz

100

95

90

85

80

70

65

60

55

50

75

EFFICIENCY (%)

0.001 0.01 0.1 1 10

MAX1842

EFFICIENCY vs. OUTPUT CURRENT

(V

IN

= 3.3V, L = 1.5µH)

MAX1842 toc12

OUTPUT CURRENT (A)

V

OUT

= 2.5V, R

TOFF

= 56kΩ,

f

PWM

= 1000kHz

V

OUT

= 2.5V, R

TOFF

= 39kΩ,

f

PWM

= 610kHz

V

OUT

= 1.8V, R

TOFF

= 43kΩ,

f

PWM

= 1050kHz

100

95

90

85

80

70

65

60

55

50

75

EFFICIENCY (%)

0.001 0.01 0.1 1 10

MAX1842

EFFICIENCY vs. OUTPUT CURRENT

(f

PWM

= 270kHz)

MAX1842 toc13

I

OUT

(A)

VIN = 3.3V, V

OUT

= 1.8V,

L = 4.7µH, R

TOFF

= 160kΩ

100

95

90

85

80

70

65

60

55

50

75

EFFICIENCY (%)

VIN = 5V, V

OUT

= 1.8V, L = 5.6µH,

R

TOFF

= 240kΩ

MAX1842

NORMALIZED OUTPUT ERROR

vs. OUTPUT CURRENT

MAX1842 toc14

0.001 0.01 0.1 1 10
OUTPUT CURRENT (A)

VIN = 3.3V, V

OUT

= 1.5V, L = 1.5µH

VIN = 5V, V

OUT

= 1.5V, L = 2.5µH

0.1

0

-0.1

-0.2

-0.3

-0.4

NORMALIZED OUTPUT ERROR (%)

0

400

200

800

600

1000

1200

0 1.0 1.50.5 2.0 2.5 3.0

MAX1842

SWITCHING FREQUENCY

vs. OUTPUT CURRENT

MAX1842 toc15

OUTPUT CURRENT (A)

FREQUENCY (kHz)

VIN = 5V, V

OUT

= 2.5V, L = 2.5µH

VIN = 3.3V, V

OUT

= 1.5V, L = 1.5µH

VIN = 5V, V

OUT

= 1.5V, L = 2.5µH

0

0

0

0

MAX1842

STARTUP AND SHUTDOWN

V

SS

2V/div

MAX1842 toc16

V

SHDN

5V/div

R

OUT

= 0.5Ω, R

TOFF

= 56kΩ

V

IN

= 3.3V, V

OUT

= 1.5V

I

INPUT

1A/div

V

OUTPUT

1V/div

1ms/div

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

8 _______________________________________________________________________________________

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

MAX1842

LOAD-TRANSIENT RESPONSE

V

OUTPUT

50mV/div

I

L

2A/div

MAX1842 toc17

10µs/div

0

MAX1842

LINE-TRANSIENT RESPONSE

V

INPUT

2V/div

V

OUTPUT

20mV/div
AC-COUPLED

MAX1842 toc18

20µs/div

I

OUT

= 2.5A, V

OUT

= 1.5V, R

TOFF

= 100kΩ, L = 2.2µH

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 the output filter capacitor and load.

5 SS Soft-Start. Connect a capacitor from SS to GND to limit inrush current during startup.

6 COMP

Integrator Compensation. Connect a capacitor from COMP to VCCfor integrator compensation. See
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 resistive 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 output voltage. See Table 3 for programming instructions.

12 V

CC

Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCCwith a 10Ω and 2.2µF low­pass filter. See Figure 1.

13, 15 PGND Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.

Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

_______________________________________________________________________________________ 9

_______________Detailed Description

The MAX1742/MAX1842 synchronous, current-mode,
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
from 1.1V to VIN. Both devices deliver up to 1A of contin­uous output current; the MAX1842 delivers bursts of out­put 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.

The MAX1742/MAX1842 optimize efficiency by operat­ing in constant-off-time mode under heavy loads and in
Maxim’s proprietary Idle Mode under light loads. A sin­gle 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 low­dropout conditions, the device operates in a 100%
duty-cycle mode, where the PMOS switch remains con­tinuously on. Idle Mode enhances light-load efficiency
by skipping cycles, thus reducing transition and gate­charge losses.

When power is drawn from a regulated supply, constant­off-time PWM architecture essentially provides constant­frequency operation. This architecture has the inherent
advantage of quick response to line and load transients.

The MAX1742/MAX1842s’ current-mode, constant-off­time PWM architecture regulates the output voltage by
changing the PMOS switch on-time relative to the con­stant off-time. Increasing the on-time increases the
peak inductor current and the amount of energy trans­ferred to the load per pulse.

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.

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 regu­lation comparator turns the PMOS switch on at the end
of each off-time, keeping the device in continuous-con­duction mode. The PMOS switch remains on until the

output is in regulation or the current limit is reached.
When the PMOS switch turns off, it remains off for the
programmed off-time (t

OFF

). To control the current
under short-circuit conditions, the PMOS switch
remains off for approximately 4 x t

OFF

when V

OUT

<

V

OUT(NOM)

/ 4.

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. Current­sense 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.

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.

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
logic-level high or connect to VCCfor normal operation.

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 pro­vided 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).

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

10 ______________________________________________________________________________________

Synchronous Rectification

In a step-down regulator without synchronous rectifica­tion, an external Schottky diode provides a path for cur­rent to flow when the inductor is discharging. Replacing
the Schottky diode with a low-resistance NMOS syn-

chronous switch reduces conduction losses and
improves efficiency.

The NMOS synchronous-rectifier switch turns on follow­ing a short delay after the PMOS power switch turns off,
thus preventing cross conduction or “shoot through.” In

Figure 2. Functional Diagram

Figure 1. Typical Circuit

INPUT

= 10µF (MAX1742)

C

IN

= 33µF (MAX1842)

C

IN

10Ω

2.2µF

470pF

R

TOFF

IN

V

CC

SHDN
COMP

TOFF

MAX1742

LX

FB

PGND

GND

FBSEL

REF

SS

L

1µF

0.01µF

OUTPUT
C

= 47µF (MAX1742)

OUT

= 150µF (MAX1842)

C

OUT

V

= 2.5V, FBSEL = V

OUT

V

= 1.8V, FBSEL = REF

OUT

= 1.5V, FBSEL = FLOATING

V

OUT

0.01µF

CC

FBSEL

FEEDBACK

COMP

V

SHDN

REF

REF

CC

G

m

470pF

10Ω

V

IN

2.2µF

1µF

NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.

SELECTION

REF

REF

GND

MAX1742
MAX1842

SUMMING

COMPARATOR

TIMER

R

SS

CURRENT

SENSE

SKIP

PWM LOGIC

AND

DRIVERS

CURRENT

SENSE

PGNDTOFF

TOFF

FB

IN

10µF

LX

V

3.0V TO 5.5V

C

IN

OUT

V

OUT

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

______________________________________________________________________________________ 11

constant-off-time mode, the synchronous-rectifier
switch turns off just prior to the PMOS power switch
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 rel­atively high.

Thermal Resistance

Junction-to-ambient thermal resistance, θJA, is highly
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1742 evaluation kit
has 0.5in2of 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 high­current pins.

Power Dissipation

Power dissipation in the MAX1742/MAX1842 is domi­nated 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:

P

DS

= C x V

IN

2

x f

PWM

where C = 2.5nF and f

PWM

is the switching frequen-

cy in PWM mode.

This number is reduced when the switching frequency
decreases as the part enters Idle Mode. Combined con­duction losses in the two power switches are approxi­mated by:

P

D

= I

OUT

2

x R

PMOS

where R

PMOS

is the on-resistance of the PMOS switch.

The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:

θ

JA

= (T

J,MAX

- T

A,MAX

) / P

D(TOT)

where: θJA= junction-to-ambient thermal resistance

T

J,MAX

= maximum junction temperature

T

A,MAX

= maximum ambient temperature

P

D(TOT)

= total losses

__________________Design Procedure

For typical applications, use the recommended compo­nent values in Tables 1 or 2. For other applications,
take the following steps:

1) Select the desired PWM-mode switching frequency;
1MHz is a good starting point. See Figure 3 for maxi­mum operating frequency.

V

OUT

(V)

R

TOFF

(kΩ)

5.6 39

L

(µH)

5 3.3

5.6

V

IN

(V)

47

5.6 755 1.8

3.9 393.3 2.5

3.9 433.3 1.8

3.9 563.3 1.5

5 2.5

Table 1. MAX1742 Recommended
Component Values (I

OUT

= 1A)

5.6 1005 1.5

f

PWM

(kHz)

850

910

610

1050

1000

770

1070

Table 2. MAX1842 Recommended
Component Values (Continuous Output
Current = 1A, Burst Output Current = 2.7A)

1180

715

940

985

570

850

800

f

PWM

(kHz)

1.55 1002.2

2.55

1.53.3 561.5

1.83.3 431.5

2.53.3 391.5

1.85 752.2

47

V

IN

(V)

2.2

3.35

L

(µH)

392.2

R

TOFF

(kΩ)

V

OUT

(V)

Figure 3. Maximum Recommended Operating Frequency vs.
Input Voltage

OPERATING FREQUENCY vs. INPUT VOLTAGE

1400

1200

1000

800

600

400

OPERATING FREQUENCY (kHz)

200

0

2.6 3.6 4.13.1 4.6 5.1 5.6

MAXIMUM RECOMMENDED

V

= 1.5V

OUT

V

= 1.8V

OUT

V

= 2.5V

OUT

VIN (V)

V

OUT

= 3.3V

MAX1842 fig03

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

12 ______________________________________________________________________________________

2) Select the constant off-time as a function of input

voltage, output voltage, and switching frequency.

3) Select R

TOFF

as a function of off-time.

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
when VFB= V

REF

. Use 50kΩ for R1. R2 is given by the

equation:

where V

REF

is typically 1.1V.

Programming the Switching Frequency

and Off-Time

The MAX1742/MAX1842 features a programmable
PWM mode switching frequency, which is set by the
input and output voltage and the value of R

TOFF

, con-

nected from TOFF to GND. R

TOFF

sets the PMOS
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:

where: t

OFF

= the programmed off-time

V

IN

= the input voltage

V

OUT

= the output voltage

V

PMOS

= the voltage drop across the internal

PMOS power switch

V

NMOS

= the voltage drop across the internal

NMOS synchronous-rectifier switch

f

PWM

= switching frequency in PWM mode

Select R

TOFF

according to the formula:

R

TOFF

= (t

OFF

- 0.07µs) (110kΩ / 1.00µs)

Recommended values for R

TOFF

range from 36kΩ to

430kΩ for off-times of 0.4µs to 4µs.

Inductor Selection

The key inductor parameters must be specified: inductor
value (L) and peak current (I

PEAK

). The following equa­tion 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 cur­rent 1.125 times the DC load current:

where: I

OUT

= maximum DC load current

LIR = ratio of peak-to-peak AC inductor current

to DC load current, typically 0.25

Figure 4. Adjustable Output Voltage

PIN

2.5

V

CC

Output voltage

1.5

1.8REF Output voltage

AdjustableGND

Resistive

divider

FB

OUTPUT

VOLTAGE

(V)

FBSEL

Unconnected Output voltage

Table 3. Output Voltage Programming

LX

MAX1742

R1 = 50kΩ
R2 = R1(V

= 1.1V

V

REF

MAX1842

/ V

OUT

REF

FB

- 1)

V

OUT

R2

R1

t

OFF

VV V

–

()

=

IN OUT PMOS

fVV V

PWM IN PMOS NMOS

−+

()

−

Vt

OUT OFF

L

=

×

I LIR

×

OUT



V

R2 R1

=−

OUT



V



REF

1






MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

______________________________________________________________________________________ 13

The peak inductor current at full load is 1.125 x I

OUT

if
the above equation is used; otherwise, the peak current
is calculated by:

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 operat­ing frequency.

Capacitor Selection

The input filter capacitor reduces peak currents and
noise at the voltage source. Use a low-ESR and low­ESL capacitor located no further than 5mm from IN.
Select the input capacitor according to the RMS input
ripple-current requirements and voltage rating:

where I

RIPPLE

= input RMS current ripple.

The output filter capacitor affects the output voltage rip­ple, output load-transient response, and feedback loop
stability. For stable operation, the MAX1742/MAX1842
requires a minimum output ripple voltage of V

RIPPLE

≥

1% x V

OUT

.

The minimum ESR of the output capacitor should be:

Stable operation requires the correct output filter capaci­tor. When choosing the output capacitor, ensure that:

Integrator Amplifier

An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C

COMP

, from COMP
to VCCcompensates the transconductance amplifier.
For stability, choose C

COMP

= 470pF.

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

decreases stability. Choose the capacitor values that
result in optimal performance.

Soft-Start

Soft-start allows a gradual increase of the internal cur­rent limit to reduce input surge currents at startup and
at exit from shutdown. A timing capacitor, CSS, placed
from SS to GND sets the rate at which the internal cur­rent 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 volt­age across the soft-start capacitor changes with time
according to the equation:

The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:

where I

LIMIT

is the current threshold from the Electrical

Characteristics.

Figure 5. Soft-Start Current Limit over Time

II

PEAK OUT

=+

Vt

×

OUT OFF

L

×

2

II

RIPPLE LOAD

VV V

OUT IN OUT

=−

()

V

IN

ESR

% >×1

t

OFF

L

t

C

C

OUT

OUT

OFF

≥µµ / 33 1742

V

OUT

t

OFF

≥µµ / 79 1842

V

OUT

FV s for the MAX

FV s for the MAX

SHDN

0

V

(V)

SS

0

(A)

I

LIMIT

0

0.7V

V

SS

At

=×4µ

C

SS

SSI

LIMIT

VV

−× .

07

SS

=

.

11

V

I

LIMIT

1.8V

I

LIMIT

t

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

14 ______________________________________________________________________________________

The constant-current source stops charging once the
voltage across the soft-start capacitor reaches 1.8V
(Figure 5).

Extended Current Limit (MAX1842)

For applications requiring occasional short bursts of
high output current (up to 2.7A), the MAX1842 provides
a higher current-limit threshold. When using the
MAX1842, choose external components capable of
withstanding its higher peak current limit.

The MAX1842 is capable of delivering large output cur­rents for limited durations, and its thermal characteris­tics allow it to operate at continuously higher output
currents. Figure 6 shows its maximum recommended
continuous output current versus ambient temperature.
Figure 7 shows the maximum recommended burst cur­rent versus the output current duty cycle at high tem­peratures.

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.

Note that if the thermal limitations of the MAX1842 are
exceeded, it will enter thermal shutdown to prevent
destructive failure.

Frequency Variation with Output Current

The operating frequency of the MAX1742/MAX1842 is
determined primarily by t

OFF

(set by R

TOFF

), VIN, and

V

OUT

as shown in the following formula:

f

PWM

= (VIN- V

OUT

- V

PMOS

) / [t

OFF(VIN

- V

PMOS

+

V

NMOS

)]

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:

∆f

PWM

= -I

OUT

x R

PMOS

/ (VINx t

OFF

)

where R

PMOS

is the resistance of the internal MOSFETs

(90mΩ typ).

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 rout­ing of traces using appropriate trace widths. The follow­ing points are in order of decreasing importance:

1) Minimize switched-current and high-current ground
loops. Connect the input capacitor’s ground, the out­put capacitor’s ground, and PGND. Connect the
resulting island to GND at only one point.

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.

Figure 6. MAX1842 Maximum Recommended Continuous
Output Current vs. Temperature

Figure 7. MAX1842 Maximum Recommended Burst Current vs.
Burst Current Duty Cycle

MAX1842

MAXIMUM RECOMMENDED CONTINUOUS

OUTPUT CURRENT vs. TEMPERATURE

2.70

2.65

2.60

2.55

2.50

2.45

OUTPUT CURRENT (A)

2.40

2.35

2.30
25 4535 55 65 75 85

TEMPERATURE (°C)

MAXIMUM RECOMMENDED BURST CURRENT

vs. BURST CURRENT DUTY CYCLE

2.7

TA = +85°C

2.6

2.5

2.4

BURST CURRENT (A)

2.3

I

IS A 100Hz SQUARE WAVE

OUT

FROM 1A TO THE BURST CURRENT

2.2

04020 60 80 100

DUTY CYCLE (%)

TA = +55°C

MAX1842 fig06

MAX1842 fig07

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

______________________________________________________________________________________ 15

3) Place the LX node components as close together
and as near to the device as possible. This reduces
resistive and switching losses as well as noise.

4) A ground plane is essential for optimum perfor­mance. 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 lay­ers for interconnections and the inner layers for an
uninterrupted ground plane. Avoid large AC currents
through the ground plane.

Chip Information

TRANSISTOR COUNT: 3662

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

MAX1742
MAX1842

QSOP

IN

LX

COMP

IN

SS

TOFF

FB

Pin Configuration

MAX1742/MAX1842

1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches

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

© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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.)

QSOP.EPS