Datasheet

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MAX1742/MAX1842

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

Synchronous Rectification and Internal Switches

________________________________________________________________ Maxim Integrated Products 1

19-1760; Rev 2; 9/05

General Description

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

. 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 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.

For similar devices that provide continuous output cur-

rents 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

= 4.5V

110mΩ On-Resistance at V

= 3V

♦ Output Voltage

2.5V, 1.8V, or 1.5V Pin Selectable

1.1V to V

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

Ordering Information

Idle Mode is a trademark of Maxim Integrated Products.

Typical Configuration

TOFF

COMP

FBSEL

SHDN

PGND

GND

REF

MAX1742

MAX1842

OUTPUT

1.1V TO

INPUT

3V TO

5.5V

0.01μF

1μF

2.2μF

470pF

10Ω

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

PART

TEMP RANGE

PIN-PACKAGE

MAX1742EEE

-40°C to +85°C

16 QSOP

MAX1742EEE+

-40°C to +85°C

16 QSOP

MAX1842EEE

-40°C to +85°C

16 QSOP

MAX1842EEE+

-40°C to +85°C

16 QSOP

+ Denotes lead-free package.

Summary of content (16 pages)

PAGE 1
9-1760; Rev 2; 9/05 KIT ATION EVALU E L B A IL AVA 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 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.
PAGE 2
MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches ABSOLUTE MAXIMUM RATINGS VCC, IN to GND ........................................................-0.3V to +6V Continuous Power Dissipation (TA = +70°C) IN to VCC .............................................................................±0.3V SSOP (derate 16.7mW/°C above +70°C; GND to PGND.....................................................................±0.3V part mounted on 1 in.2 of 1oz. copper) .
PAGE 3
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches (VIN = VCC = 3.3V, FBSEL = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Current-Limit Threshold SYMBOL ILIMIT MIN TYP MAX MAX1742 CONDITIONS 1.3 1.5 1.7 MAX1842 3.1 3.6 4.1 MAX1742 0.1 0.3 0.5 MAX1842 0.3 0.6 0.
PAGE 4
MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches ELECTRICAL CHARACTERISTICS (VIN = VCC = 3.3V, FBSEL = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.
PAGE 5
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742 EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 3.9μH) 80 75 70 85 80 75 VOUT = 1.8V, RTOFF = 43kΩ, f = 869kHz 90 85 80 75 70 65 65 60 60 60 VOUT = 1.5V, RTOFF = 56kΩ, f = 833kHz 55 VOUT = 1.5V, RTOFF = 100kΩ, f = 692kHz 0.01 0.1 1 0.001 0.01 1100 MAX1742 toc04 0.4 0.3 900 FREQUENCY (kHz) 0.1 0 -0.1 VIN = 3.3V, VOUT = 1.5V VIN = 5V, VOUT = 2.5V, L = 6μH 1000 VIN = 5V, VOUT = 1.
PAGE 6
Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1742 STARTUP AND SHUTDOWN MAX1742 LOAD-TRANSIENT RESPONSE MAX1742 toc07 MAX1742 toc06 IINPUT 0A 1A/div VOUTPUT AC-COUPLED, 50mV/div VSHDN 0V 5V/div VOUTPUT 0V 1V/div IL 0.5A/div 0V 0V VSS 2V/div 10μs/div 1ms/div MAX1742 LINE-TRANSIENT RESPONSE SUPPLY CURRENT vs.
PAGE 7
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 80 75 VOUT = 1.8V, RTOFF = 75kΩ, fPWM = 910kHz 70 65 60 50 0.001 0.01 0.1 1 75 VOUT = 1.8V, RTOFF = 43kΩ, fPWM = 1050kHz 70 55 50 0.001 VOUT = 2.5V, RTOFF = 56kΩ, fPWM = 1000kHz 0.01 0.1 1 10 OUTPUT CURRENT (A) OUTPUT CURRENT (A) MAX1842 EFFICIENCY vs. OUTPUT CURRENT (fPWM = 270kHz) MAX1842 NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT 85 80 75 VIN = 3.3V, VOUT = 1.8V, L = 4.
PAGE 8
MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1842 LINE-TRANSIENT RESPONSE MAX1842 LOAD-TRANSIENT RESPONSE MAX1842 toc18 MAX1842 toc17 VINPUT 2V/div VOUTPUT 50mV/div 0 VOUTPUT 20mV/div AC-COUPLED IL 2A/div 10μs/div 20μs/div IOUT = 2.5A, VOUT = 1.5V, RTOFF = 100kΩ, L = 2.
PAGE 9
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 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.
PAGE 10
MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches INPUT CIN = 10μF (MAX1742) CIN = 33μF (MAX1842) L IN 10Ω 2.2μF OUTPUT COUT = 47μF (MAX1742) COUT = 150μF (MAX1842) LX MAX1742 FB 470pF VCC PGND SHDN COMP FBSEL GND VOUT = 2.5V, FBSEL = VOUT = 1.8V, FBSEL = VOUT = 1.5V, FBSEL = REF 1μF TOFF SS VCC REF FLOATING 0.01μF RTOFF Figure 1. Typical Circuit 0.
PAGE 11
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches 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: PDS = C x VIN2 x fPWM where C = 2.5nF and fPWM is the switching frequency in PWM mode.
PAGE 12
MAX1742/MAX1842 1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Table 3. Output Voltage Programming FBSEL FB OUTPUT VOLTAGE (V) VCC Output voltage 2.5 Unconnected Output voltage 1.5 REF Output voltage 1.
PAGE 13
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches SHDN V × t OFF IPEAK = IOUT + OUT 2 × L 0 1.8V 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.
PAGE 14
The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 5). 2.70 2.65 2.60 2.55 2.50 2.45 2.40 2.35 2.30 25 Frequency Variation with Output Current 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.
PAGE 15
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches Pin Configuration TOP VIEW 16 LX SHDN 1 IN 2 15 PGND LX 3 IN 4 SS 5 14 LX MAX1742 MAX1842 COMP 6 TRANSISTOR COUNT: 3662 12 VCC 11 FBSEL TOFF 7 Chip Information 13 PGND 10 REF FB 8 9 GND QSOP A "+" SIGN WILL REPLACE THE FIRST PIN INDICATOR ON LEAD-FREE PACKAGES.
PAGE 16
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 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.