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19-1760; Rev 1; 2/02
KIT ATION EVALU BLE AVAILA
1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
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 90m PMOS power switch and 70m 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 ModeTM 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 1A. 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. o 1% Output Accuracy o 95% Efficiency o Internal PMOS and NMOS Switches 90m On-Resistance at VIN = 4.5V 110m On-Resistance at VIN = 3V o Output Voltage 2.5V, 1.8V, or 1.5V Pin Selectable 1.1V to VIN Adjustable o 3V to 5.5V Input Voltage Range o 600A (max) Operating Supply Current o <1A Shutdown Supply Current o Programmable Constant-Off-Time Operation o 1MHz (max) Switching Frequency 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 16-Pin QSOP Package
Features
MAX1742/MAX1842
Ordering Information
PART MAX1742EEE MAX1842EEE TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP
Applications
5V or 3.3V to Low-Voltage Conversion CPU I/O Ring Chipset Supplies Notebook and Subnotebook Computers
INPUT 3V TO 5.5V
Typical Configuration
IN 10 LX OUTPUT 1.1V TO VIN
MAX1742 FB MAX1842 VCC PGND GND SHDN COMP TOFF FBSEL REF SS 0.01F 1F
2.2F
470pF
Pin Configuration appears at end of data sheet.
Idle Mode is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products
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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 MAX1742/MAX1842
ABSOLUTE MAXIMUM RATINGS
VCC, IN to GND ........................................................-0.3V to +6V Continuous Power Dissipation (TA = +70C) IN to VCC .............................................................................0.3V SSOP (derate 16.7mW/C above +70C; GND to PGND.....................................................................0.3V part mounted on 1 in.2 of 1oz. copper) ...............................1W Operating Temperature Range ...........................-40C to +85C All Other Pins to GND.................................-0.3V to (VCC + 0.3V) LX Current (Note 1).............................................................4.7A Storage Temperature Range .............................-65C to +150C REF Short Circuit to GND Duration ............................Continuous Lead Temperature (soldering, 10s) ................................ +300C 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
(VIN = VCC = 3.3V, FBSEL = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input Voltage SYMBOL VIN, VCC FBSEL = VCC TA = +25C to +85C TA = +0C to +85C TA = +25C to +85C TA = +0C to +85C TA = +25C to +85C TA = +0C to +85C TA = +25C to +85C TA = +0C to +85C CONDITIONS MIN 3.0 2.500 2.487 1.500 1.492 1.800 1.791 1.089 1.084 VREF 2 0.4 VDO VREF VREF RON, P RON, N VIN = VCC = 3V, ILOAD = 1A TA = +25C to +85C TA = +0C to +85C IREF = -1A to +10A ILX = 0.5A ILX = 0.5A VIN = 4.5V VIN = 3V VIN = 4.5V VIN = 3V 1.089 1.084 1.100 1.100 0.5 90 110 70 80 250 1.111 1.117 2 200 250 150 200 m 2.525 2.525 1.515 1.515 1.818 1.818 1.100 1.100 TYP MAX 5.5 2.550 2.563 1.530 1.538 V 1.836 1.845 1.111 1.117 VIN V % % mV V mV FBSEL = REF UNITS V
Preset Output Voltage
VOUT
VIN = 3V to 5.5V, ILOAD = 0 to 1A for MAX1742, ILOAD = 0 to 2.5A for MAX1842, VFB = VOUT
FBSEL = unconnected
FBSEL = GND
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
VIN = VCC = 3V to 5.5V, ILOAD = 0, FBSEL = GND
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCC = 3.3V, FBSEL = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Current-Limit Threshold RMS LX Output Current Idle Mode Current Threshold Switching Frequency No-Load Supply Current Shutdown Supply Current PMOS Switch Off-Leakage Current Thermal Shutdown Threshold Undervoltage Lockout Threshold FB Input Bias Current Off-Time Default Period Off-Time Startup Period On-Time Period SS Source Current SS Sink Current SHDN Input Current SHDN Input Low Threshold SHDN Input High Threshold FBSEL Input Current FBSEL = GND FBSEL = REF FBSEL Logic Thresholds FBSEL = unconnected FBSEL = VCC 0.9 0.7 VCC - 0.2 VCC - 0.2 IIM f IIN + ICC MAX1742 MAX1842 (Note 2) VFB = 1.2V 350 <1 0.1 0.3 0.3 0.6 SYMBOL ILIMIT MAX1742 MAX1842 CONDITIONS MIN 1.3 3.1 TYP 1.5 3.6 MAX 1.7 4.1 3.1 0.5 0.9 1 600 5 15 160 2.5 0 0.9 0.24 3.8 0.4 4 VSS = 1V V SHDN = 0 to VCC 100 -1 2.0 -4 +4 0.2 1.3 0.7 VCC +0.2 V 1 0.8 5 6 2.6 60 1.00 0.30 4.5 4 tOFF 2.7 250 1.1 0.37 5.2 s s A A A V V A s UNITS A A A MHz A A A C V nA
MAX1742/MAX1842
ICC(SHDN) SHDN = GND IIN TSHDN VUVLO IFB tOFF tOFF tON ISS ISS I SHDN VIL VIH SHDN = GND Hysteresis = 15C VIN falling, hysteresis = 90mV VFB = 1.2V RTOFF = 110k RTOFF = 30.1k RTOFF = 499k FB = GND (Note 2)
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
ELECTRICAL CHARACTERISTICS
(VIN = VCC = 3.3V, FBSEL = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.) (Note 3)
PARAMETER Input Voltage SYMBOL VIN ILOAD = 0 to 1A for MAX1742, ILOAD = 0 to 2.5A for MAX1842, VFB = VOUT VIN = 3V to 5.5V, FBSEL = VCC FBSEL = unconnected FBSEL = REF FBSEL = GND CONDITIONS MIN 3.0 2.475 1.485 1.782 1.078 VREF 1.078 ILX = 0.5A ILX = 0.5A MAX1742 MAX1842 MAX1742 MAX1842 VFB = 1.2V VFB = 1.2V RTOFF = 110k 0 0.85 VIN = 4.5V VIN = 3V VIN = 4.5V VIN = 3V 1.2 2.9 0.05 0.2 MAX 5.5 2.575 1.545 1.854 1.122 VIN 1.122 200 250 150 200 1.8 4.3 0.55 1.0 600 300 1.15 A nA s A m V V V UNITS V
Preset Output Voltage
VOUT
Adjustable Output Voltage Range Reference Voltage PMOS Switch On-Resistance NMOS Switch On-Resistance Current-Limit Threshold Idle Mode Current Threshold No-Load Supply Current FB Input Bias Current Off-Time Default Period VREF RON, P RON, N ILIMIT IIM IIN + ICC IFB tOFF
VIN = VCC = 3V to 5.5V, ILOAD = 0, FBSEL = GND
Note 2: Recommended operating frequency, not production tested. Note 3: Specifications from 0C to -40C are guaranteed by design, not production tested.
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1742 EFFICIENCY vs. OUTPUT CURRENT (VIN = 5.0V, L = 6.0H)
MAX1742 toc01
MAX1742/MAX1842
MAX1742 EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 3.9H)
MAX1742 toc02
MAX1742 EFFICIENCY vs.OUTPUT CURRENT (fPWM = 270kHz)
95 90 EFFICIENCY (%) 85 80 75 70 65 60 VIN = 3.3V, VOUT = 1.8V, L = 10H, RTOFF = 160k VIN = 5V, VOUT = 1.8V, L = 15H, RTOFF = 240k
MAX1742 toc03
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.001 VOUT = 1.5V, RTOFF = 100k, f = 692kHz 0.01 0.1 1 VOUT = 2.5V, RTOFF = 47k, f = 926kHz VOUT = 1.8V, RTOFF = 75k, f = 833kHz
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.001 0.01 0.1 1 VOUT = 1.5V, RTOFF = 56k, f = 833kHz VOUT = 1.8V, RTOFF = 43k, f = 869kHz VOUT = 2.5V, RTOFF = 36k, f = 456kHz
100
55 50 0.001
0.01
0.1
1
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
MAX1742 NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT
MAX1742 toc04
MAX1742 SWITCHING FREQUENCY vs. OUTPUT CURRENT
1000 900 FREQUENCY (kHz) 800 700 600 500 400 300 200 100 0 0 0.2 0.4 0.6 0.8 1.0 OUTPUT CURRENT (A) VIN = 5V, VOUT = 1.5V, L = 6H VIN = 3.3V, VOUT = 1.5V, L = 3.9H VIN = 5V, VOUT = 2.5V, L = 6H
MAX1742 toc05
0.5 0.4 NORMALIZED OUTPUT ERROR (%) 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 0.001 0.01 0.1 1 OUTPUT CURRENT (A) VIN = 3.3V, VOUT = 1.5V VIN = 5V, VOUT = 1.5V, L = 6H
1100
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1742 STARTUP AND SHUTDOWN
MAX1742 toc06
MAX1742 LOAD-TRANSIENT RESPONSE
MAX1742 toc07
IINPUT 0A 1A/div VOUTPUT AC-COUPLED, 50mV/div
VSHDN 0V 5V/div VOUTPUT 0V 1V/div 0V VSS 2V/div 1ms/div 10s/div 0V
IL 0.5A/div
MAX1742 LINE-TRANSIENT RESPONSE
MAX1742 toc08
SUPPLY CURRENT vs. SUPPLY VOLTAGE
500 NO-LOAD SUPPLY CURRENT, IIN + ICC (A) VINPUT 2V/div 450 400 350 300 250 200 150 100 50 0 0 1 2 3 VIN (V) 4 5 6 SHUTDOWN NO LOAD
MAX1742 toc09
100 90 80 70 60 50 40 30 20 10 0 SHUTDOWN SUPPLY CURRENT, IIN + ICC (nA)
0V
VOUTPUT 20mV/div AC-COUPLED
20s/div IOUT = 1A, VOUT = 1.5V, RTOFF = 100k, L = 6H
OFF-TIME vs. RTOFF
4.5 4.0 3.5 tOFF (s) 3.0 2.5 2.0 1.5 1.0 0.5 0 0 50 100 150 200 250 300 350 400 450 500 RTOFF (k)
MAX1742 toc10
5.0
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1842 EFFICIENCY vs. OUTPUT CURRENT (VIN = 5.0V, L = 2.5H)
MAX1842 toc11
MAX1842 EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 1.5H)
95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 10 50 0.001 VOUT = 1.8V, RTOFF = 43k, fPWM = 1050kHz VOUT = 2.5V, RTOFF = 56k, fPWM = 1000kHz 0.01 0.1 1 10 VOUT = 2.5V, RTOFF = 39k, fPWM = 610kHz
MAX1842 toc12
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.001 VOUT = 1.8V, RTOFF = 75k, fPWM = 910kHz VOUT = 1.5V, RTOFF = 1OOk, fPWM = 770kHz 0.01 0.1 1 VOUT = 2.5V, RTOFF = 47k, fPWM = 1070kHz
100
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
MAX1842 EFFICIENCY vs. OUTPUT CURRENT (fPWM = 270kHz)
95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.001 0.01 0.1 IOUT (A) 1 10 -0.4 0.001 VIN = 3.3V, VOUT = 1.8V, L = 4.7H, RTOFF = 160k VIN = 5V, VOUT = 1.8V, L = 5.6H, RTOFF = 240k
MAX1842 toc13
MAX1842 NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT
VIN = 3.3V, VOUT = 1.5V, L = 1.5H NORMALIZED OUTPUT ERROR (%) 0
MAX1842 toc14
100
0.1
-0.1
-0.2 VIN = 5V, VOUT = 1.5V, L = 2.5H -0.3
0.01
0.1
1
10
OUTPUT CURRENT (A)
MAX1842 SWITCHING FREQUENCY vs. OUTPUT CURRENT
VIN = 5V, VOUT = 2.5V, L = 2.5H 1000 FREQUENCY (kHz) 800 600 400 VIN = 5V, VOUT = 1.5V, L = 2.5H 200 0 0 0.5 1.0 1.5 2.0 2.5 3.0 OUTPUT CURRENT (A)
MAX1842 toc15
MAX1842 STARTUP AND SHUTDOWN
MAX1842 toc16
1200
0
IINPUT 1A/div
VIN = 3.3V, VOUT = 1.5V, L = 1.5H
0
VSHDN 5V/div VOUTPUT 1V/div VSS 2V/div
0
0 1ms/div ROUT = 0.5, RTOFF = 56k VIN = 3.3V, VOUT = 1.5V
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1842 LOAD-TRANSIENT RESPONSE
MAX1842 toc17
MAX1842 LINE-TRANSIENT RESPONSE
MAX1842 toc18
VINPUT 2V/div VOUTPUT 50mV/div
0
IL 2A/div
VOUTPUT 20mV/div AC-COUPLED
10s/div
20s/div IOUT = 2.5A, VOUT = 1.5V, RTOFF = 100k, L = 2.2H
Pin Description
PIN 1 2, 4 3, 14, 16 5 6 7 8 9 10 11 12 13, 15 NAME SHDN IN LX SS COMP TOFF FB GND REF FBSEL VCC PGND FUNCTION Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal MOSFETs. Drive high or connect to VCC for normal operation. 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. Soft-Start. Connect a capacitor from SS to GND to limit inrush current during startup. Integrator Compensation. Connect a capacitor from COMP to VCC for integrator compensation. See 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. 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. Analog Ground Reference Output. Bypass REF to GND with a 1F capacitor. Feedback Select Input. Selects output voltage. See Table 3 for programming instructions. Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCC with a 10 and 2.2F lowpass filter. See Figure 1. Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
_______________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 0.09 PMOS power switch and a 0.07 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. 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 tOFF when VOUT < VOUT(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.
MAX1742/MAX1842
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 1A. 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 VCC for normal operation.
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
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).
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
INPUT CIN = 10F (MAX1742) CIN = 33F (MAX1842) IN 10 VCC 2.2F 470pF SHDN COMP LX L OUTPUT COUT = 47F (MAX1742) COUT = 150F (MAX1842)
MAX1742 FB
PGND GND FBSEL REF 1F TOFF SS 0.01F VOUT = 2.5V, FBSEL = VOUT = 1.8V, FBSEL = VOUT = 1.5V, FBSEL = VCC REF FLOATING
RTOFF
Figure 1. Typical Circuit
0.01F FBSEL SS FB FEEDBACK SELECTION COMP 470pF 10 VCC REF Gm REF
MAX1742 MAX1842
IN CURRENT SENSE SKIP PWM LOGIC AND DRIVERS 10F
VIN 3.0V TO 5.5V
VIN
2.2F SHDN REF 1F GND
SUMMING COMPARATOR
LX
VOUT COUT
REF
TIMER
CURRENT SENSE PGND
TOFF RTOFF
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
Figure 2. Functional Diagram
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
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
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. Junction-to-ambient thermal resistance, JA, is highly 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 80C/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.
MAX1742/MAX1842
Table 1. MAX1742 Recommended Component Values (IOUT = 1A)
VIN (V) 5 5 5 5 3.3 3.3 3.3 VOUT (V) 3.3 2.5 1.8 1.5 2.5 1.8 1.5 fPWM (kHz) 850 1070 910 770 610 1050 1000 L (H) 5.6 5.6 5.6 5.6 3.9 3.9 3.9 RTOFF (k) 39 47 75 100 39 43 56
Thermal Resistance
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. 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: PD = IOUT2 x RPMOS where RPMOS is the on-resistance of the PMOS switch. The junction-to-ambient thermal resistance required to dissipate this amount of power is calculated by: JA = (TJ,MAX - TA,MAX) / PD(TOT) where: JA = junction-to-ambient thermal resistance TJ,MAX = maximum junction temperature TA,MAX = maximum ambient temperature PD(TOT) = total losses
Table 2. MAX1842 Recommended Component Values (Continuous Output Current = 1A, Burst Output Current = 2.7A)
VIN (V) 5 5 5 5 3.3 3.3 3.3 VOUT (V) 3.3 2.5 1.8 1.5 2.5 1.8 1.5 fPWM (kHz) 800 1180 850 715 570 985 940 L (H) 2.2 2.2 2.2 2.2 1.5 1.5 1.5 RTOFF (k) 39 47 75 100 39 43 56
MAXIMUM RECOMMENDED OPERATING FREQUENCY vs. INPUT VOLTAGE
VOUT = 1.5V 1200 OPERATING FREQUENCY (kHz) 1000 800 600 400 200 0 2.6 3.1 3.6 4.1 VIN (V) 4.6 5.1 5.6 VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V
MAX1842 fig03
1400
__________________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.
Figure 3. Maximum Recommended Operating Frequency vs. Input Voltage
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
Table 3. Output Voltage Programming
PIN FBSEL VCC Unconnected REF GND FB Output voltage Output voltage Output voltage Resistive divider OUTPUT VOLTAGE (V) 2.5 1.5 1.8 Adjustable
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 RTOFF, connected 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: t OFF =
(VIN - VOUT - VPMOS ) fPWM ( VIN - VPMOS + VNMOS )
LX
VOUT
MAX1742 MAX1842
R2
FB R1 = 50k R2 = R1(VOUT / VREF - 1) VREF = 1.1V R1
where: tOFF = the programmed off-time VIN = the input voltage VOUT = the output voltage VPMOS = the voltage drop across the internal PMOS power switch VNMOS = the voltage drop across the internal NMOS synchronous-rectifier switch f PWM = switching frequency in PWM mode Select RTOFF according to the formula: RTOFF = (tOFF - 0.07s) (110k / 1.00s) Recommended values for RTOFF range from 36k to 430k for off-times of 0.4s to 4s.
Figure 4. Adjustable Output Voltage
2) Select the constant off-time as a function of input voltage, output voltage, and switching frequency. 3) Select RTOFF as a function of off-time. 4) Select the inductor as a function of output voltage, off-time, and peak-to-peak inductor current.
Inductor Selection
The key inductor parameters must be specified: inductor value (L) and peak current (IPEAK). 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: L= VOUT x t OFF IOUT x LIR
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 = VREF. Use 50k for R1. R2 is given by the equation: V R2 = R1 OUT - 1 VREF where VREF is typically 1.1V.
where: IOUT = maximum DC load current LIR = ratio of peak-to-peak AC inductor current to DC load current, typically 0.25
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
The peak inductor current at full load is 1.125 x IOUT if the above equation is used; otherwise, the peak current is calculated by: V x t OFF IPEAK = IOUT + OUT 2xL 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.
SHDN 0 1.8V VSS (V) 0 ILIMIT ILIMIT (A) 0 t 0.7V
MAX1742/MAX1842
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: IRIPPLE = ILOAD VOUT VIN - VOUT VIN
Figure 5. Soft-Start Current Limit over Time
(
)
decreases stability. Choose the capacitor values that result in optimal performance.
where IRIPPLE = 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 VRIPPLE 1% x VOUT. The minimum ESR of the output capacitor should be: ESR > 1% x L t OFF
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 4A 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: VSS = 4A x t CSS
Stable operation requires the correct output filter capacitor. When choosing the output capacitor, ensure that: t COUT OFF 33FV / s for the MAX1742 VOUT t COUT OFF 79FV / s for the MAX1842 VOUT
Integrator Amplifier
An internal transconductance amplifier fine tunes the output DC accuracy. A capacitor, CCOMP, from COMP to VCC compensates the transconductance amplifier. For stability, choose CCOMP = 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
The soft-start current limit varies with the voltage on the soft-start pin, SS, according to the equation: V - 0.7V SSILIMIT = SS x ILIMIT 1.1V where ILIMIT is the current threshold from the Electrical Characteristics.
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
The constant-current source stops charging once the voltage across the soft-start capacitor reaches 1.8V (Figure 5).
MAX1842 MAXIMUM RECOMMENDED CONTINUOUS OUTPUT CURRENT vs. TEMPERATURE
2.65 OUTPUT CURRENT (A) 2.60 2.55 2.50 2.45 2.40 2.35 2.30 25 35 45 55 65 75 85 TEMPERATURE (C)
MAX1842 fig06
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.
2.70
Figure 6. MAX1842 Maximum Recommended Continuous Output Current vs. Temperature
MAXIMUM RECOMMENDED BURST CURRENT vs. BURST CURRENT DUTY CYCLE
TA = +85C 2.6 BURST CURRENT (A)
MAX1842 fig07
2.7 TA = +55C
Frequency Variation with Output Current
The operating frequency of the MAX1742/MAX1842 is determined primarily by tOFF (set by RTOFF), VIN, and VOUT as shown in the following formula: fPWM = (VIN - VOUT - VPMOS) / [tOFF (VIN - VPMOS + VNMOS)] 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: fPWM = -IOUT x RPMOS / (VIN x tOFF) where RPMOS is the resistance of the internal MOSFETs (90m typ).
2.5
2.4
2.3 IOUT IS A 100Hz SQUARE WAVE FROM 1A TO THE BURST CURRENT 2.2 0 20 40 60 80 100 DUTY CYCLE (%)
Figure 7. MAX1842 Maximum Recommended Burst Current vs. Burst Current Duty Cycle
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.
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches
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.
Pin Configuration
TOP VIEW
SHDN 1 IN 2 LX 3 IN 4 SS 5 COMP 6 TOFF 7 16 LX 15 PGND 14 LX
MAX1742/MAX1842
MAX1742 MAX1842
13 PGND 12 VCC 11 FBSEL 10 REF 9 GND
Chip Information
TRANSISTOR COUNT: 3662
FB 8
QSOP
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1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1742/MAX1842
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
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 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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