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19-1454; Rev 2; 7/01
KIT ATION EVALU BLE AVAILA
Low-Noise, 14V Input, 1A, PWM Step-Down Converters
General Description Features
o Up to 96% Efficiency o 1A Guaranteed Output Current o 100% Duty Cycle in Dropout o 2.7V to 14V Input Range (15V Absolute Max) o 1% Accurate Reference Output o 0.24 P-Channel On-Resistance o Synchronizable Switching Frequency o Fixed-Frequency PWM Operation 300kHz (MAX1684) 600kHz (MAX1685) o 150A Normal-Mode Quiescent Current o 25A Low-Power Mode Quiescent Current o 2A Shutdown Current o Dual ModeTM Fixed 3.3V (1%) Output or Adjustable Output (1.25V to VIN) o Small 16-QSOP Package o Auxiliary Output (CVL): 3V/5mA
MAX1684/MAX1685
The MAX1684/MAX1685 are high-efficiency, internalswitch, pulse-width modulation (PWM) step-down switching regulators intended to power cellular phones, communicating PDAs, and handy-terminals. These devices deliver a guaranteed 1A output current from two lithium-ion (Li+) batteries. Their wide-input voltage range of 2.7V to 14V gives design flexibility and allows batteries to charge from a wall cube, since the ICs operate at the higher voltages that occur when the battery is removed. The output voltage is preset to 3.3V or can be externally adjusted from 1.25V to VIN. The low on-resistance power switch and built-in synchronous rectifier provide high efficiencies of up to 96%. There are four modes of operation: fixed-frequency, normal, low-power, and shutdown. The fixed-frequency PWM mode of operation offers excellent noise characteristics. The normal mode maintains high efficiency at all loads. The low-power mode is used to conserve power in standby or when full load is not required. The shutdown mode is used to power down the device for minimal current draw. The MAX1684 runs at 300kHz for applications that require highest efficiency. The MAX1685 runs at 600kHz to allow the use of smaller external components. These devices can also be synchronized to an external clock. Other features include a 100% duty cycle for low-dropout applications, an auxiliary 3V/5mA output, and a 1% accurate reference. Both devices are available in a space-saving 16-QSOP package. An evaluation kit is also available to help speed designs. For a similar device in a 10-pin MAX package with lower input voltage requirements (5.5V max), refer to the MAX1692 data sheet.
Ordering Information
PART MAX1684EEE MAX1685EEE TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP
Typical Operating Circuit
Applications
Cellular Phones Two-Way Radios and Walkie-Talkies Computer Peripherals Personal Communicators PDAs and Handy-Terminals
INPUT 2.7V TO 14V + IN AIN SHDN CVH CVL STBY SYNC/PWM BOOT LX
OUTPUT 3.3V AT 1A +
MAX1684 MAX1685
GND
Pin Configuration appears at end of data sheet.
FB
CC
REF
Dual Mode is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ 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.
Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
ABSOLUTE MAXIMUM RATINGS
AIN to AGND ............................................................-0.3 to +15V IN to PGND ................................................-0.3V to (VAIN + 0.3V) LX to PGND .................................................-0.5V to (VIN + 0.3V) PGND to AGND ..................................................................0.3V SHDN to AGND .........................................-0.3V to (VAIN + 0.3V) ILIM/SS, FB, CC, BOOT, REF to AGND ....-0.3V to (VCVL + 0.3V) CVH to IN..................................................................-6V to +0.3V CVL, STBY, SYNC/PWM to AGND............................-0.3V to +6V Reference Current ..............................................................1mA CVL Current .......................................................-1mA to +10mA LX Peak Current (Internally Limited) .....................................2.3A Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 8.3mW/C above +70C)............667mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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 = V SHDN = 6V, STBY = SYNC/PWM = CVL, VBOOT = VOUT, FB = AGND, circuit of Figure 1, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Input Voltage Range Feedback Voltage Output Voltage (3.3V Mode) Output Load Regulation Output Current Capability Output Adjust Range FB Input Current On-Resistance, P-Channel On-Resistance, N-Channel Current Limit in PWM Mode Pulse-Skipping Current Threshold Current Limit in Low-Power Mode Current Limit, N-Channel Zero Crossing Threshold ILIMLP ILIM SYNC/PWM = low STBY = low SYNC/PWM = high SYNC/PWM = low PWM mode, SYNC/PWM = high, VBOOT = 3.3V (Note 2) Quiescent Power Consumption MAX1684 MAX1685 MAX1684 MAX1685 IFB VFB VOUT VFB = VOUT, ILOAD = 0 to 1A FB = AGND, ILOAD = 0 to 1A VFB = VOUT, ILOAD = 0 to 1A VIN = 5V to 14V BOOT = AGND (Note 1) VFB = 1.4V High-side switch, ILX = 1A VIN = 6V VIN = 2.7V 1.2 285 285 0.15 -10 20 1 VREF -50 0.24 0.34 3 1.75 380 380 0.4 50 80 13 25 0.9 0.14 VIN 50 0.5 0.8 8 2.3 475 475 0.9 100 130 33 65 mW 2 0.27 SYMBOL CONDITIONS MIN 2.7 1.238 3.296 1.251 3.333 0.01 TYP MAX 14 1.264 3.368 UNITS V V V % A V nA A mA mA A mA
Low-side switch, VIN = 2.7V, ILX = 200mA
Normal mode, SYNC/PWM = low, VBOOT = 3.3V (Note 2) Low-power mode, STBY = low, VBOOT = 3.3V (Note 2)
2
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters
ELECTRICAL CHARACTERISTICS (continued)
(VIN = V SHDN = 6V, STBY = SYNC/PWM = CVL, VBOOT = VOUT, FB = AGND, circuit of Figure 1, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Quiescent Supply Current in Dropout Shutdown Supply Current LX Leakage Current Oscillator Frequency SYNC Capture Range Maximum Duty Cycle Constant-Frequency Minimum Duty Cycle Reference Output Voltage Reference Load Regulation Reference Supply Regulation CVL Regulator Output Voltage CVL Dropout Voltage CVL Undervoltage Lockout Threshold CVH with Respect to VIN BOOT Switchover Threshold Thermal Shutdown Threshold ILIM/SS Source Current Logic Input High Voltage Logic Input Low Voltage Logic Input Current SYNC/PWM Pulse Width VIH VIL VREF (Note 3) IREF = 0 -1A < IREF < 50A 2.7V < VBOOT < 5.5V VIN = 3V to 14V, BOOT = AGND, ICVL = 0 to 5mA BOOT = AGND, ICVL = 5mA BOOT = AGND, CVL falling edge, typical hysteresis is 40mV ICVH = -1mA BOOT falling edge, typical hysteresis is 0.1V Typical hysteresis is +10C (Note 4) VILIM/SS = 1.4V SHDN, STBY, SYNC/PWM SHDN, STBY, SYNC/PWM High or low period 3.3 2 0.7 -1 500 1 2.35 -5.0 2.35 2.5 -4.6 2.5 160 4 4.65 2.7 MAX1684 MAX1685 1.238 ILX fOSC SYMBOL CONDITIONS STBY = low, VIN = 2.7V SHDN = low VIN = 14V, VLX = 0 or 14V, SHDN = low MAX1684 MAX1685 MAX1684 MAX1685 260 520 180 360 100 10 20 1.251 4 0.2 3.0 1.264 15 5 3.15 120 2.6 -4.1 2.65 300 600 MIN TYP 230 2 MAX 430 6 20 340 680 350 700 UNITS A A A kHz kHz % % V mV mV V mV V V V C A V V A ns
MAX1684/MAX1685
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
ELECTRICAL CHARACTERISTICS
(VIN = V SHDN = 6V, STBY = SYNC/PWM = CVL, VBOOT = VOUT, FB = AGND, circuit of Figure 1, TA = -40C to +85C, unless otherwise noted.) (Note 5) PARAMETER Input Voltage Range Output Feedback Voltage Output Voltage (3.3V Mode) Output Current Capability Output Adjust Range FB Input Current Current Limit in PWM Mode Current Limit in Low-Power Mode IFB ILIM ILIMLP STBY = low Normal mode, SYNC/PWM = low, VBOOT = 3.3V (Note 2) Low-power mode, STBY = low, VBOOT = 3.3V (Note 2) SHDN = low fOSC MAX1684 MAX1685 IREF = 0 VIN = 3V to 14V, BOOT = AGND, ICVL = 0 to 5mA BOOT = AGND, CVL falling edge, typical hysteresis is 40mV ICVH = -1mA BOOT falling edge, typical hysteresis is 0.1V VILIM/SS = 1.4V VIH VIL SHDN, STBY, SYNC/PWM 240 480 1.232 2.7 2.4 -5.0 2.35 3.1 2 0.7 VFB VOUT VFB = VOUT, ILOAD = 0 to 1A FB = AGND, ILOAD = 0 to 1A VIN = 6V to 14V BOOT = AGND (Note 1) VFB = 1.4V SYMBOL CONDITIONS MIN 2.7 1.233 3.280 1 VREF -50 1.2 285 VIN 50 2.3 475 2 mW 0.27 6 350 700 1.268 3.15 2.6 -4.1 2.65 4.7 A kHz V V V V V A V MAX 14 1.269 3.382 UNITS V V V A V nA A mA
Quiescent Power Consumption
Shutdown Supply Current Oscillator Frequency Reference Output Voltage CVL Regulator Output Voltage CVL Undervoltage Lockout Threshold CVH with Respect to VIN BOOT Switchover Threshold ILIM/SS Source Current Logic Input High Voltage Logic Input Low Voltage
Note 1: The output adjust range with BOOT connected to VOUT is VREF to 5.5V. Connect BOOT to AGND for VOUT > 5.5V. Note 2: The quiescent power-consumption specifications include chip supply and gate-drive loss only. Divide these values by VIN (6V) to obtain quiescent currents. In normal and low-power modes, chip supply current dominates and quiescent power is proportional to VBOOT (BOOT connected to OUT). In PWM mode, gate-drive loss dominates and quiescent power is proportional to VIN (VIN - VCVH). In addition, IR losses in power switches and external components typically increase PWM quiescent power consumption by 5mW to 10mW. Note that if the device is not bootstrapped, additional power is dissipated in the CVL linear regulator. Note 3: When the duty factor (VOUT / VIN) is less than this value, the switching frequency decreases in PWM mode to maintain regulation. Note 4: Thermal shutdown is disabled in low-power mode (STBY = low) to reduce power consumption. Note 5: Specifications to -40C are guaranteed by design, not production tested.
4
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAX1684 EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V, VOUT = 1.8V, 2.5V)
MAX1684/85 toc01
MAX1684/MAX1685
MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1684/85 toc02
MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, PWM MODE)
90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 A: VIN = 4V B: VIN = 5V C: VIN = 9V D: VIN = 12V 1000 10,000 A D B C
MAX1684/85 toc03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 C B A D E A: VOUT = 2.5V LP MODE B: VOUT = 1.8V LP MODE C: VOUT = 2.5V NORM MODE D: VOUT = 1.8V NORM MODE E: VOUT = 2.5V PWM MODE F: VOUT = 1.8V PWM MODE 10 100 1000
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 1000 A: VIN = 4V LP MODE B: VIN = 12V LP MODE C: VIN = 4V NORMAL MODE D: VIN = 12V NORMAL MODE B A D C
100
F
10,000
10,000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
MAX1684/85 toc04
MAX1684 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V, PWM MODE)
D
MAX1684/85 toc05
MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, PWM MODE)
90 80 EFFICIENCY (%) 70 60 50 40 30 VIN = 9V VIN = 12V VIN = 5V VIN = 4V
MAX1684/85 toc06
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 0.1 1 A: VIN = 6V LP MODE B: VIN = 9V LP MODE C: VIN = 12V LP MODE D: VIN = 6V NORMAL MODE E: VIN = 9V NORMAL MODE F: VIN = 12V NORMAL MODE 10 100 1000 B A C F E E
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 1000 A: VIN = 6V B: VIN = 9V C: VIN =12V B C A
100
20 10 0 1
10,000
10,000
10
100
1000
10,000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1684/85 toc07
MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V PWM MODE)
MAX1684/85 toc08
MAX1685 EFFICIENCY vs. LOAD CURRENT (VOUT = 5V)
90 80 EFFICIENCY (%) 70 60 50 40 30 A: VIN = 6V LP MODE B: VIN = 9V LP MODE C: VIN = 12V LP MODE D: VIN = 6V NORMAL MODE E: VIN = 9V NORMAL MODE F: VIN =12V NORMAL MODE 0.1 1 10 100 1000 10,000 LOAD CURRENT (mA) A B E D
MAX1684/85 toc09
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 B D A
100 90 80 EFFICIENCY (%) 70 60 50 40 30 VIN =12V VIN = 6V VIN = 9V
100
C
C
F
A: VIN = 4V LP MODE B: VIN = 12V LP MODE C: VIN = 4V NORMAL MODE D: VIN = 12V NORMAL MODE 10 100 1000 10,000
20 10 0 1
20 10 0
10
100
1000
10,000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE
MAX1684/85 toc10
SAFE OPERATING AREA
MAX1684/85 toc11
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1684/85 toc12
1.8 1.7 1.6
LOAD CURRENT (A)
1400 1200 LOAD CURRENT (mA) 1000 800 600 400 200 VOUT = 3.3V
400
DROPOUT VOLTAGE (mV)
300 VOUT = 3.3V 200 VOUT = 5V 100 INDUCTOR RESISTANCE INCLUDED
1.5 1.4 1.3 1.2 1.1 PWM OR NORMAL MODE 1.0 0 2 4 6 8 10 12 14 INPUT VOLTAGE (V)
0 0 2 4 6 8 10 12 14 16 INPUT VOLTAGE (V)
0 0 200 400 600 800 1000 LOAD CURRENT (mA)
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE
MAX1684/85 toc13
MAX1684 NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (VOUT = 3.3V, PWM MODE)
MAX1684/85 toc14
PWM FIXED-FREQUENCY OPERATION AREA
14 13 INPUT VOLTAGE (V) 12 11 10 9 8 7 6 MAX1684 MAX1685
M AX1684/85 toc15
120
5.0 4.5 SUPPLY CURRENT (mA) 4.0 3.5 3.0 2.5 2.0
15
100 SUPPLY CURRENT (A)
80
NORMAL MODE
60
40 LOW-POWER MODE 20 4 6 8 10 12 14 INPUT VOLTAGE (V)
2.7 4 5 6 7 8 9 10 11 12 0 2 4 6 8 10 12 14 INPUT VOLTAGE (V) OUTPUT VOLTAGE (V)
LOAD-TRANSIENT RESPONSE
MAX1684/85 toc16
SWITCHING WAVEFORM
MAX1684/85 toc17
SWITCHING WAVEFORM
MAX1684/85 toc18
ILOAD 500mA/div
VOUT 20mV/div
VLX 5V/div
VOUT 50mV/div ILX 100mA/div ILX 100mA/div
2ms/div MAX1684, ILOAD = 0.1mA TO 1A, VOUT = 3.3V, VIN = 5V, SYNC/PWM = 3.3V
1s/div MAX1684, ILOAD = 100mA, VOUT = 3.3V, VIN = 5V, SYNC/PWM = 3.3V
1s/div MAX1684, ILOAD = 100mA, VOUT = 3.3V, VIN = 5V, SYNC/PWM = 3.3V
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.)
LINE-TRANSIENT RESPONSE
MAX1684/85 toc19
MAX1684/MAX1685
STARTUP CURRENT
MAX1684/85 toc20
VIN 5V/div 0V
VSHDN 5V/div
VOUT 100mV/div
IIN 50mA/div
2ms/div MAX1684, ILOAD = 100mA, VIN = 5V TO 10V, SYNC/PWM = 3.3V
10ms/div MAX1684, ILOAD = 100mA, VOUT = 3.3V, VIN = 5V, CILIM/SS = 0.1F, SYNC/PWM = 3.3V
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME CVH AIN IN CVL AGND REF FB CC FUNCTION High-Side MOSFET Gate Bias. Bias voltage for P-channel switch. Bypass to IN with a 0.1F capacitor. Analog Supply Voltage Input. Connect to IN with a 0.2in metal trace. Bypass to PGND with a 0.1F capacitor. Supply Voltage Input Logic Supply Voltage Output and IC Logic Supply. Sources 5mA for external loads. Bypass to AGND with 1F capacitor. Analog Ground Reference Output. 1.25V reference output supplies 10A for external loads. Bypass to AGND with 0.1F capacitor. Dual-Mode Feedback Input. Connect FB to VOUT for 1.25V output. Connect to an external resistor divider to adjust the output voltage. Connect to AGND to set output voltage to 3.3V. Integrator Capacitor Connection. Connect a 0.01F capacitor to AGND. SYNC/PWM Input: For synchronized-PWM operation, drive with TTL level, 50% square wave. Connect to CVL for PWM mode. Connect to AGND for normal mode. Current-Limit Adjust/Soft-Start Input. See the Current Limit and Soft-Start section. Standby Control Input. Connect to CVL for normal operation. Connect to AGND for low-power mode (Table 1). This pin overrides SYNC/PWM setting. Bootstrap Input. Connection for the bootstrap switch and internal feedback path. Connect BOOT to VOUT for VOUT < 5.5V. Connect BOOT to AGND for VOUT > 5.5V. Inductor Connection. Drain for internal P-channel MOSFETs. Connect inductor from LX to OUT. Active-Low Shutdown Input. Connect to ground for shutdown. SHDN can withstand the input voltage. Power Ground
9
SYNC/PWM
10 11 12 13, 14 15 16
ILIM/SS STBY BOOT LX SHDN PGND
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
0.1F
INPUT 14V MAX 22F 0.1F ON/OFF
3 2
1 CVH IN AIN LX
13, 14
L 10H* MBRS 130LT3
OUTPUT 3.3V AT 1A COUT 100F
synchronization to an external clock. The MAX1684/MAX1685 can operate in five modes. Setting the devices to operate in the appropriate mode for the intended application (Table 1) achieves highest efficiency.
PWM Control
The MAX1684/MAX1685 use an oscillator-triggered minimum/maximum on-time current-mode control scheme (Figure 2). The minimum on-time is typically 220ns unless the regulator is in dropout. The maximum on-time is 2 / f OSC, allowing operation to 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting for superior load- and line-transient response. At each falling edge of the internal oscillator, the internal P-channel MOSFET (main switch) turns on. This allows current to ramp up through the inductor to the load and stores energy in a magnetic field. The switch remains on until either the current-limit comparator trips, the maximum on-time expires, or the PWM comparator signals that the output is in regulation. When the switch turns off during the second half of each cycle, the inductor's magnetic field collapses, releasing the stored energy and forcing current through the output diode to the output filter capacitor and load. The output filter capacitor stores charge when the inductor current is high and releases it when the inductor current is low, smoothing the voltage across the load. During normal operation, the MAX1684/MAX1685 regulate the output voltage by switching at a constant frequency and modulating the power transferred to the load on each cycle using the PWM comparator. A multiinput comparator sums three weighted differential signals (the output voltage with respect to the reference, the main switch current sense, and the slope-compensation ramp) and changes states when a threshold is reached. It modulates output power by adjusting the
MAX1685 PGND
15 SHDN AGND BOOT 4 CVL
16 5 12
1F
11
STBY 9 SYNC/PWM ILIM/SS 10 CC 8
FB REF 6
7
0.1F (OPTIONAL)
0.1F 0.01F
*SUMIDA CD54-100; USE 22H FOR MAX1684
Figure 1. Standard Application Circuit
_______________Detailed Description
The MAX1684/MAX1685 step-down, PWM DC-DC converters provide an adjustable output from 1.25V to the input voltage. They accept inputs from 2.7V to 14V and deliver up to 1.6A. An internal MOSFET and synchronous rectifier reduce PC board area while maintaining high efficiency. Operation with up to 100% duty cycle minimizes dropout voltage. Fixed-frequency PWM operation reduces interference in sensitive communications and data-acquisition applications. A SYNC input allows
Table 1. Operating Modes
MODE PWM Sync PWM Normal Low Power Shutdown SYNC/PWM H Clocked L X X STBY H H H L X SHDN H H H H L FUNCTION Fixed-frequency PWM Fixed-input clock-frequency PWM PFM at light loads (<150mA); fixedfrequency PWM at heavy loads (>150mA) Low-power or standby mode Circuit disabled TYPICAL OUTPUT CAPABILITY (A) 1.6 1.6 1.6 160m 0
8
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
ILIM/SS IN AIN 4A
CVL
MAX1684 MAX1685
VH CVH
SHDN
VL
THERMAL SHUTDOWN
ON
ILIM THRESHOLD
CVL PFM CURRENT COMPARATOR ILIM COMPARATOR LX UNDERVOLTAGE COMPARATOR 2.5V PWM COMPARATOR CONTROL AND DRIVER LOGIC
REF
REF
SYNC/PWM
OSC SYNC SLOPE COMPENSATION AND STANDBY PWM MODE CONTROL NORMAL MODE LOW-POWER MODE PGND ZERO-CROSSING COMPARATOR CVL 2.5V PFM COMPARATOR GM INTEGRATOR
STBY
CC
BOOT
FB
0.125V AGND
Figure 2. Functional Diagram
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
inductor peak current during the first half of each cycle, based on the output error voltage. The MAX1684/ MAX1685s' loop gain is relatively low to enable the use of a small, low-value output filter capacitor. The 1.4% transient load regulation from 0 to 1A is compensated by an integrator circuit that lowers DC load regulation to 0.01% typical. Slope compensation accounts for the inductor-current waveform's down slope during the second half of each cycle, and eliminates the inductorcurrent staircasing characteristic of current-mode controllers at high duty cycles. For higher VIN at no load, the frequency decreases based on the following equation: f = VOUT / (VIN x 290ns) At medium- to full-load current (>100mA), V IN can increase slightly higher before the frequency decreases.
Synchronous Rectification
Although the primary rectifier is an external Schottky diode, a small internal N-channel synchronous rectifier allows PWM operation at light loads. During the second half of each cycle, when the inductor current ramps below the zero-crossing threshold or when the oscillator period ends, the synchronous rectifier turns off. This keeps excess current from flowing backward through the inductor. Choose an appropriate inductor to limit the PWM ripple current through the N-channel FET to 400mAP-P.
PFM Control
In low-power mode, the MAX1684/MAX1685 switch only as needed to service the load. This reduces the switching frequency and associated losses in the P-channel switch, the synchronous rectifier, and the external inductor. During this PFM operation, a switching cycle initiates when the PFM comparator senses that the output voltage has dropped too low. The P-channel MOSFET switch turns on and conducts current to the output-filter capacitor and load. The MAX1684/MAX1685 then wait until the PFM comparator senses a low-output voltage again. In normal mode at light load (<150mA), the device also operates in PFM. The PFM current comparator controls both entry into PWM mode and the peak switch current during PFM operation. Consequently, some jitter is normal during transition from PFM to PWM with loads around 150mA, and it has no adverse impact on regulation.
Current Limit and Soft-Start
The voltage at ILIM/SS sets the PWM current limit (ILIM = 1.75A) and the low-power current limit (ILIMLP = 380mA). The PWM current limit applies when the device is in PWM mode, in synchronized PWM mode, or delivering a heavy load in normal mode (Table 1). The ILIMLP limit applies when the device is in low-power mode. An internal 4A current source pulls ILIM/SS up to CVL. To use the maximum current-limit thresholds, leave ILIM/SS unconnected or connect it to a soft-start capacitor. Connect an external resistor from ILIM/SS to AGND to adjust the current-limit thresholds. The PWM current-limit threshold is (ILIM RILIM/SS 4A) / VREF and is adjustable from 0.5A to 1.75A. The low-power current-limit threshold is equal to (ILIMLP RILIM/SS 4A) / VREF and is adjustable from 110mA to 380mA. For example, when RILIM/SS is 156k, the PWM current limit threshold is 0.88A and the low-power current limit threshold is 0.19A. Connect a low-value capacitor from ILIM/SS to AGND to achieve soft-start, limiting inrush current. ILIM/SS internally shorts to AGND in shutdown to discharge the soft-start capacitor. Do not connect ILIM/SS to REF or CVL. Determine the soft-start duration by: tSOFT-START = CILIM/SS(1.25V / 4A) where tSOFT-START is the time from SHDN going high to the regulator being able to supply full load current. For example, a 0.1F capacitor yields 31ms of soft-start.
100% Duty-Cycle Operation
As the input voltage drops, the duty cycle increases until the P-channel MOSFET turns on continuously, achieving 100% duty cycle. Dropout voltage in 100% duty cycle is the output current multiplied by the onresistance of the internal switch and inductor, approximately 0.35V (IOUT = 1A).
Very Low Duty-Cycle Operation
Because of the P-channel minimum on-time and deadtime (duration when both switches are off), the MAX1684/MAX1685s' switching frequency must decrease in PWM or normal mode to maintain regulation at a very low duty cycle. The total P-channel ontime and dead-time is 290ns typical. As a result, the MAX1684/MAX1685 maintain fixed-frequency regulation at no load for V IN up to 10V OUT and 5V OUT , respectively (see PWM Fixed-Frequency Operation Area graph in the Typical Operating Characteristics).
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters
The output current capability for each mode is determined by the following equations: IOUTMAX = ILIM - 0.5 IRIPPLE (for PWM and normal modes) IOUTMAX = 0.5 ILIMLP (for low-power mode) where: IRIPPLE = ripple current = (VIN - VOUT) VOUT / (VIN fOSC L) ILIM = current limit in PWM mode ILIMLP = current limit in low-power mode
MAX1684/MAX1685
VOUT
MAX1684 MAX1685
FB
R1
C1
R2
Internal Low-Voltage Regulators and Bootstrap (BOOT)
The MAX1684/MAX1685 have two internal regulators (VH and VL) that generate low-voltage supplies for internal circuitry (see the Functional Diagram). The VH regulator generates -4.6V with respect to IN to supply the P-channel switch and driver. Bypass CVH to IN with a 0.1F capacitor. The VL regulator generates a 3V output at CVL to supply internal low-voltage blocks, as well as the N-channel switch and driver. Bypass CVL to AGND with a 1F capacitor. To reduce the quiescent current in low-power and normal modes, connect BOOT to OUT. After startup, when VBOOT exceeds 2.6V, the internal bootstrap switch connects CVL to BOOT. This bootstrap mechanism causes the internal circuitry to be supplied from the output and thereby reduces the input quiescent current by a factor of VOUT / VIN. Do not connect BOOT to OUT if the output voltage exceeds 5.5V. Instead, connect BOOT to AGND to keep CVL regulated at 3V. CVL has a 5mA capability to supply external logic circuitry and is disabled in shutdown mode.
Figure 3. Setting Output Voltage
Connect a small capacitor across R1 to compensate for stray capacitance at the FB pin: C1 = 5 (10-7) R2
where: R2 = 100k, use 4.7pF.
Inductor Selection
The MAX1684/MAX1685s' high switching frequency allows the use of small surface-mount inductors. Table 2 shows a selection of suitable inductors for different output voltage ranges. Calculate the minimum inductor by: L = 0.9(VOUT - 0.3V) / (IRIPPLE MAX x fOSC) where: IRIPPLE MAX = should be less than or equal to 400mA fOSC = 300kHz (MAX1684) or 600kHz (MAX1685)
Capacitor Selection
Select input and output filter capacitors to service inductor currents while minimizing voltage ripple. The input filter capacitor reduces peak currents and noise at the voltage source. The MAX1684/MAX1685s' loop gain is relatively low to enable the use of small, lowvalue output filter capacitors. Higher capacitor values provide improved output ripple and transient response. Low-ESR capacitors are recommended. Capacitor ESR is a major contributor to output ripple (usually more than 60%). Avoid ordinary aluminum electrolytic capacitors, as they typically have high ESR. Low-ESR aluminum electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact solution for spaceconstrained surface-mount designs. Do not exceed the ripple-current ratings of tantalum capacitors. Ceramic capacitors offer the lowest ESR overall. Sanyo OS-CON
11
Applications Information
Output Voltage Selection
Connect FB to AGND to select the internal 3.3V output mode. Connect BOOT to OUT in this configuration. To select an output voltage between 1.25V and VIN, connect FB to a resistor voltage-divider between the output and AGND (Figure 3). Select R2 in the 20k to 100k range. Calculate R1 as follows: R1 = R2 [( VOUT / VFB) - 1] where VFB = 1.25V.
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Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
capacitors have the lowest ESR of the high-value electrolytic types. Use ceramic and OS-CON capacitors for very compact, high-reliability, or wide-temperature applications, where expense is justified. When using very low ESR capacitors, such as ceramic or OS-CON, check for stability while examining load-transient response, and increase the output compensation capacitor if needed. Table 3 lists suppliers for the various components used with the MAX1684/MAX1685. Ensure that the minimum capacitance value and maximum ESR values are met: COUT > IOUT MAX / (VOUT AC Load Reg fOSC) RESR < 2 AC Load Reg VOUT/IOUT MAX where I OUT MAX = 1A, AC Load Reg 1.4%, and fOSC = 300kHz (MAX1684) or 600kHz (MAX1685).
Output Diode Selection
Use a 1A external Schottky diode (MBRS130LT3 or equivalent) for the output rectifier to pass inductor current during the start of the second half of each cycle. This diode operates before the internal N-channel MOSFET completely turns on and during high-current operation. Use a Schottky diode to avoid forward biasing the internal body diode of the N-channel MOSFET.
Table 2. Inductor and Minimum Output Capacitor Selection
MAX1684 (300kHz) VOUT (V) 1.25 to 2.7 2.7 to 4 4 to 6 6 to 14 L (H) 22 22 47 68 MIN COUT (F) 220 100 68 47 L (H) 10 10 22 33 MAX1685 (600kHz) MIN COUT (F) 100 47 33 22
Table 3. Component Suppliers
SUPPLIER CAPACITORS AVX Matsuo Sanyo Sprague INDUCTORS Coilcraft Murata-Erie Sumida TDK DIODES Motorola 602-303-5454 602-994-6430
VIN, MAX = 14V - |VOUT| R2 -VOUT = -1.25V R1 + 1
PHONE
FAX
0.1F
803-946-0690 714-969-2591 619-661-6835 603-224-1961
803-626-3123 714-960-6492 619-661-1055 603-224-1430
AIN VIN 22F 0.1F IN CVH
MAX1684 MAX1685
MBRS 130LT3 LX C1 L R1
-VOUT -1.25V TO -5.5V 100F
SHDN STBY CVL SYNC/PWM REF
FB R2 BOOT PGND AGND CC
847-639-6400 814-237-1431 847-956-0666 847-390-4373
847-639-1469 814-238-0490 847-956-0702 847-390-4428
1F 0.1F
ILIM/SS
0.01F
0.01F
(
)
Figure 4. Inverting Output 12 ______________________________________________________________________________________
Low-Noise, 14V Input, 1A, PWM Step-Down Converters
Inverting Output
Interchanging the ground and VOUT connections yields a negative voltage supply (Figure 4). The component selections are the same as for a positive voltage converter. The absolute maximum ratings limit the output voltage range to -1.25V to -5.5V and the maximum input voltage range to 14V - VOUT.
Pin Configuration
TOP VIEW
CVH 1 AIN 2 IN 3 CVL 4 AGND 5 REF 6 FB 7 CC 8 16 PGND 15 SHDN 14 LX
MAX1684/MAX1685
PC Board Layout
High switching frequencies and large peak currents make PC board layout a very important part of design. Poor design can result in excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which result in instability or regulation errors. Power components such as the MAX1684/MAX1685 inductor, input filter capacitor, and output filter capacitor should be placed as close together as possible, and their traces kept short, direct, and wide, Connect their ground nodes in a star-ground configuration. Keep the extra copper on the board and integrate into ground as a pseudo-ground plane. When using external feedback, the feedback network should be close to FB, within 0.2 inch (5mm), and the output voltage feedback should be tapped as close to the output capacitor as possible. Keep noisy traces, such as those from LX, away from the voltage feedback network. Separate the noisy traces by grounded copper. Place the small bypass capacitors within 0.2 inch (5mm) of their respective inputs. The MAX1684 evaluation kit manual illustrates an example PC board layout, routing, and pseudo-ground plane. Connect AIN to IN with a short (0.2 inch) metal trace or a 1 resistor and bypass AIN to PGND with a 0.1F capacitor. This acts as a lowpass filter to reduce noise at AIN.
MAX1684 MAX1685
13 LX 12 BOOT 11 STBY 10 ILIM/SS 9 SYNC/PWM
QSOP
Chip Information
TRANSISTOR COUNT: 2061
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13
Low-Noise, 14V Input, 1A, PWM Step-Down Converters MAX1684/MAX1685
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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