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General Description The MAX1684 MAX1685 are high efficiency internal switch pulse width modulation PWM step down switch ing 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 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 3V or can be externally adjusted from 25V to VIN The low on resistance power switch and built in synchro nous rectifier provide high efficiencies of up to There are four modes of operation: fixed frequency nor mal low power and shutdown The fixed frequency PWM mode of operation offers excellent noise character istics 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 cur rent draw The MAX1684 runs at 300kHz for applications that require highest efficiency The MAX1685 runs at 600kHz to allow the use of smaller external compo nents These devices can also be synchronized to an external clock Other features include a duty cycle for low dropout applications an auxiliary 3V 5mA output and a accurate reference Both devices are available in a space saving QSOP package An evaluation kit is also available to help speed designs For a similar device in a pin MAX package with lower input voltage requirements 5V max refer to the MAX1692 data sheet

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Description

Niveau: Supérieur, Doctorat, Bac+8
General Description The MAX1684/MAX1685 are high-efficiency, internal- switch, pulse-width modulation (PWM) step-down switch- ing 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 synchro- nous rectifier provide high efficiencies of up to 96%. There are four modes of operation: fixed-frequency, nor- mal, low-power, and shutdown. The fixed-frequency PWM mode of operation offers excellent noise character- istics. 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 cur- rent draw. The MAX1684 runs at 300kHz for applications that require highest efficiency. The MAX1685 runs at 600kHz to allow the use of smaller external compo- nents.

  • load current

  • low

  • khz

  • logic input

  • output voltage

  • µa

  • max1685

  • foscoscillator frequency

  • pwm


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MAX1684/MAX1685
19-1454; Rev 2; 7/01
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
General Description Features
The MAX1684/MAX1685 are high-efficiency, internal- Up to 96% Efficiency
switch, pulse-width modulation (PWM) step-down switch-
1A Guaranteed Output Currenting regulators intended to power cellular phones,
communicating PDAs, and handy-terminals. These 100% Duty Cycle in Dropout
devices deliver a guaranteed 1A output current from two
2.7V to 14V Input Range (15V Absolute Max)lithium-ion (Li+) batteries. Their wide-input voltage range
of 2.7V to 14V gives design flexibility and allows batteries ±1% Accurate Reference Output
to charge from a wall cube, since the ICs operate at the
0.24Ω P-Channel On-Resistancehigher voltages that occur when the battery is removed.
The output voltage is preset to 3.3V or can be externally Synchronizable Switching Frequency
adjusted from 1.25V to V .IN
Fixed-Frequency PWM Operation
The low on-resistance power switch and built-in synchro-
300kHz (MAX1684) nous rectifier provide high efficiencies of up to 96%.
600kHz (MAX1685)There are four modes of operation: fixed-frequency, nor-
mal, low-power, and shutdown. The fixed-frequency 150µA Normal-Mode Quiescent Current
PWM mode of operation offers excellent noise character-
25µA Low-Power Mode Quiescent Currentistics. The normal mode maintains high efficiency at all
loads. The low-power mode is used to conserve power in 2µA Shutdown Current
standby or when full load is not required. The shutdown
Dual Mode™ Fixed 3.3V (±1%) Output or mode is used to power down the device for minimal cur-
Adjustable Output (1.25V to V )rent draw. IN
The MAX1684 runs at 300kHz for applications that Small 16-QSOP Package
require highest efficiency. The MAX1685 runs at
Auxiliary Output (CVL): 3V/5mA
600kHz to allow the use of smaller external compo-
nents. These devices can also be synchronized to an
Ordering Informationexternal clock. Other features include a 100% duty
cycle for low-dropout applications, an auxiliary 3V/5mA PART TEMP RANGE PIN-PACKAGE
output, and a 1% accurate reference.
MAX1684EEE -40°C to +85°C 16 QSOP
Both devices are available in a space-saving 16-QSOP MAX1685EEE
package. An evaluation kit is also available to help
speed designs. For a similar device in a 10-pin µMAX
Typical Operating Circuitpackage with lower input voltage requirements (5.5V
max), refer to the MAX1692 data sheet.
Applications
OUTPUTINPUT
2.7V TO 14V 3.3V AT 1ACellular Phones
IN LX ++
Two-Way Radios and Walkie-Talkies
AIN MAX1684
Computer Peripherals MAX1685SHDN
GND
Personal Communicators
CVHPDAs and Handy-Terminals
CVL BOOT
STBY
SYNC/PWM
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.
EVALUATION KIT
AVAILABLELow-Noise, 14V Input, 1A, PWM
Step-Down Converters
ABSOLUTE MAXIMUM RATINGS
AIN to AGND ............................................................-0.3 to +15V CVL Current .......................................................-1mA to +10mA
IN to PGND ................................................-0.3V to (V + 0.3V) LX Peak Current (Internally Limited) .....................................2.3AAIN
LX to PGND .................................................-0.5V to (V Continuous Power Dissipation (T = +70°C)IN A
PGND to AGND ..................................................................±0.3V 16-Pin QSOP (derate 8.3mW/°C above +70°C)............667mW
SHDN to AGND .........................................-0.3V to (V + 0.3V) Operating Temperature Range ...........................-40°C to +85°CAIN
ILIM/SS, FB, CC, BOOT, REF to AGND ....-0.3V to (V Junction Temperature......................................................+150°CCVL
CVH to IN..................................................................-6V to +0.3V Storage Temperature Range .............................-65°C to +150°C
CVL, STBY, SYNC/PWM to AGND............................-0.3V to +6V Lead Temperature (soldering, 10s) .................................+300°C
Reference Current ..............................................................±1mA
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V = V = 6V, STBY = SYNC/PWM = CVL, V = V , FB = AGND, circuit of Figure 1, T = 0°C to +85°C, unless otherwiseIN SHDN BOOT OUT A
noted. Typical values are at T = +25°C.)A
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Input Voltage Range 2.7 14 V
Feedback Voltage V V = V , I = 0 to 1A 1.238 1.251 1.264 VFB FB OUT LOAD
Output Voltage (3.3V Mode) V FB = AGND, I 3.296 3.333 3.368 VOUT LOAD
Output Load Regulation V = V , I = 0 to 1A 0.01 %FB OUT LOAD
Output Current Capability V = 5V to 14V 1 AIN
Output Adjust Range BOOT = AGND (Note 1) V V VREF IN
FB Input Current I V = 1.4V -50 50 nAFB FB
V = 6V 0.24 0.5INHigh-side switch,
On-Resistance, P-Channel Ω
I = 1ALX V = 2.7V 0.34 0.8IN
On-Resistance, N-Channel Low-side switch, V = 2.7V, I = 200mA 38 ΩIN LX
Current Limit in PWM Mode I 1.2 1.75 2.3 ALIM
Pulse-Skipping Current Threshold SYNC/PWM = low 285 380 475 mA
Current Limit in Low-Power
I STBY = low 285 380 475 mALIMLPMode
Current Limit, N-Channel SYNC/PWM = high 0.15 0.4 0.9 A
MAX1684 -10 50 100
Zero Crossing Threshold SYNC/PWM = low mA
MAX1685 20 80 130
PWM mode, MAX1684 13 33
SYNC/PWM = high,
V = 3.3V BOOT
MAX1685 25 65(Note 2)
Quiescent Power Consumption mW
Normal mode, SYNC/PWM = low,
0.9 2
V = 3.3V (Note 2)BOOT
Low-power mode, STBY = low,
0.14 0.27
VBOOT
2 _______________________________________________________________________________________
MAX1684/MAX1685MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
ELECTRICAL CHARACTERISTICS (continued)
(V = V = 6V, STBY = SYNC/PWM = CVL, V = V , FB = AGND, circuit of Figure 1, T = 0°C to +85°C, unless otherwiseIN SHDN BOOT OUT A
noted. Typical values are at T = +25°C.)A
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Quiescent Supply Current
STBY = low, V = 2.7V 230 430 µAINin Dropout
Shutdown Supply Current SHDN = low 26 µA
LX Leakage Current I V = 14V, V = 0 or 14V, SHDN = low 20 µALX IN LX
MAX1684 260 300 340
Oscillator Frequency f kHzOSC
MAX1685 520 600 680
MAX1684 180 350
SYNC Capture Range kHz
MAX1685 360 700
Maximum Duty Cycle 100 %
MAX1684 10Constant-Frequency Minimum
(Note 3) %
Duty Cycle MAX1685 20
Reference Output Voltage V I = 0 1.238 1.251 1.264 VREF REF
Reference Load Regulation -1µA < I < 50µA 415 mVREF
Reference Supply Regulation 2.7V < V < 5.5V 0.2 5 mVBOOT
V = 3V to 14V, BOOT = AGND,INCVL Regulator Output Voltage 2.7 3.0 3.15 V
I = 0 to 5mACVL
CVL Dropout Voltage BOOT = AGND, I = 5mA 120 mVCVL
CVL Undervoltage Lockout BOOT = AGND, CVL falling edge,
2.35 2.5 2.6 V
Threshold typical hysteresis is 40mV
CVH with Respect to V I = -1mA -5.0 -4.6 -4.1 VIN CVH
BOOT falling edge,
BOOT Switchover Threshold 2.35 2.5 2.65 V
typical hysteresis is 0.1V
Thermal Shutdown Threshold Typical hysteresis is +10°C (Note 4) 160 °C
ILIM/SS Source Current V = 1.4V 3.3 4 4.65 µAILIM/SS
Logic Input High Voltage V 2 VIH
SHDN, STBY, SYNC/PWM
Logic Input Low Voltage V 0.7 VIL
Logic Input Current -1 1 µASHDN, STBY
SYNC/PWM Pulse Width High or low period 500 ns
_______________________________________________________________________________________ 3Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
ELECTRICAL CHARACTERISTICS
(V = V = 6V, STBY = SYNC/PWM = CVL, V = V , FB = AGND, circuit of Figure 1, T = -40°C to +85°C, unless other-IN SHDN BOOT OUT A
wise noted.) (Note 5)
PARAMETER SYMBOL CONDITIONS MIN MAX UNITS
Input Voltage Range 2.7 14 V
Output Feedback Voltage V V = V , I = 0 to 1A 1.233 1.269 VFB FB OUT LOAD
Output Voltage (3.3V Mode) V FB = AGND, I 3.280 3.382 VOUT LOAD
Output Current Capability V = 6V to 14V 1 AIN
Output Adjust Range BOOT = AGND (Note 1) V V VREF IN
FB Input Current I V = 1.4V -50 50 nAFB FB
Current Limit in PWM Mode I 1.2 2.3 ALIM
Current Limit in Low-Power
I STBY = low 285 475 mALIMLPMode
Normal mode, SYNC/PWM = low,
2
V = 3.3V (Note 2)BOOT
Quiescent Power Consumption mW
Low-power mode, STBY = low,
0.27
VBOOT
Shutdown Supply Current 6 µASHDN = low
MAX1684 240 350
Oscillator Frequency f kHzOSC
MAX1685 480 700
Reference Output Voltage I = 0 1.232 1.268 VREF
V = 3V to 14V, BOOT = AGND, INCVL Regulator Output Voltage 2.7 3.15 V
I = 0 to 5mACVL
CVL Undervoltage Lockout BOOT = AGND, CVL falling edge,
2.4 2.6 V
Threshold typical hysteresis is 40mV
CVH with Respect to V I = -1mA -5.0 -4.1 VIN CVH
BOOT Switchover Threshold BOOT falling edge, typical hysteresis is 0.1V 2.35 2.65 V
ILIM/SS Source Current V = 1.4V 3.1 4.7 µAILIM/SS
Logic Input High Voltage V 2IH
SHDN, STBY, SYNC/PWM V
Logic Input Low Voltage V 0.7IL
Note 1: The output adjust range with BOOT connected to V is V to 5.5V. Connect BOOT to AGND for V > 5.5V.OUT REF OUT
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 V (BOOT connected to OUT). In PWM mode, gate-drive loss dominates and quiescent power is propor-BOOT
tional to V ✕ (V - V ). In addition, IR losses in power switches and external components typically increase PWM quies-IN IN CVH
cent 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 (V / V ) is less than this value, the switching frequency decreases in PWM mode to maintainOUT IN
regulation.
Note 4: Thermal shutdown is disabled in low-power mode (STBY = low) to reduce power consumption.
Note 5: Specifications to -40°C are guaranteed by design, not production tested.
4 _______________________________________________________________________________________
MAX1684/MAX1685MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
Typical Operating Characteristics
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)A
MAX1684 MAX1684 MAX1684
EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT
(V = 3.3V, V = 1.8V, 2.5V) (V = 3.3V) (V = 3.3V, PWM MODE)IN OUT OUT OUT
100 100 100
CA90 90 90
80 8080 CB AD
70 A 70 70
D D
60 60 60 B
50 C E 50 50
B40 40 40A: V = 2.5V LP MODE OUT
B: V = 1.8V LP MODE30 F OUT 30 30
A: V = 4V LP MODEIN A: V = 4V C: V = 2.5V NORM MODE INOUT
20 20 B: V = 12V LP MODE 20IN B: V = 5V D: V = 1.8V NORM MODE INOUT
C: V = 4V NORMAL MODE C: V = 9V INE: V = 2.5V PWM MODE IN10 OUT 10 10D: V = 12V NORMAL MODE D: V = 12V INF: V = 1.8V PWM MODE INOUT
0 0 0
0.1 1 10 100 1000 10,000 0.1 1 10 100 1000 10,000 1 10 100 1000 10,000
LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA)
MAX1684 MAX1685MAX1684
EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENTEFFICIENCY vs. LOAD CURRENT
(V = 3.3V, PWM MODE)(V = 5V) OUTOUT (V = 5V, PWM MODE)OUT
100 100100
D
B 909090
A V = 4VE IN8080E80 AC
7070
F
70 60 V = 5VIN60
60 5050
B 40C4050 A: V = 6V LP MODE IN V = 9VIN
B: V = 9V LP MODE 30IN 30
40 C: V = 12V LP MODEIN 20A: V = 6V 20 IN V = 12VD: V = 6V NORMAL MODE ININ
B: V = 9V 30 INE: V = 9V NORMAL MODEIN 1010 C: V =12V INF: V = 12V NORMAL MODEIN
20 00
0.1 1 10 100 1000 10,000 1 10 100 1000 10,0001 10 100 1000 10,000
LOAD CURRENT (mA) LOAD CURRENT (mA)LOAD CURRENT (mA)
MAX1685 MAX1685 MAX1685
EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENTEFFICIENCY vs. LOAD CURRENT
(V = 5V PWM MODE) (V = 5V)OUT OUT(V = 3.3V)OUT
100100100 BAC
90 9090 A
D80 80 E80 C FV = 6VIND 70 7070 V = 9V B IN
606060
505050
404040 A: V = 6V LP MODEIN
B: V = 9V LP MODE30 30 IN30
C: V = 12V LP MODEA: V = 4V LP MODE ININ V =12VIN 2020 20 D: V = 6V NORMAL MODEB: V = 12V LP MODE ININ
E: V = 9V NORMAL MODEC: V = 4V NORMAL MODE 10 IN10 IN 10
F: V =12V NORMAL MODED: V = 12V NORMAL MODE ININ
0 00
0.1 1 10 100 1000 10,0001 10 100 1000 10,0000.1 1 10 100 1000 10,000
LOAD CURRENT (mA) LOAD CURRENT (mA)LOAD CURRENT (mA)
_______________________________________________________________________________________ 5
EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%)
MAX1684/85 toc07 MAX1684/85 toc04 MAX1684/85 toc01
EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%)
MAX1684/85 toc08 MAX1684/85 toc05
MAX1684/85 toc02
EFFICIENCY (%)
EFFICIENCY (%) EFFICIENCY (%)
MAX1684/85 toc09 MAX1684/85 toc06 MAX1684/85 toc03Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)A
MAXIMUM LOAD CURRENT
SAFE OPERATING AREA DROPOUT VOLTAGE vs. LOAD CURRENTvs. INPUT VOLTAGE
14001.8 400
V = 3.3VOUT
1.7 1200
3001.6 1000
1.5 V = 3.3VOUT800
2001.4
600
V = 5VOUT1.3
400
1.2 100
2001.1
INDUCTOR RESISTANCE INCLUDEDPWM OR NORMAL MODE
0 01.0
4062 8 10 12 14 16 0 200 400 600 800 100004268101214
INPUT VOLTAGE (V) LOAD CURRENT (mA)INPUT VOLTAGE (V)
MAX1684
NO-LOAD SUPPLY CURRENT
NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE PWM FIXED-FREQUENCY
vs. INPUT VOLTAGE (V = 3.3V, PWM MODE) OPERATION AREAOUT
5.0120 15
14
4.5
100 13 MAX1684
12
4.0 MAX1685
80 11
NORMAL MODE
3.5 10
60 9
3.0
8
40 7
2.5LOW-POWER MODE
6
20 2.0 2.7
486101214 465 7 8 9 10 11 12 02468 10 12 14
INPUT VOLTAGE (V) INPUT VOLTAGE (V) OUTPUT VOLTAGE (V)
SWITCHING WAVEFORM SWITCHING WAVEFORMLOAD-TRANSIENT RESPONSE
MAX1684/85 toc17MAX1684/85 toc16 MAX1684/85 toc18
ILOAD VOUT
500mA/div V20mV/div LX
5V/div
VOUT
50mV/div
ILX ILX
100mA/div 100mA/div
2ms/div 1µs/div 1µs/div
MAX1684, I = 100mA, V = 3.3V, V = 5V,MAX1684, I = 0.1mA TO 1A, V = 3.3V, V = 5V, LOAD OUT IN MAX1684, I = 100mA, V = 3.3V, V = 5V,LOAD OUT IN LOAD OUT IN
SYNC/PWM = 3.3V SYNC/PWM = 3.3VSYNC/PWM = 3.3V
6 _______________________________________________________________________________________
MAX1684/MAX1685
SUPPLY CURRENT (µA) LOAD CURRENT (A)
MAX1684/85 toc13
MAX1684/85 toc10
LOAD CURRENT (mA)
SUPPLY CURRENT (mA)
MAX1684/85 toc14 MAX1684/85 toc11
INPUT VOLTAGE (V)
DROPOUT VOLTAGE (mV)
M AX1684/85 toc15
MAX1684/85 toc12MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
Typical Operating Characteristics (continued)
(Circuit of Figure 1, T = +25°C, unless otherwise noted.)A
LINE-TRANSIENT RESPONSE STARTUP CURRENT
MAX1684/85 toc20MAX1684/85 toc19
VSHDNVIN
5V/div5V/div
0V
VOUT IIN100mV/div
50mA/div
10ms/div2ms/div
MAX1684, I = 100mA, V = 3.3V,MAX1684, I = 100mA, V = 5V TO 10V, LOAD OUTLOAD IN
SYNC/PWM = 3.3V V = 5V, C = 0.1µF, SYNC/PWM = 3.3VIN ILIM/SS
Pin Description
PIN NAME FUNCTION
1 CVH High-Side MOSFET Gate Bias. Bias voltage for P-channel switch. Bypass to IN with a 0.1µF capacitor.
Analog Supply Voltage Input. Connect to IN with a 0.2in metal trace. Bypass to PGND with a 0.1µF
2 AIN
capacitor.
3 IN Supply Voltage Input
Logic Supply Voltage Output and IC Logic Supply. Sources 5mA for external loads. Bypass to AGND
4 CVL
with 1µF capacitor.
5 AGND Analog Ground
Reference Output. 1.25V reference output supplies 10µA for external loads. Bypass to AGND with 0.1µF
6 REF
capacitor.
Dual-Mode Feedback Input. Connect FB to V for 1.25V output. Connect to an external resistor dividerOUT7 FB
to adjust the output voltage. Connect to AGND to set output voltage to 3.3V.
8 CC Integrator Capacitor Connection. Connect a 0.01µF capacitor to AGND.
SYNC/PWM Input:
For synchronized-PWM operation, drive with TTL level, 50% square wave.
9 SYNC/PWM
Connect to CVL for PWM mode.
Connect to AGND for normal mode.
10 ILIM/SS 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
11 STBY
(Table 1). This pin overrides SYNC/PWM setting.
Bootstrap Input. Connection for the bootstrap switch and internal feedback path. Connect BOOT to VOUT12 BOOT
for V < 5.5V. Connect BOOT to AGND for V > 5.5V.OUT OUT
13, 14 LX Inductor Connection. Drain for internal P-channel MOSFETs. Connect inductor from LX to OUT.
15 SHDN Active-Low Shutdown Input. Connect to ground for shutdown. SHDN can withstand the input voltage.
16 PGND Power Ground
_______________________________________________________________________________________ 7Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
synchronization to an external clock. The
MAX1684/MAX1685 can operate in five modes. Setting
0.1µF
the devices to operate in the appropriate mode for the
intended application (Table 1) achieves highest effi-
L ciency.1INPUT OUTPUT
10µH*14V MAX 3.3V AT 1ACVH 13, 143
IN LX PWM Control
COUT22µF 2 The MAX1684/MAX1685 use an oscillator-triggered min-MBRSAIN 100µF
130LT3 imum/maximum on-time current-mode control scheme
16 (Figure 2). The minimum on-time is typically 220ns0.1µF PGNDMAX1685
5 unless the regulator is in dropout. The maximum on-time
AGNDON/OFF 15
SHDN is 2 / f , allowing operation to 100% duty cycle.OSC
4 12CVL Current-mode feedback provides cycle-by-cycle currentBOOT
11 limiting for superior load- and line-transient response.1µF STBY
79 At each falling edge of the internal oscillator, the inter-SYNC/PWM FB
nal P-channel MOSFET (main switch) turns on. This
ILIM/SS CC REF allows current to ramp up through the inductor to the
10 8 6
load and stores energy in a magnetic field. The switch
0.1µF 0.1µF remains on until either the current-limit comparator0.01µF(OPTIONAL)
trips, the maximum on-time expires, or the PWM com-
parator signals that the output is in regulation. When
*SUMIDA the switch turns off during the second half of each
CD54-100;
cycle, the inductor’s magnetic field collapses, releasing
USE 22µH FOR MAX1684
the stored energy and forcing current through the out-
put diode to the output filter capacitor and load. The
Figure 1. Standard Application Circuit
output filter capacitor stores charge when the inductor
current is high and releases it when the inductor cur-
_______________Detailed Description rent is low, smoothing the voltage across the load.
The MAX1684/MAX1685 step-down, PWM DC-DC con- During normal operation, the MAX1684/MAX1685 regu-
verters provide an adjustable output from 1.25V to the late the output voltage by switching at a constant fre-
input voltage. They accept inputs from 2.7V to 14V and quency and modulating the power transferred to the
deliver up to 1.6A. An internal MOSFET and synchro- load on each cycle using the PWM comparator. A multi-
nous rectifier reduce PC board area while maintaining input comparator sums three weighted differential sig-
high efficiency. Operation with up to 100% duty cycle nals (the output voltage with respect to the reference,
minimizes dropout voltage. Fixed-frequency PWM oper- the main switch current sense, and the slope-compen-
ation reduces interference in sensitive communications sation ramp) and changes states when a threshold is
and data-acquisition applications. A SYNC input allows reached. It modulates output power by adjusting the
Table 1. Operating Modes
TYPICAL
MODE SYNC/PWM STBY SHDN FUNCTION OUTPUT
CAPABILITY (A)
PWM H H H Fixed-frequency PWM 1.6
Sync PWM Clocked H H Fixed-input clock-frequency PWM 1.6
PFM at light loads (<150mA); fixed-
Normal L H H 1.6
frequency PWM at heavy loads (>150mA)
Low Power X L H Low-power or standby mode 160m
Shutdown X X L Circuit disabled 0
8 _______________________________________________________________________________________
MAX1684/MAX1685MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
ILIM/SS
CVL
MAX1684IN 4µA MAX1685
CVHAIN
VH
ILIM THRESHOLDTHERMALSHDN ONVL SHUTDOWN
CVL
PFM ILIM
CURRENT COMPARATOR
REF
REF COMPARATOR
LX
CONTROL
UNDERVOLTAGE AND
COMPARATOR DRIVER
LOGIC2.5V
PWM
COMPARATOR
OSC
SYNC/PWM
SYNC
SLOPE COMPENSATION PGNDAND
STANDBY ZERO-CROSSINGPWM MODECONTROL COMPARATOR
NORMAL MODE
STBY
LOW-POWER MODE
CVL
2.5V
CC
PFM
COMPARATOR
BOOT
GM
INTEGRATOR
FB
0.125V
AGND
Figure 2. Functional Diagram
_______________________________________________________________________________________ 9Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
inductor peak current during the first half of each cycle, For higher V at no load, the frequency decreasesIN
based on the output error voltage. The MAX1684/ based on the following equation:
MAX1685s’ loop gain is relatively low to enable the use f = V / (V × 290ns)OUT IN
of a small, low-value output filter capacitor. The 1.4%
At medium- to full-load current (>100mA), V canINtransient load regulation from 0 to 1A is compensated
increase slightly higher before the frequency decreas-by an integrator circuit that lowers DC load regulation
es.to 0.01% typical. Slope compensation accounts for the
inductor-current waveform’s down slope during the Synchronous Rectification
second half of each cycle, and eliminates the inductor- Although the primary rectifier is an external Schottky
current staircasing characteristic of current-mode con- diode, a small internal N-channel synchronous rectifier
trollers at high duty cycles. allows PWM operation at light loads. During the second
half of each cycle, when the inductor current rampsPFM Control
below the zero-crossing threshold or when the oscilla-In low-power mode, the MAX1684/MAX1685 switch
tor period ends, the synchronous rectifier turns off. Thisonly as needed to service the load. This reduces the
keeps excess current from flowing backward throughswitching frequency and associated losses in the
the inductor. Choose an appropriate inductor to limitP-channel switch, the synchronous rectifier, and the
the PWM ripple current through the N-channel FET toexternal inductor. During this PFM operation, a switch-
400mA .P-Ping cycle initiates when the PFM comparator senses
that the output voltage has dropped too low. The Current Limit and Soft-Start
P-channel MOSFET switch turns on and conducts cur- The voltage at ILIM/SS sets the PWM current limit
rent to the output-filter capacitor and load. The (I = 1.75A) and the low-power current limit (I =LIM LIMLP
MAX1684/MAX1685 then wait until the PFM comparator 380mA). The PWM current limit applies when the
senses a low-output voltage again. device is in PWM mode, in synchronized PWM mode,
In normal mode at light load (<150mA), the device also or delivering a heavy load in normal mode (Table 1).
operates in PFM. The PFM current comparator controls The I limit applies when the device is in low-powerLIMLP
both entry into PWM mode and the peak switch current mode. An internal 4µA current source pulls ILIM/SS up
during PFM operation. Consequently, some jitter is nor- to CVL. To use the maximum current-limit thresholds,
mal during transition from PFM to PWM with loads leave ILIM/SS unconnected or connect it to a soft-start
around 150mA, and it has no adverse impact on regu- capacitor. Connect an external resistor from ILIM/SS to
lation. AGND to adjust the current-limit thresholds.
✕ ✕The PWM current-limit threshold is (I RLIM ILIM/SS100% Duty-Cycle Operation
4µA) / V and is adjustable from 0.5A to 1.75A. REFAs the input voltage drops, the duty cycle increases
until the P-channel MOSFET turns on continuously, The low-power current-limit threshold is equal to (ILIMLP
✕ ✕achieving 100% duty cycle. Dropout voltage in 100% R 4µA) / V and is adjustable from 110mAILIM/SS REF
duty cycle is the output current multiplied by the on- to 380mA.
resistance of the internal switch and inductor, approxi- For example, when R is 156kΩ, the PWM currentILIM/SS
mately 0.35V (I = 1A).OUT limit threshold is 0.88A and the low-power current limit
threshold is 0.19A.Very Low Duty-Cycle Operation
Connect a low-value capacitor from ILIM/SS to AGNDBecause of the P-channel minimum on-time and dead-
to achieve soft-start, limiting inrush current. ILIM/SStime (duration when both switches are off), the
MAX1684/MAX1685s’ switching frequency must internally shorts to AGND in shutdown to discharge the
soft-start capacitor. Do not connect ILIM/SS to REF ordecrease in PWM or normal mode to maintain regula-
CVL. Determine the soft-start duration by:tion at a very low duty cycle. The total P-channel on-
time and dead-time is 290ns typical. As a result, the t = C (1.25V / 4µA)SOFT-START ILIM/SS
MAX1684/MAX1685 maintain fixed-frequency regula-
where t is the time from SHDN going high totion at no load for V up to 10V and 5V ,IN OUT OUT
the regulator being able to supply full load current. Forrespectively (see PWM Fixed-Frequency Operation
example, a 0.1µF capacitor yields 31ms of soft-start.Area graph in the Typical Operating Characteristics).
10 ______________________________________________________________________________________
MAX1684/MAX1685