For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
General Description
The MAX1684/MAX1685 are high-efficiency, internalswitch, 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 Li-Ion 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
mode, normal mode, low-power mode, and shutdown
mode. 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 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 µMAX
package with lower input voltage requirements (5.5V
max), see the MAX1692 data sheet.
Applications
Cellular Phones
2-Way Radios and Walkie-Talkies
Computer Peripherals
Personal Communicators
PDAs and Handy-Terminals
Features
♦ Up to 96% Efficiency
♦ 1A Guaranteed Output Current
♦ 100% Duty Cycle in Dropout
♦ 2.7V to 14V Input Range (15V absolute max)
♦ ±1% Accurate Reference Output
♦ 0.24Ω P-Channel On-Resistance
♦ Synchronizable Switching Frequency
♦ Fixed-Frequency PWM Operation
300kHz (MAX1684)
600kHz (MAX1685)
♦ 150µA Normal Mode Quiescent Current
♦ 25µA Low-Power Mode Quiescent Current
♦ 2µA Shutdown Current
♦ Dual Mode™ Fixed 3.3V (±1%) Output or
Adjustable Output (1.25V to V
IN
)
♦ Small 16-QSOP Package
♦ Auxiliary Output (CVL): 3V/5mA
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
________________________________________________________________
Maxim Integrated Products
1
MAX1684
MAX1685
LXIN
AIN
GND
OUTPUT
3.3V AT 1A
INPUT
2.7V TO 14V
BOOT
SHDN
CVH
CVL
STBY
SYNC/PWM
FB CC REF
+
+
19-1454; Rev 0; 4/99
PART
MAX1684EEE
MAX1685EEE
-40°C to +85°C
-40°C to +85°C
TEMP. RANGE PIN-PACKAGE
16 QSOP
16 QSOP
EVALUATION KIT MANUAL
FOLLOWS DATA SHEET
Typical Operating Circuit
Ordering Information
Pin Configuration appears at end of data sheet.
Dual Mode is a trademark of Maxim Integrated Products.
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN= V
SHDN
= +6V, STBY = SYNC/PWM = CVL, V
BOOT
= V
OUT
, FB = AGND, circuit of Figure 1, TA= 0°C to +85°C, unless other-
wise noted. Typical values are at T
A
= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
AIN to AGND ............................................................-0.3 to +15V
IN to PGND................................................-0.3V to (V
AIN
+ 0.3V)
LX to PGND .................................................-0.5V to (V
IN
+ 0.3V)
PGND to AGND ..................................................................±0.3V
SHDN to AGND .........................................-0.3V to (V
AIN
+ 0.3V)
ILIM/SS, FB, CC, BOOT, REF to AGND.....-0.3V to (V
CVL
+ 0.3V)
CVH to IN..................................................................-6V to +0.3V
CVL, STBY, SYNC/PWM to AGND............................-0.3V to +6V
Reference Current..............................................................±1mA
CVL Current ..........................................................10mA to -1mA
LX Peak Current (Internally Limited).....................................2.3A
Continuous Power Dissipation (T
A
= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)............667mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10sec )............................+300°C
V
FB
= V
OUT
, I
LOAD
= 0 to 1A
Low-power mode, STBY = low,
V
BOOT
= 3.3V (Note 2)
Normal mode, SYNC/PWM = low,
V
BOOT
= 3.3V (Note 2)
PWM mode,
SYNC/PWM = high,
V
BOOT
= 3.3V
(Note 2)
SYNC/PWM = high
Low-side switch, VIN= 2.7V, ILX= 200mA
High-side switch,
ILX= 1A
VFB= V
OUT
, I
LOAD
= 0 to 1A
VIN= 5V to 14V
BOOT = AGND (Note 1)
SYNC/PWM = low
VFB= 1.4V
CONDITIONS
0.14 0.27
Quiescent Power Consumption
0.9 2
25 65
mW
13 33
mA
40 80 120
Zero Crossing Threshold
20 50 80
VV
FB
Feedback Voltage 1.238 1.251 1.264
V2.7 14Input Voltage Range
A0.15 0.4 0.9Current Limit, N-Channel
A1.2 1.75 2.3I
LIM
Current Limit in PWM Mode
Ω38On-Resistance, N-Channel
0.24 0.5
%0.01Output Load Regulation
A1Output Current Capability
VV
REF
V
IN
Output Adjust Range
nA-50 50I
FB
FB Input Current
UNITSMIN TYP MAXSYMBOLPARAMETER
FB = AGND, I
LOAD
= = 0 to 1A VV
OUT
Output Voltage (3.3V mode) 3.3 3.333 3.365
VIN= 6V
VIN= 2.7V
Ω
0.34 0.8
On-Resistance, P-Channel
MAX1684
MAX1685
MAX1684
MAX1685
SYNC/PWM = low mA285 380 475Pulse-Skipping Current Threshold
STBY = low
mA285 380 475I
LIMLP
Current Limit in Low-Power
Mode
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VIN= V
SHDN
= +6V, STBY = SYNC/PWM = CVL, V
BOOT
= V
OUT
, FB = AGND, circuit of Figure 1, TA= 0°C to +85°C, unless other-
wise noted. Typical values are at T
A
= +25°C.)
STBY = low, VIN= 2.7V
CONDITIONS
µA230 430
Quiescent Supply Current
in Dropout
UNITSMIN TYP MAXSYMBOLPARAMETER
SHDN = low
µA26Shutdown Supply Current
MAX1684 260 300 340
MAX1684 180 350
VIN= 14V, VLX= 0 or 14V, SHDN = low
SYNC Capture Range
MAX1685
kHz
520 600 680
f
OSC
Oscillator Frequency
µA20I
LX
LX Leakage Current
%100Maximum Duty Cycle
(Note 3) %
20
MAX1685
Constant-Frequency Minimum
Duty Cycle
10
-1µA < I
REF
< 50µA mV415Reference Load Regulation
VIN= 3V to 14V, BOOT = AGND,
I
CVL
= 0 to 5mA
V2.7 3.0 3.15
I
REF
= 0
CVL Regulator Output Voltage
2.7V < V
BOOT
< 5.5V mV0.2 5Reference Supply Regulation
V1.238 1.251 1.264V
REF
Reference Output Voltage
kHz
360 700
MAX1684
MAX1685
BOOT = AGND, I
CVL
= 5mA mV120CVL Dropout Voltage
Typical hysteresis is 10°C (Note 4) °C160Thermal Shutdown Threshold
BOOT falling edge,
typical hysteresis is 0.1V
V2.4 2.5 2.6BOOT Switchover Threshold
I
CVH
= -1mA V-5.0 -4.6 -4.1CVH with Respect to V
IN
SHDN, STBY, SYNC/PWM
V2V
IH
Logic Input High Voltage
V
ILIM/SS
= 1.4V µA3.3 4 4.65ILIM/SS Source Current
High or low period ns500SYNC/PWM Pulse Width
SHDN, STBY, SYNC/PWM
µA-1 1Logic Input Current
0.7V
IL
Logic Input Low Voltage
BOOT = AGND, CVL falling edge,
typical hysteresis is 40mV
V2.4 2.5 2.6
CVL Undervoltage Lockout
Threshold
V
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(VIN= V
SHDN
= +6V, STBY = SYNC/PWM = CVL, V
BOOT
= V
OUT
, FB = AGND, circuit of Figure 1, TA= -40°C to +85°C, unless other-
wise noted.) (Note 5)
Note 1: The output adjust range with BOOT connected to V
OUT
is V
REF
to 5.5V. Connect BOOT to AGND for V
OUT
> 5.5V.
Note 2: The quiescent power-consumption specifications include chip supply and gate-drive loss only. Divide these values by V
IN
(= 6V) to obtain quiescent currents. In normal and low-power modes, chip supply current dominates and quiescent power
is proportional to V
BOOT
(BOOT connected to OUT). In PWM mode, gate-drive loss dominates and quiescent power is pro-
portional to V
IN
· (V
IN
- V
CVH
). 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 will be dissipated in the CVL linear regulator.
Note 3: When the duty factor (V
OUT
/ VIN) is less than this value, the switching frequency decreases in PWM mode to maintain regu-
lation.
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.
CVL Undervoltage Lockout
Threshold
2.4 2.6 V
BOOT = AGND, CVL falling edge,
typical hysteresis is 40mV
Logic Input Low Voltage V
IL
0.7
Logic Input High Voltage V
IH
2
V
SHDN, STBY, SYNC/PWM
ILIM/SS Source Current 3.1 4.7 µAV
ILIM/SS
= 1.4V
0.27
Normal mode, SYNC/PWM = low,
V
BOOT
= 3.3V (Note 2)
BOOT Switchover Threshold 2.4 2.6 VBOOT falling edge, typical hysteresis is 0.1V
CVH with Respect to V
IN
-5.0 -4.1 VI
CVH
= -1mA
CVL Regulator Output Voltage 2.7 3.15 V
VIN= 3V to 14V, BOOT = AGND,
I
CVL
= 0 to 5mA
Reference Output Voltage 1.236 1.266 VI
REF
= 0
Oscillator Frequency f
OSC
480 700
kHz
MAX1685
240 350MAX1684
Shutdown Supply Current 6 µA
SHDN = low
Quiescent Power Consumption
2
mW
Current Limit in Low-Power
Mode
I
LIMLP
285 475 mA
STBY = low
Output Voltage (3.3V mode) V
OUT
3.295 3.375 V
PARAMETER SYMBOL MIN MAX UNITS
FB Input Current I
FB
-50 50 nA
Output Adjust Range V
REF
V
IN
V
Output Current Capability 1 A
Input Voltage Range 2.7 14 V
Output Feedback Voltage V
FB
1.236 1.266 V
Current Limit in PWM Mode I
LIM
1.2 2.3 A
CONDITIONS
VFB= 1.4V
BOOT = AGND (Note 1)
VIN= 6V to 14V
FB = AGND, I
LOAD
= = 0 to 1A
VFB= V
OUT
, I
LOAD
= 0 to 1A
Low-power mode, STBY = low,
V
BOOT
= 3.3V (Note 2)
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
_______________________________________________________________________________________
5
100
0
0.1 1 10 100 1,000 10,000
MAX1684
EFFICIENCY vs. LOAD CURRENT
(V
IN
= 3.3V, V
OUT
= 1.8V, 2.5V)
20
10
MAX1684/85-01
LOAD CURRENT (mA)
EFFICIENCY (%)
40
30
60
80
90
70
50
A: V
OUT
= 2.5V LP MODE
B: V
OUT
= 1.8V LP MODE
C: V
OUT
= 2.5V NORM MODE
D: V
OUT
= 1.8V NORM MODE
E: V
OUT
= 2.5V PWM MODE
F: V
OUT
= 1.8V PWM MODE
A
D
E
B
F
C
100
0
0.1 1 10 100 1,000 10,000
MAX1684
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 3.3V)
20
10
MAX1684/85-02
LOAD CURRENT (mA)
EFFICIENCY (%)
40
30
60
80
90
70
50
A: VIN = 4V LOW-POWER MODE
B: V
IN
= 12V LOW-POWER MODE
C: V
IN
= 4V NORMAL MODE
D: V
IN
= 12V NORMAL MODE
D
C
A
B
1 10010 1,000 10,000
MAX1684
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 3.3V, PWM MODE)
MAX1684/85-03
LOAD CURRENT (mA)
EFFICIENCY (%)
100
0
20
10
50
40
30
70
60
90
80
A: VIN = 4V
B: V
IN
= 5V
C: V
IN
= 9V
D: V
IN
= 12V
C
B
A
D
100
20
0.1 1 10 100 1,000 10,000
MAX1684
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 5V)
40
30
MAX1684/85-04
LOAD CURRENT (mA)
EFFICIENCY (%)
50
70
90
80
60
A: VIN = 6V LP MODE
B: V
IN
= 9V LP MODE
C: V
IN
= 12V LP MODE
D: V
IN
= 6V NORMAL MODE
E: V
IN
= 9V NORMAL MODE
F: V
IN
= 12V NORMAL MODE
E
B
F
E
D
A
C
100
0
0.1 1 10 100 1,000 10,000
MAX1685
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 3.3V)
20
10
MAX1684/85-07
LOAD CURRENT (mA)
EFFICIENCY (%)
40
30
60
80
90
70
50
A: VIN = 4V LP MODE
B: V
IN
= 12V LP MODE
C: V
IN
= 4V NORMAL MODE
D: V
IN
= 12V NORMAL MODE
D
C
B
A
1 10010 1,000 10,000
MAX1684
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 5V, PWM MODE)
MAX1684/85-05
LOAD CURRENT (mA)
EFFICIENCY (%)
100
0
20
10
50
40
30
70
60
90
80
A: VIN = 6V
B: V
IN
= 9V
C: V
IN
=12V
C
B
A
1 10010 1,000 10,000
MAX1685
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 3.3V, PWM MODE)
MAX1684/85-06
LOAD CURRENT (mA)
EFFICIENCY (%)
100
0
20
10
50
40
30
70
60
90
80
VIN = 5V
VIN =9V
VIN = 12V
VIN = 4V
1 10010 1,000 10,000
MAX1685
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 5V PWM MODE)
MAX1684/85-08
LOAD CURRENT (mA)
EFFICIENCY (%)
100
0
20
10
50
40
30
70
60
90
80
VIN = 6V
VIN = 9V
VIN =12V
100
0
0.1 1 10 100 1,000 10,000
MAX1685
EFFICIENCY vs. LOAD CURRENT
(V
OUT
= 5V)
20
10
MAX1684/85-09
LOAD CURRENT (mA)
EFFICIENCY (%)
40
30
60
80
90
70
50
A: VIN = 6V LOW-POWER MODE
B: V
IN
= 9V LOW-POWER MODE
C: V
IN
= 12V LOW-POWER MODE
D: V
IN
= 6V NORMAL MODE
E: V
IN
= 9V NORMAL MODE
F: V
IN
=12V NORMAL MODE
A
F
E
D
C
B
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
1.0
1.2
1.1
1.4
1.3
1.7
1.6
1.5
1.8
042 6 8101214
MAXIMUM LOAD CURRENT
vs. INPUT VOLTAGE
MAX1684/85-10
INPUT VOLTAGE (V)
LOAD CURRENT (A)
PWM OR NORMAL MODE
0
200
600
400
1,000
1,200
800
1,400
062 8 10 12 14 16
SAFE OPERATING AREA
MAX1684/85-11
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
V
OUT
= 3.3V
4
0
100
300
200
400
0 200 400 600 800 1,000
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1684/85-12
LOAD CURRENT (mA)
DROPOUT VOLTAGE (mV)
V
OUT
= 3.3V
V
OUT
= 5V
INDUCTOR RESISTANCE INCLUDED
0
100
50
200
150
250
300
4 6785 9 10 11 12 13 14 15
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (V
OUT
= 3.3V)
MAX1684/85-13
INPUT VOLTAGE (V)
SUPPLY CURRENT (µA)
NORMAL MODE
LOW-POWER MODE
SCHOTTKY LEAKAGE
CURRENT INCLUDED
I
LOAD
500mA/div
V
OUT
50mV/div
2ms/div
LOAD-TRANSIENT RESPONSE
MAX1684/85-16
MAX1684, I
LOAD
= 0.1mA TO 1A, V
OUT
= 3.3V, VIN = 5V,
SYNC/PWM = 3.3V
2.0
2.5
3.0
3.5
4.0
4.5
5.0
465 7 8 9 10 11 12
MAX1684
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
(V
OUT
= 3.3V, PWM MODE)
MAX1684/85-14
INPUT VOLTAGE (V)
SUPPLY CURRENT (mA)
2.7
8
7
6
10
9
14
13
12
11
15
02468101214
PWM FIXED-FREQUENCY
OPERATION AREA
M AX1684/85-15
OUTPUT VOLTAGE (V)
INPUT VOLTAGE (V)
MAX1685
MAX1684
V
OUT
20mV/div
I
LX
100mA/div
1µs/div
SWITCHING WAVEFORM
MAX1684/85-17
MAX1684, I
LOAD
= 100mA, V
OUT
= 3.3V, VIN = 5V,
SYNC/PWM = 3.3V
V
LX
5V/div
I
LX
100mA/div
1µs/div
SWITCHING WAVEFORM
MAX1684/85-18
MAX1684, I
LOAD
= 100mA, V
OUT
= 3.3V, VIN = 5V,
SYNC/PWM = 3.3V
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
_______________________________________________________________________________________ 7
Pin Description
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25°C, unless otherwise noted.)
V
IN
5V/div
0V
V
OUT
100mV/div
2ms/div
LINE-TRANSIENT RESPONSE
MAX1684/85-19
MAX1684, I
LOAD
= 100mA, VIN = 5V TO 10V,
SYNC/PWM = 3.3V
V
SHDN
5V/div
I
IN
50mA/div
10ms/div
START-UP CURRENT
MAX1684/85-20
MAX1684, I
LOAD
= 100mA, V
OUT
= 3.3V,
V
IN
= 5V, C
ILIM/SS
= 0.1µF, SYNC/PWM = 3.3V
PIN
High-Side MOSFET Gate Bias. Bias voltage for P-channel switch. Bypass to IN with a 0.1µF capacitor.CVH1
FUNCTIONNAME
Analog Supply Voltage Input. Connect to IN with a 0.2 in. metal trace. Bypass to PGND with a 0.1µF
capacitor.
AIN2
Logic Supply Voltage Output and IC Logic Supply. Sources 5mA for external loads. Bypass to AGND
with 1µF capacitor.
CVL4
Supply Voltage InputIN3
Reference Output. 1.25V reference output supplies 10µA for external loads. Bypass to AGND with 0.1µF
capacitor.
REF6
Integrator Capacitor Connection. Connect a 0.01µF capacitor to AGND.CC8
Dual Mode Feedback Input. Connect FB to V
OUT
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.
FB7
Analog GroundAGND5
Current-Limit Adjust/Soft-Start Input. See
Current Limit and Soft-Start
section.ILIM/SS10
Bootstrap Input. Connection for the bootstrap switch and internal feedback path. Connect BOOT to V
OUT
for V
OUT
< 5.5V. Connect BOOT to AGND for V
OUT
> 5.5V.
BOOT12
Standby Control Input. Connect to CVL for normal operation. Connect to AGND for low-power mode
(Table 1). This pin overrides SYNC/PWM setting.
STBY
11
Active-Low Shutdown Input. Connect to ground for shutdown. SHDN can withstand the input voltage.SHDN
15
Power GroundPGND16
Inductor Connection. Drain for internal P-channel MOSFETs. Connect inductor from LX to OUT.LX13, 14
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.
SYNC/PWM9
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
8 _______________________________________________________________________________________
_______________Detailed Description
The MAX1684/MAX1685 step-down, pulse-width-modulation (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 appli-
cations. A SYNC input allows 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
LX
13, 14
L
10µH*
MBRS
130LT3
16
5
12
7
IN
CVH
AIN
PGND
AGND
OUTPUT
3.3V AT 1A
*SUMIDA
CD54-100;
USE 22µH FOR MAX1684
INPUT
14V MAX
BOOT
FB
SHDN
CVL
4
15
2
3
1
11
1µF
0.1µF
0.1µF
22µF
C
OUT
100µF
0.1µF 0.1µF
0.01µF
ON/OFF
9
STBY
SYNC/PWM
ILIM/SS CC
6810
REF
(OPTIONAL)
Figure 1. Standard Application Circuit
Table 1. Operating Modes
Fixed-Frequency PWMHPWM
MODE FUNCTIONSYNC/PWM
H
STBY
H
SHDN
1.6A
TYPICAL
OUTPUT
CAPABILITY
H H 1.6AFixed-Input Clock-Frequency PWMClockedSync PWM
H H 1.6A
PFM at light loads (<150mA). Fixedfrequency PWM at heavy loads (>150mA).
LNormal
L H 160mALow-Power or Standby Mode.XLow Power
X L 0Circuit DisabledXShutdown
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
_______________________________________________________________________________________ 9
MAX1684
MAX1685
SYNC
&
STANDBY
CONTROL
REF
2.5V
4µA
2.5V
SLOPE COMPENSATION
OSC
GM
INTEGRATOR
PFM
COMPARATOR
PWM MODE
NORMAL MODE
LOW-POWER MODE
VL
THERMAL
SHUTDOWN
UNDERVOLTAGE
COMPARATOR
PWM
COMPARATOR
PFM
CURRENT
COMPARATOR
ILIM
COMPARATOR
ZERO-CROSSING
COMPARATOR
CVL
ON
CVL
ILIM/SS
IN
AIN
SHDN
CVL
REF
SYNC/PWM
STBY
CC
ILIM THRESHOLD
CONTROL
& DRIVER
LOGIC
VH
CVH
LX
PGND
BOOT
FB
AGND
0.125V
Figure 2. Functional Diagram
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
10 ______________________________________________________________________________________
inductor peak current during the first half of each cycle,
based on the output error voltage. The MAX1684/
MAX1685’s loop gain is relatively low to enable the use
of a small, low-valued 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.
PFM Control
In low-power mode, the MAX1684/MAX1685 devices
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.
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 (I
OUT
= 1A).
Very Low Duty-Cycle Operation
Because of the P-channel minimum on-time and deadtime (duration when both switches are off), the
MAX1684/MAX1685’s switching frequency must
decrease in PWM or normal mode to maintain regulation
at a very low duty cycle. The total P-channel on-time and
dead-time is 290ns typical. As a result, the MAX1684/
MAX1685 will maintain fixed-frequency regulation at no
load for VINup to 10V
OUT
and 5V
OUT
, respectively (see
PWM Fixed-Frequency Operation Area graph in the
Typical Operating Characteristics
). For higher VINat no
load, the frequency decreases based on the following
equation:
f = V
OUT
/ (VIN· 290ns)
At medium- to full-load current (>100mA), VINcan
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.
Current Limit and Soft-Start
The voltage at ILIM/SS sets the PWM current limit
(I
LIM
= 1.75A) and the low-power current limit (I
LIMLP
=
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 I
LIMLP
limit applies when the device is in low-power
mode. An internal 4µA 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 (I
LIM
· R
ILIM/SS
·
4µA) / V
REF
and is adjustable from 0.5A to 1.75A.
The low-power current-limit threshold is equal to (I
LIMLP
·
R
ILIM/SS
· 4µA) / V
REF
and is adjustable from 110mA to
380mA.
For example, when R
ILIM/SS
is 156kΩ, 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:
t
SOFT-START
= C
ILIM/SS
(1.25V / 4µA)
where t
SOFT-START
is the time from SHDN going high to
the regulator being able to supply full load current. For
example, a 0.1µF capacitor yields 31ms of soft-start.
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
______________________________________________________________________________________ 11
The output current capability for each mode is determined by the following equations:
I
OUTMAX
= I
LIM
- 0.5 · I
RIPPLE
(for PWM and normal
modes)
I
OUTMAX
= 0.5 · I
LIMLP
(for low-power mode)
where:
I
RIPPLE
= ripple current =
(VIN- V
OUT
) · V
OUT
/ (VIN· f
OSC
· L)
I
LIM
= current limit in PWM mode
I
LIMLP
= current limit in low-power mode.
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 (
Functional Diagram
). The VH regulator
generates -4.6V with respect to IN to supply the Pchannel switch and driver. Bypass CVH to IN with a
0.1µF 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 1µF capacitor.
To reduce the quiescent current in low-power and normal modes, connect BOOT to OUT. After start-up,
when V
BOOT
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 V
OUT
/ 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.
Apply an external supply voltage, such as +3.3V or +5V,
to BOOT to achieve a bootstrap effect when the output
voltage is too low to bootstrap the device (below 2.7V).
CVL has a 5mA capability to supply external logic circuitry and is disabled in shutdown mode.
________________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 20kΩ to
100kΩ range. Calculate R1 as follows:
R1 = R2 [( V
OUT
/ VFB) - 1]
where VFB= 1.25V.
Connect a small capacitor across R1 to compensate for
stray capacitance at the FB pin.
For R2 = 100kΩ, use 4.7pF.
Inductor Selection
The MAX1684/MAX1685’s 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(V
OUT
- 0.3V) / (I
RIPPLE MAX
· f
OSC
)
where:
I
RIPPLE MAX
= should be less than or equal to 400mA
f
OSC
= 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/MAX1685’s loop gain
is relatively low, to enable the use of small, low-value
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 space-constrained
surface-mount designs. Do not exceed the ripple-current ratings of tantalum capacitors. Ceramic capacitors
)
MAX1684
MAX1685
FB
V
OUT
R1 C1
R2
Figure 3. Setting Output Voltage
−
7
5 (10
C1
=
R
2
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
MAX1684
MAX1685
LXAIN
IN
CVH
FB
L
C1
MBRS
130LT3
R1
R2
BOOT
PGND
AGND
CC
SHDN
STBY
0.1µF
22µF
0.1µF
100µF
-V
OUT
-1.25V
TO -5.5V
-V
OUT
= -1.25V
( )
V
IN
1µF
0.1µF
CVL
SYNC/PWM
REF
ILIM/SS
0.01µF
0.01µF
R2
+ 1
R1
VIN, MAX = 14V - |V
OUT|
Figure 4. Inverting Output
Table 2. Inductor and Minimum Output Capacitor Selection
Table 3. Component Suppliers
22 220
MAX1684 (300kHz)
1.25 to 2.7
SUPPLIER PHONE
10 100
MAX1685 (600kHz)
L
(µH)
MIN C
OUT
(µF)
L
(µH)
MIN C
OUT
(µF)
V
OUT
(V)
22 100 10 472.7 to 4
47 68 22 334 to 6
68 47 33 226 to 14
CAPACITORS
FAX
803-626-3123803-946-0690AVX
714-960-6492714-969-2591Matsuo
619-661-1055619-661-6835Sanyo
INDUCTORS
847-956-0702847-956-0666Sumida
847-639-1469847-639-6400Coilcraft
814-238-0490814-237-1431Murata-Erie
847-390-4428847-390-4373TDK
DIODES
602-994-6430602-303-5454Motorola
603-224-1430603-224-1961Sprague
847-956-0702847-956-0666Sumida
offer the lowest ESR overall. Sanyo OS-CON 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 lowESR 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:
C
OUT > IOUT MAX
/ (V
OUT
· AC Load Reg · f
OSC
)
R
ESR
< 2 · AC Load Reg · V
OUT/IOUT MAX
where I
OUT MAX
= 1A, AC Load Reg ≅ 1.4%, and
f
OSC
= 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.
12 ______________________________________________________________________________________
Inverting Output
Interchanging the ground and V
OUT
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 - V
OUT
.
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
1Ω resistor and bypass AIN to PGND with a 0.1µF capacitor. This acts as a lowpass filter to reduce noise at AIN.
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
______________________________________________________________________________________ 13
Pin Configuration
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
CVH PGND
SHDN
LX
LX
BOOT
STBY
ILIM/SS
SYNC/PWM
TOP VIEW
MAX1684
MAX1685
QSOP
AIN
IN
REF
CVL
AGND
FB
CC
Chip Information
TRANSISTOR COUNT: 2061
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
14 ______________________________________________________________________________________
Package Information
QSOP.EPS
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
______________________________________________________________________________________ 15
NOTES
MAX1684/MAX1685
Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
NOTES
16 ______________________________________________________________________________________
Loading...