19-1898; Rev 0; 2/01
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
MAX1856
General Description
The MAX1856 offers a low-cost solution for generating a
SLIC (ringer and off-hook) power supply. Using standard
off-the-shelf transformers from multiple vendors, the
MAX1856 generates various output voltages: -24V and
-72V (dual output) for both ringer and off-hook supplies for
voice-enabled broadband consumer premises equipment
(CPE), -48V for IP phones and routers, -5V and -15V (sin-
gle or dual output) for DSL CO line drivers, or negative
voltages as high as -185V for MEMS bias supplies. The
output voltages are adjusted with an external voltage
divider.
Due to its wide operating voltage range, the MAX1856
operates from a low-cost, unregulated DC power supply
for cost-sensitive applications like xDSL, cable modems,
set-top boxes, LMDS, MMDS, WLL, and FTTH CPE. The
MAX1856 provides low audio-band noise for talk battery
and a sturdy output capable of handling the ring trip con-
ditions for ring battery.
The operating frequency can be set between 100kHz and
500kHz with an external resistor in free-running mode. For
noise-sensitive applications, the MAX1856’s operating fre-
quency can be synchronized to an external clock over its
operating frequency range.
The flyback topology allows operation close to 50% duty
cycle, offering high transformer utilization, low ripple cur-
rent, and less stress on input and output capacitors.
Internal soft-start minimizes startup stress on the input
capacitor, without any external components.
The MAX1856’s current-mode control scheme does not
require external loop compensation. The low-side current-
sense resistor provides accurate current-mode control
and overcurrent protection.
o
Low-Cost, Off-the-Shelf Transformer
o
3V to 28V Input Range
o
Low Audio-Band Noise on Talk Battery
o
Effectively Handles Ring Trip Transients
o
Powers 2-, 4-, or 12-Line Equipment
o
High Efficiency Extends Battery Life During
Life-Line Support Conditions
o
Adjustable 100kHz to 500kHz Switching
Frequency
o
Clock Synchronization
o
Internal Soft-Start
o
Current-Mode PWM and Idle Mode™ Control
Scheme
o
Logic-Level Shutdown
o
10-Pin µMAX Package
Features
Ordering Information
PART
MAX1856EUB
TEMP. RANGE
-40°C to +85°C
PIN-PACKAGE
10 µMAX
Typical Operating Circuit
INPUT
3V TO 28V
OUT2
-72V
Applications
VoIP Ringer and Off-Hook Voltage Generators
Cable and DSL Modems
Set-Top Boxes
Wireless Local Loop
FTTH
LMDS/MMDS
Routers
Industrial Power Supplies
CO DSL Line Driver Supplies
MEMS Bias Supplies
V
CC
SYNC/SHDN
LDO
EXT
FREQ
CS
1
2
2
OUT1
-24V
MAX1856
GND
FB
PGND
REF
2
Idle Mode is a trademark of Maxim Integrated Products.
________________________________________________________________
Maxim Integrated Products
1
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
MAX1856
ABSOLUTE MAXIMUM RATINGS
V
CC
, SYNC/SHDN to GND .....................................-0.3V to +30V
PGND to GND .......................................................-0.3V to +0.3V
LDO, FREQ, FB, CS to GND.....................................-0.3V to +6V
EXT, REF to GND......................................-0.3V to (V
LDO
+ 0.3V)
LDO Output Current............................................-1mA to +20mA
LDO Short Circuit to GND ...............................................<100ms
REF Short Circuit to GND ...........................................Continuous
Continuous Power Dissipation (T
A
= +70°C)
10-Pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°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.
ELECTRICAL CHARACTERISTICS
(V
CC
= SYNC/SHDN, V
CC
= 5V, V
LDO
= 5V, R
OSC
= 200kΩ,
T
A
= 0°C to +85°C.
Typical values are at T
A
= +25°C, unless otherwise noted.)
PARAMETER
PWM CONTROLLER
Operating Input Voltage Range
FB Input Current
Load Regulation
Line Regulation
Current-Limit Threshold
CS Input Current
Idle Mode Current-Sense
Threshold
V
CC
Supply Current (Note 1)
Shutdown Supply Current
REFERENCE AND LDO REGULATOR
LDO Output Voltage
Undervoltage Lockout
Threshold
REF to FB Voltage (Note 2)
REF Load Regulation
REF Undervoltage Lockout
Threshold
OSCILLATOR
R
OSC
= 100kΩ
±1%
Oscillator Frequency
f
OSC
R
OSC
= 200kΩ
±1%
R
OSC
= 500kΩ
±1%
R
OSC
= 100kΩ
±1%
Maximum Duty Cycle
D
R
OSC
= 200kΩ
±1%
R
OSC
= 500kΩ
±1%
425
225
85
86
87
86
500
250
100
90
90
90
575
275
115
94
93
94
%
kHz
V
LDO
V
UVLO
V
REF
R
LDO
= 400Ω
5V
≤
V
CC
≤
28V
3V
≤
V
CC
≤
28V
4.50
2.65
2.40
1.225
2.50
1.250
-2
1.0
1.1
5.00
5.50
5.50
2.60
1.275
-10
1.2
V
V
V
mV
V
I
CC
V
FB
= -0.05V, V
CC
= 3V to 28V
SYNC/SHDN = GND, V
CC
= 28V
V
CS
I
CS
CS = GND
5
15
250
3.5
V
CC
I
FB
3
V
CC
= V
LDO
V
FB
= -0.05V
V
CS
= 0 to 100mV for 0 to I
LOAD(MAX)
Typically 0.0074% per % duty factor on EXT
85
2.7
1
0.013
0.0074
100
115
1
25
400
6
28
5.5
50
V
V
nA
%/mV
%/%
mV
µA
mV
µA
µA
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
V
LDO
falling edge, 1% hysteresis (typ)
R
REF
= 10kΩ, C
REF
= 0.22µF
I
REF
= 0 to 400µA
Rising edge, 1% hysteresis (typ)
2
_______________________________________________________________________________________
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
ELECTRICAL CHARACTERISTICS (continued)
(V
CC
= SYNC/SHDN, V
CC
= 5V, V
LDO
= 5V, R
OSC
= 200kΩ,
T
A
= 0°C to +85°C.
Typical values are at T
A
= +25°C, unless otherwise noted.)
PARAMETER
Minimum EXT Pulse Width
Minimum SYNC Input Signal
Duty Cycle
Minimum SYNC Input
Low Pulse Width
Maximum SYNC Input
Rise/Fall Time
SYNC Input Frequency Range
SYNC/SHDN Falling Edge to
Shutdown Delay
SYNC/SHDN Input High Voltage
SYNC/SHDN Input Low Voltage
SYNC/SHDN Input Current
EXT Sink/Source Current
EXT On-Resistance
I
EXT
R
ON(EXT)
f
SYNC
t
SHDN
V
IH
V
IL
V
SYNC/SHDN
= 5V
V
SYNC/SHDN
= 28V
EXT forced to 2V
EXT high or low
0.5
1.5
1
2
5
2.0
0.45
3.0
10
100
50
SYMBOL
CONDITIONS
MIN
TYP
290
20
50
200
500
45
200
MAX
UNITS
ns
%
ns
ns
kHz
µs
V
V
µA
A
Ω
MAX1856
ELECTRICAL CHARACTERISTICS
(V
CC
= SYNC/SHDN, V
CC
= 5V, V
LDO
= 5V, R
OSC
= 200kΩ,
T
A
= -40°C to +85°C,
unless otherwise noted.) (Note 3)
PARAMETER
PWM CONTROLLER
Operating Input Voltage Range
FB Input Current
Current-Limit Threshold
CS Input Current
V
CC
Supply Current (Note 1)
Shutdown Supply Current
REFERENCE AND LDO REGULATOR
LDO Output Voltage
REF to FB Voltage (Note 2)
REF Load Regulation
REF Undervoltage Lockout
Threshold
OSCILLATOR
R
OSC
= 100kΩ
±1%
Oscillator Frequency
f
OSC
R
OSC
= 200kΩ
±1%
R
OSC
= 500kΩ
±1%
425
222
85
575
278
115
kHz
V
LDO
V
REF
R
LDO
= 400Ω
5V
≤
V
CC
≤
28V
3V
≤
V
CC
≤
28V
4.50
2.65
1.22
5.50
5.50
1.28
-10
1.0
1.2
V
V
mV
V
V
CC
I
FB
V
CS
I
CS
I
CC
CS = GND
V
FB
= -0.05V, V
CC
= 3V to 28V
SYNC/SHDN = GND, V
CC
= 28V
3
V
CC
= V
LDO
V
FB
= -0.05V
85
2.7
28
5.5
50
115
1
400
6
V
V
nA
mV
µA
µA
µA
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
R
REF
= 10kΩ, C
REF
= 0.22µF
I
REF
= 0 to 400µA
Rising edge, 1% hysteresis (typ)
_______________________________________________________________________________________
3
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
MAX1856
ELECTRICAL CHARACTERISTICS (continued)
(V
CC
= SYNC/SHDN, V
CC
= 5V, V
LDO
= 5V, R
OSC
= 200kΩ,
T
A
= -40°C to +85°C,
unless otherwise noted.) (Note 3)
PARAMETER
Maximum Duty Cycle
Minimum SYNC Input Signal
Duty Cycle
Minimum SYNC Input
Low Pulse Width
SYNC Input Frequency Range
SYNC/SHDN Input High Voltage
SYNC/SHDN Input Low Voltage
SYNC/SHDN Input Current
EXT On-Resistance
R
ON(EXT)
f
SYNC
V
IH
V
IL
V
SYNC/SHDN
= 5V
V
SYNC/SHDN
= 28V
EXT high or low
100
2.0
0.45
3.0
10
5
SYMBOL
D
CONDITIONS
R
OSC
= 100kΩ
±1%
R
OSC
= 200kΩ
±1%
R
OSC
= 500kΩ
±1%
MIN
86
87
86
TYP
MAX
94
93
94
45
200
500
%
ns
kHz
V
V
µA
Ω
%
UNITS
Note 1:
This is the V
CC
current consumed when active, but not switching, so the gate-drive current is not included.
Note 2:
The reference output voltage (VREF) is measured with respect to FB. The difference between REF and FB is guaranteed to
be within these limits to ensure output voltage accuracy.
Note 3:
Specifications to -40°C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, V
CC
= V
SYNC/SHDN
= 12V, V
OUT1
= -24V, V
OUT2
= -72V, R
OSC
= 200kΩ, unless otherwise noted.)
-24V OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1856 toc01
-72V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT
MAX1856 toc02
EFFICIENCY vs. LOAD CURRENT
(-24V OUTPUT)
MAX1856 toc03
-22.0
-72.0
100
V
IN
= 12V
-22.5
V
IN
= 5V
V
OUT1
(V)
-72.5
90
EFFICIENCY (%)
V
IN
= 5V
V
OUT2
(V)
-23.0
-73.0
V
IN
= 12V
-73.5
V
IN
= 5V
V
IN
= 24V
80
V
IN
= 24V
-23.5
V
IN
= 12V
V
IN
= 24V
70
-24.0
V
OUT1
= -24V
V
OUT2
= -72V
I
OUT2
= 5mA
0
100
200
300
400
500
600
700
-74.0
V
OUT1
= -24V
V
OUT2
= -72V
I
OUT2
= 5mA
0
100
200
300
400
500
600
700
60
V
OUT1
= -24V
V
OUT2
= -72V
I
OUT2
= 5mA
0
100
200
300
400
500
600
700
-24.5
I
OUT1
(mA)
-74.5
I
OUT1
(mA)
50
I
OUT1
(mA)
4
_______________________________________________________________________________________
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
Typical Operating Characteristics (continued)
(Circuit of Figure 1, V
CC
= V
SYNC/SHDN
= 12V, V
OUT1
= -24V, V
OUT2
= -72V, R
OSC
= 200kΩ, unless otherwise noted.)
-72V OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1856 toc04
MAX1856
-24V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT
MAX1856 toc05
EFFICIENCY vs. LOAD CURRENT
(-72V OUTPUT)
MAX1856 toc06
-70
-23.0
100
V
IN
= 12V
V
IN
= 5V
-71
V
OUT2
(V)
V
IN
= 5V
V
IN
= 12V
-23.2
V
IN
= 5V
V
IN
= 12V
90
EFFICIENCY (%)
-72
V
OUT1
(V)
-23.4
80
V
IN
= 24V
70
V
IN
= 24V
-23.6
-73
V
OUT1
= -24V
V
OUT2
= -72V
I
OUT1
= 5mA
-74
0
50
100
150
200
250
I
OUT2
(mA)
-23.8
V
IN
= 24V
V
OUT1
= -24V
V
OUT2
= -72V
I
OUT1
= 5mA
150
200
250
60
V
OUT1
= -24V
V
OUT2
= -72V
I
OUT1
= 5mA
0
50
100
150
200
250
-24.0
0
50
100
I
OUT2
(mA)
50
I
OUT2
(mA)
-24V OUTPUT VOLTAGE
vs. INPUT VOLTAGE
MAX1856 toc07
-72V OUTPUT VOLTAGE
vs. INPUT VOLTAGE
MAX1856 toc08
DUAL-OUTPUT EFFICIENCY
vs. INPUT VOLTAGE
V
OUT1
= -24V
I
OUT1
= 100mA
V
OUT2
= -72V
I
OUT2
= 100mA
MAX1856 toc09
-23.0
V
OUT1
= -24V
I
OUT1
= 100mA
V
OUT2
= -72V
I
OUT2
= 100mA
-71.0
V
OUT1
= -24V
I
OUT1
= 100mA
V
OUT2
= -72V
I
OUT2
= 100mA
100
-23.2
-71.4
90
EFFICIENCY (%)
V
OUT1
(V)
V
OUT1
(V)
-23.4
-71.8
80
V
OUT1
= -24V
I
OUT1
= 50mA
V
OUT2
= -72V
I
OUT2
= 50mA
-23.6
V
OUT1
= -24V
I
OUT1
= 50mA
V
OUT2
= -72V
I
OUT2
= 50mA
-72.2
V
OUT1
= -24V
I
OUT1
= 50mA
V
OUT2
= -72V
I
OUT2
= 50mA
0
5
10
V
IN
(V)
15
20
25
70
-23.8
-72.6
60
-24.0
0
5
10
V
IN
(V)
15
20
25
-73.0
50
0
5
10
V
IN
(V)
15
20
25
-48V OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1856 toc10
EFFICIENCY vs. LOAD CURRENT
(-48V OUTPUT)
MAX1856 toc11
SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1856 toc12
-46.5
V
IN
= 12V
-46.9
V
IN
= 5V
100
250
90
V
IN
= 5V
EFFICIENCY (%)
SUPPLY CURRENT (µA)
V
IN
= 12V
200
V
OUT2
(V)
-47.3
V
IN
= 24V
-47.7
80
150
70
V
IN
= 24V
100
-48.1
V
OUT
= -48V
FIGURE 4
-48.5
0
100
200
I
OUT2
(mA)
300
400
60
V
OUT
= -48V
FIGURE 4
50
0
100
200
I
OUT2
(mA)
300
400
50
CURRENT INTO V
CC
PIN
R
OSC
= 500kΩ
0
0
5
10
15
20
25
30
INPUT VOLTAGE (V)
_______________________________________________________________________________________
5
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