MAXIM MAX1856 User Manual

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 (sin­gle 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 con­ditions 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 fre­quency 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 cur­rent, 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 current­sense resistor provides accurate current-mode control
and overcurrent protection.

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

Features

♦ Low-Cost, Off-the-Shelf Transformer

♦ 3V to 28V Input Range

♦ Low Audio-Band Noise on Talk Battery

♦ Effectively Handles Ring Trip Transients

♦ Powers 2-, 4-, or 12-Line Equipment

♦ High Efficiency Extends Battery Life During

Life-Line Support Conditions

♦ Adjustable 100kHz to 500kHz Switching

Frequency

♦ Clock Synchronization

♦ Internal Soft-Start

♦ Current-Mode PWM and Idle Mode™ Control

Scheme

♦ Logic-Level Shutdown

♦ 10-Pin µMAX Package

MAX1856

Wide Input Range, Synchronizable,

PWM SLIC Power Supply

________________________________________________________________ Maxim Integrated Products 1

Typical Operating Circuit

Ordering Information

19-1898; Rev 0; 2/01

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.

Idle Mode is a trademark of Maxim Integrated Products.

PART TEMP. RANGE PIN-PACKAGE

MAX1856EUB -40°C to +85°C 10 µMAX

INPUT
3V TO 28V

V

CC

SYNC/SHDN

LDO

FREQ

GND

PGND

OUT2

-72V

1

EXT

CS

MAX1856

FB

REF

2

2

2

OUT1

-24V

MAX1856

Wide Input Range, Synchronizable,
PWM SLIC Power Supply

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VCC= SYNC/SHDN, V

CC

= 5V, V

LDO

= 5V, R

OSC

= 200kΩ, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)

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.

VCC, 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

PWM CONTROLLER

Operating Input Voltage Range V

FB Input Current I

Load Regulation VCS = 0 to 100mV for 0 to I

Line Regulation Typically 0.0074% per % duty factor on EXT 0.0074 %/%

Current-Limit Threshold V

CS Input Current I

Idle Mode Current-Sense
Threshold

VCC Supply Current (Note 1) I
Shutdown Supply Current SYNC/SHDN = GND, VCC = 28V 3.5 6 µA

REFERENCE AND LDO REGULATOR

LDO Output Voltage V

Undervoltage Lockout
Threshold

REF to FB Voltage (Note 2) V

REF Load Regulation I

REF Undervoltage Lockout
Threshold

OSCILLATOR

Oscillator Frequency f

Maximum Duty Cycle D

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

CC

FB

CS

CS

CC

LDO

V

UVLO

REF

OSC

328V

VCC = V

VFB = -0.05V 1 50 nA

CS = GND 1 µA

VFB = -0.05V, VCC = 3V to 28V 250 400 µA

R

V

R

Rising edge, 1% hysteresis (typ) 1.0 1.1 1.2 V

R

R

R

R

R

R

LDO

LOAD(MAX)

= 400Ω

LDO

falling edge, 1% hysteresis (typ) 2.40 2.50 2.60 V

LDO

= 10kΩ, C

REF

= 0 to 400µA -2 -10 mV

REF

= 100kΩ ±1% 425 500 575

OSC

= 200kΩ ±1% 225 250 275

OSC

= 500kΩ ±1% 85 100 115

OSC

= 100kΩ ±1% 86 90 94

OSC

= 200kΩ ±1% 87 90 93

OSC

= 500kΩ ±1% 86 90 94

OSC

5V ≤ VCC ≤ 28V 4.50 5.00 5.50
3V ≤ V

= 0.22µF 1.225 1.250 1.275 V

REF

≤ 28V 2.65 5.50

CC

2.7 5.5 V

0.013 %/mV

85 100 115 mV

51525mV

V

kHz

%

MAX1856

Wide Input Range, Synchronizable,

PWM SLIC Power Supply

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VCC= SYNC/SHDN, V

CC

= 5V, V

LDO

= 5V, R

OSC

= 200kΩ, TA= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)

ELECTRICAL CHARACTERISTICS

(VCC= SYNC/SHDN, V

CC

= 5V, V

LDO

= 5V, R

OSC

= 200kΩ, TA= -40°C to +85°C, unless otherwise noted.) (Note 3)

Minimum EXT Pulse Width 290 ns

Minimum SYNC Input Signal
Duty Cycle

Minimum SYNC Input
Low Pulse Width

Maximum SYNC Input
Rise/Fall Time

SYNC Input Frequency Range f

SYNC/SHDN Falling Edge to
Shutdown Delay

SYNC/SHDN Input High Voltage V
SYNC/SHDN Input Low Voltage V

SYNC/SHDN Input Current

EXT Sink/Source Current I

EXT On-Resistance R

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

SYNC

t

SHDN

IH

IL

EXT

ON(EXT)

20 45 %

50 200 ns

200 ns

100 500 kHz

50 µs

2.0 V

0.45 V

V

SYNC/SHDN

V

SYNC/SHDN

EXT forced to 2V 1 A
EXT high or low 2 5 Ω

= 5V 0.5 3.0

= 28V 1.5 10

PWM CONTROLLER

Operating Input Voltage Range V

FB Input Current I

Current-Limit Threshold V

CS Input Current I

V
Shutdown Supply Current SYNC/SHDN = GND, VCC = 28V 6 µA

REFERENCE AND LDO REGULATOR

LDO Output Voltage V

REF to FB Voltage (Note 2) V

REF Load Regulation I

REF Undervoltage Lockout
Threshold

OSCILLATOR

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Supply Current (Note 1) I

CC

CC

FB

CS

CS

CC

LDO

REF

OSC

328V

VCC = V

VFB = -0.05V 50 nA

CS = GND 1 µA

VFB = -0.05V, VCC = 3V to 28V 400 µA

R

R

Rising edge, 1% hysteresis (typ) 1.0 1.2 V

R

R

R

LDO

= 400Ω

LDO

= 10kΩ, C

REF

= 0 to 400µA -10 mV

REF

= 100kΩ ±1% 425 575

OSC

= 200kΩ ±1% 222 278Oscillator Frequency f

OSC

= 500kΩ ±1% 85 115

OSC

REF

5V ≤ VCC ≤ 28V 4.50 5.50
3V ≤ V

= 0.22µF 1.22 1.28 V

≤ 28V 2.65 5.50

CC

2.7 5.5 V

85 115 mV

µA

V

kHz

MAX1856

Wide Input Range, Synchronizable,
PWM SLIC Power Supply

4 _______________________________________________________________________________________

)

ELECTRICAL CHARACTERISTICS (continued)

(VCC= SYNC/SHDN, V

CC

= 5V, V

LDO

= 5V, R

OSC

= 200kΩ, TA= -40°C to +85°C, unless otherwise noted.) (Note 3)

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 (V

REF

) 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, VCC= V

SYNC/SHDN

= 12V, V

OUT1

= -24V, V

OUT2

= -72V, R

OSC

= 200kΩ, unless otherwise noted.)

-24.5

-24.0

-23.0

-23.5

-22.5

-22.0

0 200100 300 400 500 600 700

-24V OUTPUT VOLTAGE
vs. LOAD CURRENT

MAX1856 toc01

I

OUT1

(mA)

V

OUT1

(V)

VIN = 5V

VIN = 12V

VIN = 24V

V

OUT1

= -24V

V

OUT2

= -72V

I

OUT2

= 5mA

-74.5

-74.0

-73.0

-73.5

-72.5

-72.0

0 200100 300 400 500 600 700

-72V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT

MAX1856 toc02

I

OUT1

(mA)

V

OUT2

(V)

VIN = 5V

VIN = 24V

V

OUT1

= -24V

V

OUT2

= -72V

I

OUT2

= 5mA

VIN = 12V

50

60

70

90

80

100

0 100 200 300 400 500 600 700

EFFICIENCY vs. LOAD CURRENT

(-24V OUTPUT)

MAX1856 toc03

I

OUT1

(mA)

EFFICIENCY (%)

V

OUT1

= -24V

V

OUT2

= -72V

I

OUT2

= 5mA

VIN = 5V

VIN = 24V

VIN = 12V

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Minimum SYNC Input Signal
Duty Cycle

Minimum SYNC Input
Low Pulse Width

SYNC Input Frequency Range f
SYNC/SHDN Input High Voltage V
SYNC/SHDN Input Low Voltage V

SYNC/SHDN Input Current

EXT On-Resistance R

SYNC

IH

IL

ON(EXT

R

= 100kΩ ±1% 86 94

OSC

R

= 200kΩ ±1% 87 93Maximum Duty Cycle D

OSC

= 500kΩ ±1% 86 94

R

OSC

V

SYNC/SHDN

V

SYNC/SHDN

= 5V 3.0

= 28V 10

EXT high or low 5 Ω

%

45 %

200 ns

100 500 kHz

2.0 V

0.45 V

µA

MAX1856

Wide Input Range, Synchronizable,

PWM SLIC Power Supply

_______________________________________________________________________________________ 5

Typical Operating Characteristics (continued)

(Circuit of Figure 1, VCC= V

SYNC/SHDN

= 12V, V

OUT1

= -24V, V

OUT2

= -72V, R

OSC

= 200kΩ, unless otherwise noted.)

-74

-73

-72

-71

-70

0 50 100 150 200 250

-72V OUTPUT VOLTAGE
vs. LOAD CURRENT

MAX1856 toc04

I

OUT2

(mA)

V

OUT2

(V)

V

OUT1

= -24V

V

OUT2

= -72V

I

OUT1

= 5mA

VIN = 5V

VIN = 24V

VIN = 12V

-24.0

-23.8

-23.4

-23.6

-23.2

-23.0

010050 150 200 250

-24V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT

MAX1856 toc05

I

OUT2

(mA)

V

OUT1

(V)

V

OUT1

= -24V

V

OUT2

= -72V

I

OUT1

= 5mA

VIN = 5V

VIN = 24V

VIN = 12V

50

60

70

80

90

100

010050 150 200 250

EFFICIENCY vs. LOAD CURRENT

(-72V OUTPUT)

MAX1856 toc06

I

OUT2

(mA)

EFFICIENCY (%)

V

OUT1

= -24V

V

OUT2

= -72V

I

OUT1

= 5mA

VIN = 5V

VIN = 24V

VIN = 12V

-24V OUTPUT VOLTAGE
vs. INPUT VOLTAGE

-23.0

V

= -24V

OUT1

= 100mA

I

VIN (V)

V
I

OUT1

OUT2

OUT2

= -72V

= 100mA

-23.2

-23.4

(V)

OUT1

V

-23.6

-23.8

-24.0

V

= -24V

OUT1

= 50mA

I

OUT1

= -72V

V

OUT2

= 50mA

I

OUT2

0105152025

MAX1856 toc07

-72V OUTPUT VOLTAGE

-71.0

-71.4

-71.8

(V)

OUT1

V

-72.2

V

OUT1

= 50mA

I

-72.6

-73.0

OUT1

V

OUT2

= 50mA

I

OUT2

0105152025

vs. INPUT VOLTAGE

V

OUT1

= 100mA

I

OUT1

V

OUT2

= 100mA

I

OUT2

= -24V

= -72V

VIN (V)

100

= -24V

MAX1856 toc08

90

= -72V

80

70

EFFICIENCY (%)

60

50

0105152025

DUAL-OUTPUT EFFICIENCY

vs. INPUT VOLTAGE

V

OUT1

= 100mA

I

OUT1

V

OUT2

= 100mA

I

OUT2

V

= -24V

OUT1

= 50mA

I

OUT1

= -72V

V

OUT2

= 50mA

I

OUT2

VIN (V)

= -24V

= -72V

MAX1856 toc09

-48V OUTPUT VOLTAGE

-46.5

VIN = 5V

-46.9

-47.3

(V)

OUT2

V

-47.7

-48.1

-48.5
0 100 200 300 400

SUPPLY CURRENT

vs. INPUT VOLTAGE

vs. LOAD CURRENT

EFFICIENCY vs. LOAD CURRENT

(-48V OUTPUT)

100

250

VIN = 12V

MAX1856 toc10

90

VIN = 12V

MAX1856 toc11

200

VIN = 5V

80

150

VIN = 24V

70

EFFICIENCY (%)

V

= -48V

OUT

FIGURE 4

I

(mA)

OUT2

60

50

0 100 200 300 400

I

OUT2

(mA)

VIN = 24V

V

OUT

FIGURE 4

= -48V

100

SUPPLY CURRENT (µA)

50

CURRENT INTO VCC PIN

R

0

0105 15202530

OSC

INPUT VOLTAGE (V)

MAX1856 toc12

= 500kΩ

MAX1856

Wide Input Range, Synchronizable,
PWM SLIC Power Supply

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 1, VCC= V

SYNC/SHDN

= 12V, V

OUT1

= -24V, V

OUT2

= -72V, R

OSC

= 200kΩ, unless otherwise noted.)

1.240

1.250

1.245

1.255

1.260

REFERENCE VOLTAGE

vs. REFERENCE CURRENT

MAX1856 toc16

REFERENCE CURRENT (µA)

V

REF

(V)

0 200100 300 500

400

1.240

1.250

1.245

1.255

1.260

REFERENCE VOLTAGE

vs. TEMPERATURE

MAX1856 toc17

TEMPERATURE (°C)

V

REF

(V)

-40 10-15 35 85

60

NO LOAD

1000

100

100 1000

SWITCHING FREQUENCY vs. R

OSC

MAX1856 toc18

R

OSC

(kΩ)

SWITCHING FREQUENCY (kHz)

0

200

100

400

300

500

600

-40 10-15 35 60 85

SWITCHING FREQUENCY

vs. TEMPERATURE

MAX1856 toc19

TEMPERATURE (°C)

SWITCHING FREQUENCY (kHz)

R

OSC

= 100kΩ

R

OSC

= 200kΩ

R

OSC

= 500kΩ

70

0

0.1 1 10

EXT RISE/FALL TIME

vs. CAPACITANCE

10

MAX1856 toc20

CAPACITANCE (nF)

EXT RISE/FALL TIME (ns)

30

50

60

40

20

t

RISE

, VIN = 5V

t

RISE

, VIN = 3.3V

t

FALL

, VIN = 5V

t

FALL

, VIN = 3.3V

150

190

270

230

310

350

-40 10-15 35 60 85

SUPPLY CURRENT

vs. TEMPERATURE

MAX1856 toc13

TEMPERATURE (°C)

SUPPLY CURRENT (µA)

R

OSC

= 100kΩ

R

OSC

= 200kΩ

R

OSC

= 500kΩ

CURRENT INTO VCC PIN

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0105 15202530

SHUTDOWN CURRENT

vs. INPUT VOLTAGE

MAX1856 toc14

INPUT VOLTAGE (V)

SHUTDOWN CURRENT (µA)

CURRENT INTO VCC PIN

R

OSC

= 500kΩ

SYNC/SHDN = GND

0

50

150

100

200

250

0426810

LDO DROPOUT VOLTAGE

vs. LOAD CURRENT

MAX1856 toc15

I

LDO

(mA)

DROPOUT VOLTAGE (V)

VIN = 5V

VIN = 3.3V