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PDF MAX1856 Data sheet ( Hoja de datos )
| Número de pieza | MAX1856 | |
| Descripción | Wide Input Range / Synchronizable / PWM SLIC Power Supply | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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19-1898; Rev 0; 2/01
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
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.
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
Idle Mode is a trademark of Maxim Integrated Products.
Features
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
PART
MAX1856EUB
Ordering Information
TEMP. RANGE
-40°C to +85°C
PIN-PACKAGE
10 µMAX
Typical Operating Circuit
INPUT
3V TO 28V
VCC
SYNC/SHDN
LDO
FREQ
EXT
CS
MAX1856
GND
PGND
FB
REF
1
2
2
2
OUT2
-72V
OUT1
-24V
________________________________________________________________ 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.
1 page  
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VCC = VSYNC/SHDN = 12V, VOUT1 = -24V, VOUT2 = -72V, ROSC = 200kΩ, unless otherwise noted.)
-72V OUTPUT VOLTAGE
vs. LOAD CURRENT
-70
-71 VIN = 5V
VIN = 12V
-72
-73
-74
0
-23.0
-23.2
-23.4
VIN = 24V
VOUT1 = -24V
VOUT2 = -72V
IOUT1 = 5mA
50 100 150 200 250
IOUT2 (mA)
-24V OUTPUT VOLTAGE
vs. INPUT VOLTAGE
VOUT1 = -24V
IOUT1 = 100mA
VOUT2 = -72V
IOUT2 = 100mA
-23.6
-23.8
VOUT1 = -24V
IOUT1 = 50mA
VOUT2 = -72V
IOUT2 = 50mA
-24.0
0
5 10 15 20 25
VIN (V)
-48V OUTPUT VOLTAGE
vs. LOAD CURRENT
-46.5
-46.9 VIN = 5V
VIN = 12V
-47.3
VIN = 24V
-47.7
-24V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT
-23.0
-23.2
-23.4 VIN = 5V
VIN = 12V
-23.6
-23.8
-24.0
0
-71.0
-71.4
-71.8
VIN = 24V
VOUT1 = -24V
VOUT2 = -72V
IOUT1 = 5mA
50 100 150 200 250
IOUT2 (mA)
-72V OUTPUT VOLTAGE
vs. INPUT VOLTAGE
VOUT1 = -24V
IOUT1 = 100mA
VOUT2 = -72V
IOUT2 = 100mA
-72.2
-72.6
VOUT1 = -24V
IOUT1 = 50mA
VOUT2 = -72V
IOUT2 = 50mA
-73.0
0
5 10 15 20 25
VIN (V)
EFFICIENCY vs. LOAD CURRENT
(-48V OUTPUT)
100
90
VIN = 5V
80
VIN = 12V
70 VIN = 24V
EFFICIENCY vs. LOAD CURRENT
(-72V OUTPUT)
100
90 VIN = 5V
VIN = 12V
80
VIN = 24V
70
60
50
0
100
90
80
70
60
VOUT1 = -24V
VOUT2 = -72V
IOUT1 = 5mA
50 100 150 200 250
IOUT2 (mA)
DUAL-OUTPUT EFFICIENCY
vs. INPUT VOLTAGE
VOUT1 = -24V
IOUT1 = 100mA
VOUT2 = -72V
IOUT2 = 100mA
VOUT1 = -24V
IOUT1 = 50mA
VOUT2 = -72V
IOUT2 = 50mA
50
0
250
5 10 15 20
VIN (V)
SUPPLY CURRENT
vs. INPUT VOLTAGE
25
200
150
100
-48.1
-48.5
0
VOUT = -48V
FIGURE 4
100 200 300
IOUT2 (mA)
400
60
50
0
VOUT = -48V
FIGURE 4
100 200 300
IOUT2 (mA)
400
50
0
0
CURRENT INTO VCC PIN
ROSC = 500kΩ
5 10 15 20 25 30
INPUT VOLTAGE (V)
_______________________________________________________________________________________ 5
5 Page 
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
100kHz to 500kHz frequency, which is set by a resistor
(ROSC) connected from FREQ to GND. The relationship
between fOSC and ROSC is:
ROSC
=
50MΩ × kHz
ƒOSC(kHz)
Thus, a 250kHz operating frequency, for example, is
set with ROSC = 200kΩ. At higher frequencies, the
magnetic components will be smaller. Peak currents
and, consequently, resistive losses will be lower at the
higher switching frequency. However, core losses, gate
charge currents, and switching losses increase with
higher switching frequencies.
Rising clock edges on SYNC/SHDN are interpreted as
synchronization input. If the sync signal is lost while
SYNC/SHDN is high, the internal oscillator takes over at
the end of the last cycle, and the frequency is returned
to the rate set by ROSC. If the signal is lost with
SYNC/SHDN low, the IC waits for 50µs before shutting
down. This maintains output regulation even with inter-
mittent sync signals. When an external sync signal is
used, Idle Mode switchover at the 15mV current-sense
threshold is disabled so that Idle Mode only occurs at
very light loads. Also, ROSC should be set for a fre-
quency 15% below the SYNC clock rate:
ROSC(SYNC)
=
50MΩ × kHz
0.85 ƒOSC(kHz)
Setting the Output Voltage
Set the output voltage using two external resistors form-
ing a resistive divider to FB between the output and
REF. First select a value for R3 between 3.3kΩ and
100kΩ. R1 is then given by:
R1
=
R3
VOUT
VREF
For a dual output as shown in Figure 1, a split feedback
technique is recommended. Since the feedback volt-
age threshold is 0, the total feedback current is:
ITOTAL
=
IR1
+
IR2
=
VREF
R3
Since the feedback resistors are connected to the ref-
erence, ITOTAL must be <400µA so that VREF is guaran-
teed to be in regulation (see Electrical Characteristics
Table). Therefore, select R3 so the total current value is
between 200µA and 250µA as shown in Figure 1. To
ensure that the MAX1856 regulates both outputs with
the same degree of accuracy over load, select the
feedback resistors (R1 and R2) so their current ratio
(IR1:IR2) equals the output power ratio (POUT1:POUT2)
under full load:
IR1 = VOUT1IOUT1
IR2 VOUT2 IOUT2
Once R3 and the dual feedback currents (IR1 and IR2)
are determined from the two equations above, use the
following two equations to determine R1 and R2:
IR1 =
VOUT1
R1
and
IR2
=
VOUT2
R2
Selecting the Transformer
The MAX1856 PWM controller works with economical
off-the-shelf transformers. The transformer selection
depends on the input-to-output voltage ratio, output
current capacity, duty cycle, and oscillator frequency.
Table 1 shows recommended transformers for the typi-
cal applications, and Table 2 gives some recommend-
ed suppliers.
Transformer Turns Ratio
The transformer turns ratio is a function of the input-to-
output voltage ratio and maximum duty cycle. Under
steady-state conditions, the change in flux density dur-
ing the on-time must equal the return change in flux
density during the off-time (or flyback period):
VINtON = VOUTtOFF
NP NS
For example, selecting a 50% duty cycle for the stan-
dard application circuit (Figure 1) and a +12V input
voltage, the -72V output requires a 1:6 turns ratio, and
the -24V output requires a 1:2 turns ratio. Therefore, a
transformer with a 1:2:2:2 turns ratio was selected.
Primary inductance
The average input current at maximum load can be cal-
culated as:
IIN(DC)
=
VOUT IOUT(MAX)
ηVIN(MIN)
where η = efficiency. For VOUT = -24V, IOUT(MAX) =
400mA, and VIN(MIN) = 10.8V as shown in Figure 1, this
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet MAX1856.PDF ] | |
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