MAX1856EUB+T ,Wide-Input-Range, Synchronizable, PWM SLIC Power SupplyFeaturesThe MAX1856 offers a low-cost solution for generating a♦ Low-Cost, Off-the-Shelf Transforme ..
MAX1857EUA47 ,500mA / Low-Dropout / Ripple-Rejecting LDO in MAXELECTRICAL CHARACTERISTICS(V = +5.25V, V = 4.75V, SHDN = IN, SET = GND, T = 0°C to +85°C, unless ot ..
MAX1857EUA47+T ,500mA, Low-Dropout, Ripple-Rejecting LDO in µMAXApplications Ordering InformationNotebook ComputersPART* TEMP. RANGE PIN-PACKAGECellular and Cordle ..
MAX1857EUA47+T ,500mA, Low-Dropout, Ripple-Rejecting LDO in µMAXELECTRICAL CHARACTERISTICS(V = +5.25V, V = 4.75V, SHDN = IN, SET = GND, T = 0°C to +85°C, unless ot ..
MAX1858A ,Dual 180° Out-of-Phase Buck Controllers with Sequencing/Prebias Startup and PORApplicationsMAX1858AREF 6 19 VLNetwork Power Supplies MAX1875AGND 7 18 PGNDMAX1876ATelecom Power Su ..
MAX1858AEEG ,Dual 180 Out-of-Phase Buck Controllers with Sequencing/Prebias Startup and PORApplicationsMAX1858AREF 6 19 VLNetwork Power Supplies MAX1875AGND 7 18 PGNDMAX1876ATelecom Power Su ..
MAX4764EBC+T ,Low-Voltage, Dual SPDT, Audio Clickless Switches with Negative Rail CapabilityFeaturesThe MAX4762–MAX4764/MAX4764A/MAX4765 dual♦ Distortion-Free Negative Signal Throughput DownS ..
MAX4764ETB ,Low-Voltage / Dual SPDT / Audio Clickless Switches with Negative Rail CapabilityFeaturesThe MAX4762–MAX4765 dual SPDT (single-pole/double-♦ Distortion-Free Negative Signal Through ..
MAX4764ETB+ ,Low-Voltage, Dual SPDT, Audio Clickless Switches with Negative Rail CapabilityFeaturesThe MAX4762–MAX4764/MAX4764A/MAX4765 dual♦ Distortion-Free Negative Signal Throughput DownS ..
MAX4764ETB+T ,Low-Voltage, Dual SPDT, Audio Clickless Switches with Negative Rail CapabilityApplications♦ Available in µMAX, TDFN, Thin QFN, and UCSPPackagesCell PhonesPDAs and Handheld Devic ..
MAX4764ETB+T ,Low-Voltage, Dual SPDT, Audio Clickless Switches with Negative Rail CapabilityFeaturesThe MAX4762–MAX4764/MAX4764A/MAX4765 dual♦ Distortion-Free Negative Signal Throughput DownS ..
MAX4764EUB ,Low-Voltage / Dual SPDT / Audio Clickless Switches with Negative Rail CapabilityApplications♦ 0.01% Total Harmonic DistortionCell Phones♦ Available in µMAX, TDFN, Thin QFN, and UC ..
MAX1856EUB+-MAX1856EUB+T
Wide-Input-Range, Synchronizable, PWM SLIC Power Supply
General DescriptionThe 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.
ApplicationsVoIP 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
FeaturesLow-Cost, Off-the-Shelf Transformer3V to 28V Input RangeLow Audio-Band Noise on Talk BatteryEffectively Handles Ring Trip TransientsPowers 2-, 4-, or 12-Line EquipmentHigh Efficiency Extends Battery Life During
Life-Line Support ConditionsAdjustable 100kHz to 500kHz Switching
FrequencyClock SynchronizationInternal Soft-StartCurrent-Mode PWM and Idle Mode™Control
SchemeLogic-Level Shutdown10-Pin µMAX Package
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power SupplyEXT2
LDO
FREQ
INPUT
3V TO 28V
GND
REF
VCC
PGND
SYNC/SHDN
MAX1856
OUT2
-72V
OUT1
-24V
ypical Operating Circuit
Ordering Information19-1898; Rev 0; 2/01
PARTTEMP. RANGEPIN-PACKAGEMAX1856EUB-40°C to +85°C 10 µMAX
Idle Mode is a trademark of Maxim Integrated Products.
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= SYNC/SHDN,VCC = 5V, VLDO = 5V, ROSC= 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/SHDNto 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 (VLDO+ 0.3V)
LDO Output Current............................................-1mA to +20mA
LDO Short Circuit to GND...............................................<100ms
REF Short Circuit to GND...........................................Continuous
Continuous Power Dissipation (TA= +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
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
PWM CONTROLLER328VOperating Input Voltage RangeVCCVCC = VLDO2.75.5V
FB Input CurrentIFBVFB = -0.05V150nA
Load RegulationVCS = 0 to 100mV for 0 to ILOAD(MAX)0.013%/mV
Line RegulationTypically 0.0074% per % duty factor on EXT0.0074%/%
Current-Limit ThresholdVCS85100115mV
CS Input CurrentICSCS = GND1µA
Idle Mode Current-Sense
Threshold51525mV
VCC Supply Current (Note 1)ICCVFB = -0.05V, VCC = 3V to 28V250400µA
Shutdown Supply CurrentSYNC/SHDN = GND, VCC = 28V3.56µA
REFERENCE AND LDO REGULATOR5V ≤ VCC ≤ 28V4.505.005.50LDO Output VoltageVLDORLDO = 400Ω3V ≤ VCC ≤ 28V2.655.50V
Undervoltage Lockout
ThresholdVUVLOVLDO falling edge, 1% hysteresis (typ)2.402.502.60V
REF to FB Voltage (Note 2)VREFRREF = 10kΩ, CREF = 0.22µF1.2251.2501.275V
REF Load RegulationIREF = 0 to 400µA-2-10mV
REF Undervoltage Lockout
ThresholdRising edge, 1% hysteresis (typ)1.01.11.2V
OSCILLATORROSC = 100kΩ ±1%425500575
ROSC = 200kΩ ±1%225250275Oscillator FrequencyfOSC
ROSC = 500kΩ ±1%85100115
kHz
ROSC = 100kΩ ±1%869094
ROSC = 200kΩ ±1%879093Maximum Duty CycleD
ROSC = 500kΩ ±1%869094
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
ELECTRICAL CHARACTERISTICS (continued)(VCC= SYNC/SHDN,VCC = 5V, VLDO = 5V, ROSC= 200kΩ, TA
= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSMinimum EXT Pulse Width290ns
Minimum SYNC Input Signal
Duty Cycle2045%
Minimum SYNC Input
Low Pulse Width50200ns
Maximum SYNC Input
Rise/Fall Time200ns
SYNC Input Frequency RangefSYNC100500kHz
SYNC/SHDN Falling Edge to
Shutdown DelaytSHDN50µs
SYNC/SHDN Input High VoltageVIH2.0V
SYNC/SHDN Input Low VoltageVIL0.45V
VSYNC/SHDN = 5V0.53.0SYNC/SHDN Input CurrentVSYNC/SHDN = 28V1.510µA
EXT Sink/Source CurrentIEXTEXT forced to 2V1A
EXT On-ResistanceRON(EXT)EXT high or low25Ω
ELECTRICAL CHARACTERISTICS(VCC= SYNC/SHDN,VCC = 5V, VLDO = 5V, ROSC= 200kΩ, TA
= -40°C to +85°C, unless otherwise noted.) (Note 3)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
PWM CONTROLLER328VOperating Input Voltage RangeVCCVCC = VLDO2.75.5V
FB Input CurrentIFBVFB = -0.05V50nA
Current-Limit ThresholdVCS85115mV
CS Input CurrentICSCS = GND1µA
VCC Supply Current (Note 1)ICCVFB = -0.05V, VCC = 3V to 28V400µA
Shutdown Supply CurrentSYNC/SHDN = GND, VCC = 28V6µA
REFERENCE AND LDO REGULATOR5V ≤ VCC ≤ 28V4.505.50LDO Output VoltageVLDORLDO = 400Ω3V ≤ VCC ≤ 28V2.655.50V
REF to FB Voltage (Note 2)VREFRREF = 10kΩ, CREF = 0.22µF1.221.28V
REF Load RegulationIREF = 0 to 400µA-10mV
REF Undervoltage Lockout
ThresholdRising edge, 1% hysteresis (typ)1.01.2V
OSCILLATORROSC = 100kΩ ±1%425575
ROSC = 200kΩ ±1%222278Oscillator FrequencyfOSC
ROSC = 500kΩ ±1%85115
kHz
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSROSC = 100kΩ ±1%8694
ROSC = 200kΩ ±1%8793Maximum Duty CycleD
ROSC = 500kΩ ±1%8694
Minimum SYNC Input Signal
Duty Cycle45%
Minimum SYNC Input
Low Pulse Width200ns
SYNC Input Frequency RangefSYNC100500kHz
SYNC/SHDN Input High VoltageVIH2.0V
SYNC/SHDN Input Low VoltageVIL0.45V
VSYNC/SHDN = 5V3.0SYNC/SHDN Input CurrentVSYNC/SHDN = 28V10µA
EXT On-ResistanceRON(EXT)EXT high or low5Ω
ELECTRICAL CHARACTERISTICS (continued)(VCC= SYNC/SHDN,VCC = 5V, VLDO = 5V, ROSC= 200kΩ, TA
= -40°C to +85°C, unless otherwise noted.) (Note 3)
Note 1:This is the VCCcurrent 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, VCC= VSYNC/SHDN= 12V, VOUT1= -24V, VOUT2= -72V, ROSC= 200kΩ, unless otherwise noted.)
-24V OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1856 toc01
IOUT1 (mA)
OUT1
(V)
VIN = 5V
VIN = 12VVIN = 24V
VOUT1 = -24V
VOUT2 = -72V
IOUT2 = 5mA
-72V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT
MAX1856 toc02
IOUT1 (mA)
OUT2
(V)
VIN = 5VVIN = 24V
VOUT1 = -24V
VOUT2 = -72V
IOUT2 = 5mA
VIN = 12V
EFFICIENCY vs. LOAD CURRENT
(-24V OUTPUT)
MAX1856 toc03
IOUT1 (mA)
EFFICIENCY (%)
VOUT1 = -24V
VOUT2 = -72V
IOUT2 = 5mA
VIN = 5V
VIN = 24V
VIN = 12V
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply-24V OUTPUT VOLTAGE
vs. INPUT VOLTAGE
MAX1856 toc07
VIN (V)
OUT1
(V)
VOUT1 = -24V
IOUT1 = 100mA
VOUT2 = -72V
IOUT2 = 100mA
VOUT1 = -24V
IOUT1 = 50mA
VOUT2 = -72V
IOUT2 = 50mA
-72V OUTPUT VOLTAGE
vs. INPUT VOLTAGE
MAX1856 toc08
VIN (V)
OUT1
(V)
VOUT1 = -24V
IOUT1 = 100mA
VOUT2 = -72V
IOUT2 = 100mA
VOUT1 = -24V
IOUT1 = 50mA
VOUT2 = -72V
IOUT2 = 50mA
DUAL-OUTPUT EFFICIENCY
vs. INPUT VOLTAGE
MAX1856 toc09
VIN (V)
EFFICIENCY (%)
VOUT1 = -24V
IOUT1 = 100mA
VOUT2 = -72V
IOUT2 = 100mA
VOUT1 = -24V
IOUT1 = 50mA
VOUT2 = -72V
IOUT2 = 50mA
-48V OUTPUT VOLTAGE
vs. LOAD CURRENT
MAX1856 toc10
IOUT2 (mA)
OUT2
(V)
VOUT = -48V
FIGURE 4
VIN = 5V
VIN = 24V
VIN = 12V
EFFICIENCY vs. LOAD CURRENT
(-48V OUTPUT)
MAX1856 toc11
IOUT2 (mA)
EFFICIENCY (%)
VOUT = -48V
FIGURE 4
VIN = 5V
VIN = 24V
VIN = 12V
SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX1856 toc12
INPUT VOLTAGE (V)
SUPPLY CURRENT (
CURRENT INTO VCC PIN
ROSC = 500kΩ
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
MAX1856 toc04
IOUT2 (mA)
OUT2
(V)
VOUT1 = -24V
VOUT2 = -72V
IOUT1 = 5mA
VIN = 5V
VIN = 24V
VIN = 12V
-24V CROSS-REGULATION VOLTAGE
vs. LOAD CURRENT
MAX1856 toc05
IOUT2 (mA)
OUT1
(V)
VOUT1 = -24V
VOUT2 = -72V
IOUT1 = 5mA
VIN = 5V
VIN = 24V
VIN = 12V
EFFICIENCY vs. LOAD CURRENT
(-72V OUTPUT)
MAX1856 toc06
IOUT2 (mA)
EFFICIENCY (%)
VOUT1 = -24V
VOUT2 = -72V
IOUT1 = 5mA
VIN = 5V
VIN = 24V
VIN = 12V
MAX1856
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.)
REFERENCE VOLTAGE
vs. REFERENCE CURRENT
MAX1856 toc16
REFERENCE CURRENT (μA)
REF
(V)200100300500400
REFERENCE VOLTAGE
vs. TEMPERATURE
MAX1856 toc17
TEMPERATURE (°C)
REF
(V)
NO LOAD1000
SWITCHING FREQUENCY vs. ROSC
MAX1856 toc18
ROSC (kΩ)
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY
vs. TEMPERATURE
MAX1856 toc19
TEMPERATURE (°C)
SWITCHING FREQUENCY (kHz)
ROSC = 100kΩ
ROSC = 200kΩ
ROSC = 500kΩ
EXT RISE/FALL TIME
vs. CAPACITANCE
MAX1856 toc20
CAPACITANCE (nF)
EXT RISE/FALL TIME (ns)
tRISE, VIN = 5V
tRISE, VIN = 3.3V
tFALL, VIN = 5V
tFALL, VIN = 3.3V
SUPPLY CURRENT
vs. TEMPERATURE
MAX1856 toc13
TEMPERATURE (°C)
SUPPLY CURRENT (
ROSC = 100kΩ
ROSC = 200kΩ
ROSC = 500kΩ
CURRENT INTO VCC PIN
SHUTDOWN CURRENT
vs. INPUT VOLTAGE
MAX1856 toc14
INPUT VOLTAGE (V)
SHUTDOWN CURRENT (
CURRENT INTO VCC PIN
ROSC = 500kΩ
SYNC/SHDN = GND
LDO DROPOUT VOLTAGE
vs. LOAD CURRENT
MAX1856 toc15
ILDO (mA)
DROPOUT VOLTAGE (V)
VIN = 5V
VIN = 3.3V
A. VOUT = -48V, IOUT = 200mA, 50mV/div
B. ILP, 2A/div
CIRCUIT OF FIGURE 4
HEAVY-LOAD SWITCHING WAVEFORMMAX1856 toc23
-47.2V
-47.0V
-47.1V
2.0μs/div
A. VOUT = -48V, IOUT = 20mA, 20mV/div
B. ILP, 2A/div
CIRCUIT OF FIGURE 4
LIGHT-LOAD SWITCHING WAVEFORMMAX1856 toc24
-47.80V
-47.72V
-47.76V
4μs/div
MAX1856
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.)
A. IOUT = 20mA TO 200mA, 200mA/div
B. VOUT, = -48V, 500mV/div
C. ILP, 2A/div
CIRCUIT OF FIGURE 4
LOAD TRANSIENTMAX1856 toc25
-47.6V
1ms/div
200mA
-47.1V
-48.1V
RINGER TO TALK-BATTERY CROSSTALK
MAX1856 toc27
FREQUENCY (Hz)
(dB)
TALK-BATTERY OUTPUT
RINGER OUTPUT
A. VIN = 10V TO 14V, 2Vdiv
B. VOUT = -48V, IOUT = 200mA, 100mV/div
CIRCUIT OF FIGURE 4
LINE TRANSIENTMAX1856 toc26
14V
10V
400μs/div
12V
-47V
A. VSYNC/SHDN = 0 TO 5V, 5V/div
B. ILP, 2A/div
C. VOUT = -48V, ROUT = 2.4kΩ, 20V/div
CIRCUIT OF FIGURE 4
EXITING SHUTDOWNMAX1856 toc21
-60V
-20V
-40V
4ms/div
A. VSYNC/SHDN = 5V TO 0, 5V/div
B. VEXT, 5V/div
C. ILP, 2A/div
VOUT = -48V, ROUT = 240Ω
CIRCUIT OF FIGURE 4
ENTERING SHUTDOWNMAX1856 toc22
10μs/div
MAX1856
Detailed DescriptionThe MAX1856 current-mode PWM controller uses an
inverting flyback configuration that is ideal for generat-
ing the high negative voltages required for SLIC power
supplies. Optimum conversion efficiency is maintained
over a wide range of loads by employing both PWM
operation and Maxim’s proprietary Idle Mode control to
minimize operating current at light loads. Other features
include shutdown, adjustable internal operating fre-
quency or synchronization to an external clock, soft-
start, adjustable current limit, and a wide (3V to 28V)
input range.
PWM ControllerThe heart of the MAX1856 current-mode PWM con-
troller is a BiCMOS multi-input comparator that simulta-
neously processes the output-error signal, the
current-sense signal, and a slope-compensation ramp
(Figure 2). The main PWM comparator is direct sum-
ming, lacking a traditional error amplifier and its associ-
ated phase shift. The direct-summing configuration
approaches ideal cycle-by-cycle control over the out-
put voltage since there is no conventional error amplifi-
er in the feedback path.
In PWM mode, the controller uses fixed-frequency, cur-
rent-mode operation where the duty ratio is set by the
input-to-output voltage ratio and the transformer’s turn
ratio. The current-mode feedback loop regulates peak
inductor current as a function of the output error signal.
At light loads, the controller enters Idle Mode. During
Idle Mode, switching pulses are provided only as nec-
essary to supply the load, and operating current is min-
imized to provide the best light-load efficiency. The
minimum-current comparator threshold is 15mV, or
15% of the full-load value (IMAX) of 100mV. When the
controller is synchronized to an external clock, Idle
Mode occurs only at very light loads.
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
Pin Description
PINNAMEFUNCTION1LDO5V Linear Regulator Output. The regulator powers all of the internal circuitry, including the EXT gate
driver. Bypass LDO to GND with a 1µF or greater ceramic capacitor.FREQ
Oscillator Frequency Set Input. A resistor from FREQ to GND sets the oscillator from 100kHz (ROSC =
500kΩ) to 500kHz (ROSC = 100kΩ): fOSC = 50MΩ-kHz / ROSC. The MAX1856 still requires ROSC when
an external clock is connected to SYNC/SHDN.GNDAnalog GroundREF1.25V Reference Output. REF can source up to 400µA. Bypass to GND with a 2.2µF ceramic capacitor.FBFeedback Input. The feedback voltage threshold is 0.CSPositive Current-Sense Input. Connect a current-sense resistor (RCS) between CS and PGND.PGNDPower GroundEXTExternal MOSFET Gate-Driver Output. EXT swings from LDO to PGND.
9VCCInput Supply to the Linear Regulator. VCC accepts inputs up to 28V. Bypass to PGND with a 1µF
ceramic capacitor.SYNC/SHDN
Shutdown Control and Synchronization Input. There are three operating modes:• SYNC/SHDN low: shutdown modeSYNC/SHDN high: the DC-to-DC controller operates with the oscillator frequency set at FREQ by
ROSCSYNC/SHDN clocked: the DC-to-DC controller operates with the oscillator frequency set by the SYNC
clock input. The conversion cycles initiate on the rising edge of the input clock signal. However, the
MAX1856 still requires ROSC when SYNC/SHDN is externally clocked.
Low-Dropout Regulator (LDO)All MAX1856 functions, including EXT, are internally
powered from the on-chip, low-dropout 5V regulator.
The regulator input is at VCC, while its output is at LDO.
The VCC-to-LDO dropout voltage is typically 200mV
(300mV max at 12mA), so that when VCCis <5.2V,
VLDOis typically VCC- 200mV. When the LDO is in
dropout, the MAX1856 still operates with VCCas low as
3V (as long as the LDO exceeds 2.7V), but with
reduced amplitude FET drive at EXT. The maximum
VCCinput voltage is 28V.
LDO can supply up to 12mA to power the IC, supply
gate charge through EXT to the external FET, and sup-
ply small external loads. When driving particularly large
FETs at high switching rates, little or no LDO current
may be available for external loads. For example, when
switched at 500kHz, a large FET with 20nC gate charge
requires 20nC ✕500kHz, or 10mA.
Soft-StartThe MAX1856 features a “digital” soft-start that is pre-
set and requires no external capacitor. Upon startup,
the peak inductor current increments from 1/5th of the
value set by RCS, to the full current-limit value in five
steps over 1024 cycles of fOSCor fSYNC. Additionally,
the oscillator runs at 1/3 the normal operating frequen-
cy (fOSC/3) until the output voltage reaches 20% of its
nominal value (VFB≤1.0V). See the Typical Operating
Characteristicsfor a scope picture of the soft-start
operation. Soft-start is implemented: 1) when power is
first applied to the IC, 2) when exiting shutdown with
power already applied, and 3) when exiting undervolt-
age lockout. The MAX1856’s soft-start sequence does
not start until VLDOreaches 2.5V.
Design ProcedureThe MAX1856 can operate within a wide input voltage
range from 3V to 28V. This allows it to be used with wall
adapters. In applications driven by low-power, low-cost
and low input and output ripple current requirements,
the MAX1856 flyback topology can be used to gener-
ate various levels of output voltages and multiple out-
puts.
Communications over the Internet interface with a stan-
dard telephone connection, which includes the
Subscriber Line Interface Circuit (SLIC). The SLIC
requires a negative power supply for the audio and
ringer functions. The circuits discussed here are
designed for these applications. The following design
discussions are related to the standard application cir-
MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power SupplyEXT2
LDO
FREQ
*INPUT
4.5V TO 24V
D1, D2:Central Semiconductor CMR1U-02
M1: International Rectifier IRLL2705
T1: Coiltronics CTX01-14853
ROSC
200kΩ
1μF
CLDO
1μFRCS
33mΩ
CIN
(2x) 10μF
25V
GND
REF
VCC
PGND
SYNC/SHDN
MAX1856
OUT2
-72V4D2
1nF
100Ω
10Ω
470Ω
5.11kΩ
681kΩ
174kΩ
CREF
2.2μF
100μF
Sanyo
100MV100AX
330μF
Sanyo
35MV330AX
100pF
OUT1
-24V
CFB
1nF
*INPUT RANGE LIMITED BY OUTPUT POWER REQUIREMENTS.
SEE MAXIMUM OUTPUT POWER AND TYPICAL OPERATING CHARACTERISTICS.
Figure 1. Standard Application Circuit
