MAX1864TEEE-T ,xDSL/Cable Modem Triple/Quintuple Output Power SuppliesFeaturesThe MAX1864/MAX1865 power-supply controllers are ♦ 4.5V to 28V Input Voltage Rangedesigned ..
MAX186ACAP ,Low-Power, 8-Channel, Serial 12-Bit ADCsFeaturesThe MAX186/MAX188 are 12-bit data-acquisition sys- ' 8-Channel Single-Ended or 4-Channel te ..
MAX186ACAP+ ,Low-Power, 8-Channel, Serial 12-Bit ADCsEVALUATION KIT AVAILABLE MAX186/MAX188Low-Power, 8-Channel,Serial 12-Bit ADCs
MAX186ACAP+ ,Low-Power, 8-Channel, Serial 12-Bit ADCsApplicationsCH1 2 19 SCLKPortable Data LoggingCSCH2 3 18Data-Acquisition CH3 417 DINMAX186High-Accu ..
MAX186ACPP ,Low-Power, 8-Channel, Serial 12-Bit ADCsMAX186/MAX18819-0123; Rev. 4; 8/96Low-Power, 8-Channel,Serial 12-Bit ADCs _______________
MAX186ACPP+ ,Low-Power, 8-Channel, Serial 12-Bit ADCsEVALUATION KIT AVAILABLE MAX186/MAX188Low-Power, 8-Channel,Serial 12-Bit ADCs
MAX4782EUE+ ,High-Speed, Low-Voltage, 0.7Ω CMOS Analog Switches/MultiplexersApplicationsOrdering InformationBattery-Operated EquipmentPART TEMP RANGE PIN-PACKAGEAudio Signal R ..
MAX4783ETE+ ,High-Speed, Low-Voltage, 0.7Ω CMOS Analog Switches/MultiplexersMAX4781/MAX4782/MAX478319-2522; Rev 3; 2/05High-Speed, Low-Voltage, 0.7Ω CMOS AnalogSwitches/Multip ..
MAX4784EUE ,0.7 M Low-Voltage, Quad 2:1 Analog MultiplexersApplicationsPower RoutingBattery-Powered SystemsOrdering InformationAudio and Video Signal RoutingL ..
MAX4785EXK+T ,50mA/100mA Current-Limit SwitchesELECTRICAL CHARACTERISTICS(V = 2.3V to 5.5V, T = -40°C to +85°C, unless otherwise noted. Typical va ..
MAX4785EXK+T ,50mA/100mA Current-Limit SwitchesFeaturesThe MAX4785–MAX4788 family of switches feature inter-♦ Guaranteed Current Limit: 50mA, 100m ..
MAX4786EXS+T ,50mA/100mA Current-Limit SwitchesELECTRICAL CHARACTERISTICS(V = 2.3V to 5.5V, T = -40°C to +85°C, unless otherwise noted. Typical va ..
MAX1864TEEE+-MAX1864TEEE+T-MAX1864TEEE-T
xDSL/Cable Modem Triple/Quintuple Output Power Supplies
General DescriptionThe MAX1864/MAX1865 power-supply controllers are
designed to address cost-conscious applications such
as cable modem Consumer Premise Equipment (CPE),
xDSL CPE, and set-top boxes. Operating off a low-cost,
unregulated DC supply (such as a wall adapter output),
the MAX1864 generates three positive outputs, and the
MAX1865 generates four positive outputs and one neg-
ative output to provide a cost-effective system power
supply.
The MAX1864 includes a current-mode synchronous
step-down controller and two positive regulator gain
blocks. The MAX1865 has one additional positive gain
block and one negative regulator gain block. The main
synchronous step-down controller generates a
high-current output that is preset to 3.3V or adjustable
from 1.236V to 0.8 ✕VINwith an external resistive-
divider. The 100kHz/200kHz operating frequency
allows the use of low-cost aluminum-electrolytic capaci-
tors and low-cost power magnetics. Additionally, the
MAX1864/MAX1865 step-down controllers sense the
voltage across the low-side MOSFET’s on-resistance to
efficiently provide the current-limit signal, eliminating
the need for costly current-sense resistors.
The MAX1864/MAX1865 generate additional supply
rails at low cost. The positive regulator gain blocks use
an external PNP pass transistor to generate low-voltage
rails directly from the main step-down converter (such
as 2.5V or 1.8V from the main 3.3V output) or higher
voltages using coupled windings from the step-down
converter (such as 5V, 12V, or 15V). The MAX1865’s
negative gain block uses an external NPN pass transis-
tor in conjunction with a coupled winding to generate
-5V, -12V, or -15V.
All output voltages are externally adjustable, providing
maximum flexibility. Additionally, the MAX1864/
MAX1865 feature soft-start for the step-down converter
and all the positive linear regulators, and have a power-
good output that monitors all of the output voltages.
ApplicationsxDSL, Cable, and ISDN Modems
Set-Top Boxes
Wireless Local Loop
Features4.5V to 28V Input Voltage RangeMaster DC-DC Step-Down Converter
Preset 3.3V or Adjustable (1.236V to 0.8 ✕ VIN)
Output Voltage
Fixed-Frequency (100kHz/200kHz) PWM
Controller
No Current-Sense Resistor
Adjustable Current Limit
95% EfficientTwo (MAX1864)/Four (MAX1865) Analog
Gain Blocks
Positive Analog Blocks Drive Low-Cost PNP
Pass Transistors to Build Positive Linear
Regulators
Negative Analog Block (MAX1865) Drives a
Low-Cost NPN Pass Transistor to Build a
Negative Linear RegulatorPower-Good IndicatorSoft-Start Ramp for All Positive Regulators
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power SuppliesPOKIN
BST
GND
ILIM
TOP VIEW
MAX1864
16 QSOPCOMP
OUT
FB2
FB3
Pin Configurations
Ordering Information19-2030; Rev 0; 4/01
EVALUATION KITAVAILABLE
PARTTEMP.
RANGE
PIN-
PACKAGE
fOSC
(kHz)
MAX1864TEEE-40°C to +85°C16 QSOP200
MAX1864UEEE-40°C to +85°C16 QSOP100
MAX1865TEEP-40°C to +85°C20 QSOP200
MAX1865UEEP-40°C to +85°C20 QSOP100
Pin Configurations continued at end of data sheet.
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VIN= 12V, ILIM = FB = GND, VBST- VLX= 5V, 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.
IN, B2, B3, B4 to GND............................................-0.3V to +30V
B5 to OUT...............................................................-20V to +0.3V
VL, POK, FB, FB2, FB3, FB4, FB5 to GND...............-0.3V to +6V
LX to BST..................................................................-6V to +0.3V
BST to GND............................................................-0.3V to +36V
DH to LX....................................................-0.3V to (VBST+ 0.3V)
DL, OUT, COMP, ILIM to GND......................-0.3V to (VL+ 0.3V)
VL Output Current...............................................................50mA
VL Short Circuit to GND...................................................≤100ms
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........666mW
20-Pin QSOP (derate 9.1mW/°C above +70°C)...........727mW
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
GENERALOperating Input Voltage Range
(Note 1)VIN4.528V
MAX18641.02
Quiescent Supply CurrentIIN
VFB = 0, VOUT = 4V,
VFB2 = VFB3 = VFB4 = 1.5V,
VFB5 = -0.1VMAX18651.43
VL REGULATOROutput VoltageVL6V < VIN < 28V, 0.1mA < ILOAD < 20mA4.755.005.25V
Power-Supply RejectionPSRRVIN = 6V to 28V3%
Undervoltage Lockout Trip LevelVUVLOVL rising, 3% hysteresis (typ)3.23.53.8V
Minimum Bypass CapacitanceCBYP(MIN)10mΩ < ESR < 500mΩ1µF
DC-DC CONTROLLEROutput Voltage (Preset Mode)VOUTFB = GND3.2723.3143.355V
Typical Output Voltage Range
(Adjustable Mode) (Note 2)VOUT1.2360.8 x VINV
FB Set Voltage
(Adjustable Mode)VSETFB = COMP1.2211.2361.252V
FB Dual Mode™ Threshold50100150mV
FB Input Leakage CurrentIFBVFB = 1.5V0.01100nA
FB to COMP TransconductancegmFB = COMP, ICOMP = ±5µA70100140µS
Current-Sense Amplifier Voltage
GainALIMVIN - VLX = 250mV4.464.95.44V/V
Current-Limit Threshold
(Internal Mode)VVALLEYVILIM = 5.0V190250310mV
Current-Limit Threshold
(External Mode)VVALLEYVILIM = 2.5V440530620mV
Dual Mode is a trademark of Maxim Integrated Products, Inc.
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSMAX186_T160200240Switching FrequencyfOSCMAX186_U80100120kHz
Maximum Duty CycleDMAX778290%
Soft-Start PeriodtSOFT10241/fOSC
Soft-Start StepsVREF/64V
DH Output Low VoltageISINK = 10mA, measured from DH to LX0.1V
DH Output High VoltageISOURCE = 10mA, measured from BST to DH0.1V
DL Output Low VoltageISINK = 10mA, measured from DL to GND0.1V
DL Output High VoltageISOURCE = 10mA, measured from DL to GNDVL - 0.1V
DH, DL On-Resistance310Ω
Output Drive CurrentSourcing or sinking, VDH or VDL = VL/20.5A
LX, BST Leakage CurrentVBST = VLX = VIN = 28V, VFB = 1.5V0.0320µA
POSITIVE ANALOG GAIN BLOCKSFB2, FB3, FB4 Regulation
Voltage
VB2 = VB3 = VB4 = 5V,
IB2 = IB3 = IB4 = 1mA (sink)1.2261.2401.257V
FB2, FB3, FB4 to B_
TransconductanceΔVFB_VB2 = VB3 = VB4 = 5V, IB2 = IB3 = IB4 =
0.5mA to 5mA (sink)-1-1.75%
Feedback Input Leakage
CurrentIFB_VFB2 = VFB3 = VFB4 = 1.5V0.01100nA
VB2 = VB3 = VB4 = 2.5V1023Driver Sink CurrentIB_VFB2 = VFB3 =
VFB4 = 1.188VVB2 = VB3 = VB4 = 4.0V26mA
NEGATIVE ANALOG GAIN BLOCKFB5 Regulation VoltageVB5 = VOUT - 2V, VOUT = 3.5V, IB5 = 1mA
(source)-20-5+10mV
FB5 to B5 TransconductanceΔVFB5VB5 = 0, IB5 = 0.5mA to 5mA (source)-13-20mV
Feedback Input Leakage
CurrentIFB5VFB5 = -100mV0.01100nA
Driver Source CurrentIB5VFB5 = 200mV, VB5 = VOUT - 2.0V, VOUT =
3.5V1025mA
POWER GOOD (POK)OUT Trip Level (Preset Mode)FB = GND, falling edge, 1% hysteresis (typ)2.8833.12V
FB Trip Level (Adjustable Mode)Falling edge, 1% hysteresis (typ)1.0701.1141.159V
FB2, FB3, FB4 Trip LevelFalling edge, 1% hysteresis (typ)1.0701.1141.159V
FB5 Trip LevelRising edge, 35mV hysteresis (typ)368500632mV
POK Output Low LevelISINK = 1mA0.4V
POK Output High LeakageVPOK = 5V1µA
THERMAL PROTECTION (Note 3)Thermal ShutdownRising temperature160°C
Thermal Shutdown Hysteresis15°C
ELECTRICAL CHARACTERISTICS (continued)(VIN= 12V, ILIM = FB = GND, VBST- VLX= 5V, TA
= 0°C to +85°C. Typical values are at TA= +25°C, unless otherwise noted.)
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
PARAMETERSYMBOLCONDITIONSMINMAXUNITS
GENERALOperating Input Voltage Range
(Note 1)VIN4.528V
MAX18642
Quiescent Supply CurrentIIN
VFB = 0, VOUT = 4V, VFB2
= VFB3 = VFB4 = 1.5V,
VFB5 = -0.1VMAX18653
VL REGULATOROutput VoltageVL6V < VIN < 28V, 0.1mA < ILOAD <20mA4.755.25V
Power-Supply RejectionPSRRVIN = 6V to 28V3%
Undervoltage Lockout Trip LevelVUVLOVL rising, 3% hysteresis (typ)34V
DC-DC CONTROLLEROutput Voltage (Preset Mode)VOUTFB = GND3.2473.380V
Feedback Set Voltage
(Adjustable Mode)VSETFB = COMP1.2111.261V
Current-Sense Amplifier Voltage
GainALIMVIN - VLX = 250mV4.125.68V/V
Current-Limit Threshold
(Internal Mode)VVALLEYVILIM = 5V150350mV
Current-Limit Threshold
(External Mode)VVALLEYVILIM = 2.5V400660mV
MAX186_T160240Switching FrequencyfOSCMAX186_U80120kHz
Maximum Duty CycleDMAX7490%
POSITIVE ANALOG GAIN BLOCKSFB2, FB3, FB4 Regulation
Voltage
VB2 = VB3 = VB4 = 5V, IB2 = IB3 = IB4 =
1mA (sink)1.2151.265V
FB2, FB3, FB4 to B_
TransconductanceΔVFB_VB2 = VB3 = VB4 = 5V, IB2 = IB3 = IB4 =
0.5mA to 5mA (sink)-2.25%
NEGATIVE ANALOG GAIN BLOCKFB5 Regulation VoltageVB5 = VOUT - 2V, VOUT = 3.5V, IB5 = 1mA
(source)-25+10mV
FB5 to B5 TransconductanceΔVFB5VB5 = 0, IB5 = 0.5mA to 5mA (source)-30mV
ELECTRICAL CHARACTERISTICS(VIN= 12V, ILIM = FB = GND, VBST- VLX= 5V, TA
= -40°C to +85°C, unless otherwise noted.) (Note 4)
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Note 1:Connect VL to IN for operation with VIN< 5V.
Note 2:See Output Voltage Selectionsection.
Note 3:The internal 5V linear regulator (VL) powers the thermal shutdown block. Shorting VL to GND disables thermal shutdown.
Note 4:Specifications to -40°C are guaranteed by design, not production tested.
ELECTRICAL CHARACTERISTICS (continued)
(VIN= 12V, ILIM = FB = GND, VBST- VLX= 5V, TA= -40°C to +85°C, unless otherwise noted.) (Note 4)
PARAMETERSYMBOLCONDITIONSMINMAXUNITS
POWER GOOD (POK)
OUT Trip Level (Preset Mode)FB = GND, falling edge, 1% hysteresis (typ)2.853.15V
FB Trip Level
(Adjustable Mode)Falling edge, 1% hysteresis (typ)1.0581.17V
FB2, FB3, FB4 Trip LevelFalling edge, 1% hysteresis (typ)1.0581.17V
FB5 Trip LevelRising edge, 35mV hysteresis (typ)325675mV
EFFICIENCY vs. LOAD CURRENT
(PRESET MODE)MAX1864/65 toc01
LOAD CURRENT (A)
EFFICIENCY (%)
VIN = 6.5V
VIN = 8V
VIN = 12V
VIN = 18V
VIN = 24V
VOUT = 3.3V
OUTPUT VOLTAGE vs. LOAD CURRENT
(PRESET MODE)
MAX1864/65 toc02
LOAD CURRENT (A)
OUTPUT VOLTAGE (V)
EFFICIENCY vs. LOAD CURRENT
(ADJUSTABLE MODE)
MAX1864/65 toc03
LOAD CURRENT (A)
EFFICIENCY (%)
VIN = 6.5V
VIN = 8V
VIN = 12V
VIN = 18V
VIN = 24V
VOUT = 5.0V
Typical Operating Characteristics(Circuit of Figure 1, VIN= 12V, VOUT= 3.3V, TA = +25°C, unless otherwise noted.)
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies2μs/div
SWITCHING WAVEFORMS
(STEP-DOWN CONVERTER)3.35V
1.5A
MAX1864/65 toc07
3.30V
A. VOUT = 3.3V (PRESET), IOUT = 1A, 50mV/div
B. INDUCTOR CURRENT, 500mA/div
C. VLX, 10V/div
VIN = 12V
10V
0.5A
1ms/div
SOFT-STARTMAX1864/65 toc08
A. VL, 5V/div
B. VOUT = 3.3V (PRESET), 2V/div
C. INDUCTOR CURRENT, 1A/div
VIN = 0 TO 12V
POSITIVE LINEAR REGULATOR BASE-
DRIVE CURRENT vs. BASE-DRIVE VOLTAGE
MAX1864/65 toc09
BASE VOLTAGE (V)
BASE-DRIVE SINK CURRENT (mA)
VFB_ = 1.0V
VFB_ = 0.96VREF
B2, B3 AND B4
(MAX1865) ONLY
Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 12V, VOUT= 3.3V, TA = +25°C, unless otherwise noted.)
OUPUT VOLTAGE vs. LOAD CURRENT
(ADJUSTABLE MODE)
MAX1864/65 toc04
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
INTERNAL 5V LINEAR REGULATOR
vs. LOAD CURRENT
MAX1864/65 toc05
LOAD CURRENT (mA)
VL (V)
1ms/div
LOAD TRANSIENT
(STEP-DOWN CONVERTER)3.5V
3.1V
MAX1864/65 toc06
3.3V
A. VOUT = 3.3V (PRESET), 200mV/div
B. IOUT = 10mA TO 1A, 500mA/div
VIN = 12V
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 12V, VOUT= 3.3V, TA = +25°C, unless otherwise noted.)
POSITIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. LOAD CURRENT
(QLDO = 2N3905)
MAX18664/65 toc10
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)1000
VSUP(POS) = 5.0V
VSUP(POS) = 3.3V
POSITIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
(QLDO = 2N3905)
MAX1864/65 toc11
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
IOUT2 = 1mA
IOUT2 = 100mA
POSITIVE LINEAR REGULATOR
POWER-SUPPLY REJECTION RATIO
(QLDO = 2N3905)
MAX1864/65 toc12
FREQUENCY (kHz)
PSRR (dB)
IOUT2 = 50mA
POSITIVE LINEAR REGULATOR
LOAD TRANSIENT
(QLDO = 2W3905)MAX1864/65 toc13
100mA
2.457V
2.467V
A. IOUTZ = 1mA TO 100mA, 50mA/div
B. VOUTZ = 2.5V, 5mV/div
CLDO(POS) = 10μF CERAMIC, VSUP(POS) = 3.3V
CIRCUIT OF FIGURE 1
10μs/div
POSITIVE LINEAR REGUALTOR
OUTPUT VOLTAGE vs. LOAD CURRENT
(QLDO = TIP30)
MAX1864/65 toc14
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
VSUP(POS) = 5.0V
VSUP(POS) = 3.3V
POSITIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
(QLDO = TIP30)
MAX1864/65 toc15
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
IOUT2 = 1mA
IOUT2 = 100mA
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 12V, VOUT= 3.3V, TA = +25°C, unless otherwise noted.)
NEGATIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. LOAD CURRENT
(QLDO = TIP29)
MAX1864/65 toc19
LOAD CURRENT (mA)
OUTPUT VOLTAGE (V)
VSUP(NEG) = -15V
VOUT3 = 5V
NEGATIVE LINEAR REGULATOR
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
(QLDO = TIP29)
MAX1864/65 toc20
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE (V)
ILDO(NEG) = 100mA
ILDO(NEG) = 1mA
POSITIVE LINEAR REGULATOR
POWER-SUPPLY REJECTION RATIO
(QLDO = TIP30)
MAX1864/65 toc16
FREQUENCY (kHz)
PSRR (dB)
IOUT2 = 150mA
POSITIVE LINEAR REGULATOR
LOAD TRANSIENT
(QLDO = TIP30)MAX1864/65 toc17
250mA
2.453V
2.473V
A. IOUT2 = 10mA TO 250mA, 200mA/div
B. VOUT2 = 2.5V, 10mV/div
CLDO(POS) = 10μF CERAMIC, VSUP(POS) = 3.3V
CIRCUIT OF FIGURE 1
10μs26810
NEGATIVE LINEAR REGULATOR BASE-
DRIVE CURRENT vs. BASE-DRIVE VOLTAGEMAX1864/65 toc18
VOUT - VB5 (V)
BASE-DRIVE SOURCE CURRENT (mA)
VFB5 = 250mV
VFB5 = 50mV
VOUT = 5.0V
VOUT = 3.3V
B5 (MAX1865) ONLY
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Pin Description
PIN
MAX1864MAX1865NAMEFUNCTIONPOK
Open-Drain Power-Good Output. POK is low when the output voltage is more than
10% below the regulation point. POK is high impedance when the output is in
regulation. Connect a resistor between POK and VL for logic-level voltages.2COMPCompensation Pin. Connect a series RC to GND to compensate the control loop.
Typical values are 47kΩ and 8.2nF.3OUTRegulated Output Voltage High-Impedance Sense Input. Internally connected to a
resistive-divider and negative gain block (MAX1865).FB
Dual Mode Switching-Regulator Feedback Input. Connect to GND for the preset 3.3V
output. Connect to a resistive-divider from output to FB to GND to adjust the output
voltage between 1.236V and 0.8 ✕ VIN. The feedback set point is 1.236V.B2
Open-Drain Output PNP Transistor Driver (Regulator #2). Internally connected to the
drain of a DMOS. B2 connects to the base of an external PNP pass transistor to form a
positive linear regulator.6FB2
Analog Gain-Block Feedback Input (Regulator #2). Connect to a resistive-divider
between the positive linear regulator’s output and GND to adjust the output voltage.
The feedback set point is 1.24V.B3
Open-Drain Output PNP Transistor Driver (Regulator #3). Internally connected to the
drain of a DMOS. B3 connects to the base of an external PNP pass transistor to form a
positive linear regulator.8FB3
Analog Gain-Block Feedback Input (Regulator #3). Connect to a resistive-divider
between the positive linear regulator’s output and GND to adjust the output voltage.
The feedback set point is 1.24V.B4
Open-Drain Output PNP Transistor Driver (Regulator #4). Internally connected to the
drain of a DMOS. B4 connects to the base of an external PNP pass transistor to form a
positive linear regulator.10FB4
Analog Gain-Block Feedback Input (Regulator #4). Connect to a resistive-divider
between the positive linear regulator’s output and GND to adjust the output voltage.
The feedback set point is 1.24V.
—11B5
Open-Drain Output NPN Transistor Driver (Regulator #5). Internally connected to the
drain of a P-channel MOSFET. B5 connects to the base of an external NPN pass
transistor to form a negative linear regulator.12FB5
Analog Gain-Block Feedback Input (Regulator #5). Connect to a resistive-divider
between the negative linear regulator’s output and a positive reference voltage,
typically one of the positive linear regulator outputs, to adjust the output voltage. The
feedback set point is at GND.
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Detailed DescriptionThe MAX1864/MAX1865 power-supply controllers pro-
vide system power for cable and xDSL modems. The
main step-down DC-DC controller operates in a cur-
rent-mode pulse-width-modulation (PWM) control
scheme to ease compensation requirements and pro-
vide excellent load- and line-transient response.
The MAX1864 includes two analog gain blocks to regu-
late two additional positive auxiliary output voltages,
and the MAX1865 includes four analog gain blocks to
regulate three additional positive and one negative aux-
iliary output voltages. The positive regulator gain blocks
can be used to generate low-voltage rails directly from
the main step-down converter or higher voltages using
coupled windings from the step-down converter. The
negative gain block can be used in conjunction with a
coupled winding to generate -5V, -12V, or -15V.
DC-DC ControllerThe MAX1864/MAX1865 step-down converters use a
pulse-width-modulated (PWM) current-mode control
scheme (Figure 2). An internal transconductance
amplifier establishes an integrated error voltage at the
COMP pin. The heart of the current-mode PWM con-
troller is an open-loop comparator that compares the
integrated voltage-feedback signal against the ampli-
fied current-sense signal plus the slope compensation
ramp. At each rising edge of the internal clock, the
high-side MOSFET turns-on until the PWM comparator
trips or the maximum duty cycle is reached. During this
on-time, current ramps up through the inductor, sourc-
ing current to the output and storing energy in a mag-
netic field. The current-mode feedback system
regulates the peak inductor current as a function of the
output voltage error signal. Since the average inductor
current is nearly the same as the peak inductor current
(assuming that the inductor value is relatively high to
minimize ripple current), the circuit acts as a switch-
mode transconductance amplifier. It pushes the output
LC filter pole, normally found in a voltage-mode PWM,
to a higher frequency. To preserve inner loop stability
and eliminate inductor stair-casing, a slope-compensa-
tion ramp is summed into the main PWM comparator.
During the second-half of the cycle, the high-side MOS-
FET turns off and the low-side N-channel MOSFET turns
on. Now the inductor releases the stored energy as its
current ramps down, providing current to the output.
Therefore, the output capacitor stores charge when the
inductor current exceeds the load current and dis-
charges when the inductor current is lower, smoothing
Pin Description (continued)
PIN
MAX1864MAX1865NAMEFUNCTION13ILIM
Dual Mode Current-Limit Adjustment Input. Connect to VL for the default 250mV
current-limit threshold. In adjustable mode, the current-limit threshold voltage is 1/5th
the voltage present at ILIM. Connect to a resistive-divider between VL and GND to
adjust VILIM between 1V and 2.5V. The logic threshold for switchover to the 250mV
default value is approximately VL - 1V.14GNDGround15DLLow-Side Gate-Driver Output. DL swings between GND and VL.16LXInductor Connection. Used for current sense between IN and LX, and used for current
limit between LX and GND.17DHHigh-Side Gate-Driver Output. DH swings between LX and BST.18BSTBoost Flying Capacitor Connection. Connect BST to the external boost diode and
capacitor as shown in the standard application circuit (Figures 1 and 6).19VL
Internal 5V Linear-Regulator Output. Supplies the IC and powers the DL low-side gate
driver and external boost diode and capacitor. Bypass with a 1µF or greater ceramic
capacitor to GND.20INInput Supply Voltage, 4.5V to 28V. Bypass to GND with a 1µF or greater ceramic
capacitor close to the IC.
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Suppliesthe voltage across the load. Under overload conditions,
when the inductor current exceeds the selected cur-
rent-limit (see the Setting the Current Limitsection), the
high-side MOSFET is not turned on at the rising edge of
the clock and the low-side MOSFET remains on to let
the inductor current ramp down.
The MAX1864/MAX1865 operate in a forced-PWM
mode, so even under light loads the controller main-
tains a constant switching frequency to minimize cross-
regulation errors in applications that use a transformer.
The low-side gate-drive waveform is the complement of
the high-side gate-drive waveform, which causes the
inductor current to reverse under light loads.
Current-Sense AmplifierThe MAX1864/MAX1865s’ current-sense circuit ampli-
fies (AV= 5) the current-sense voltage generated by
the high-side MOSFET’s on-resistance (RDS(ON)✕
IINDUCTOR). This amplified current-sense signal and
the internal slope compensation signal are summed
together (VSUM) and fed into the PWM comparator’s
inverting input. The PWM comparator turns-off the high-
side MOSFET when VSUMexceeds the integrated feed-
back voltage (VCOMP). Place the high-side MOSFET no
further than 5mm from the controller, and connect IN
and LX to the MOSFET using Kelvin sense connections
to guarantee current-sense accuracy and improve
stability.
BST
1μF
CENTRAL CMPSH-3
1μF
CBST
0.1μF
RPOK
100kΩ
CCOMP
COUT
470μF
CIN
470μFVOUT = 3.3V
INPUT
9V TO 18V
OUT
GND
ILIM
POK
COMP
MAX1864
8.2nF
RCOMP
47kΩ
RDH
10ΩRDL
10Ω
RBE2
220Ω
10μFR1
10kΩVOUT2 = 2.5V
300mA
10μF
10kΩ
RBE3
220ΩQ2
10μF
30kΩ
10kΩ
10μF
VOUT3 = 5.0V
100mA
470μF
NIHON EP05Q03L
FB2
FB3
CBE3
4700pF
CBE2
2200pF
NL, NH:INTERNATIONAL RECTIFIER
IRF7303
Q1:TIP30
Q2:2N3905
Figure 1. Standard MAX1864 Application Circuit
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power SuppliesBIAS
VREF
1.114VTHERMAL
SHDN
3.5V
VL LDO
BST
GND
100kΩ
400kΩ
ILIM
0.9 ✕ VL
POK
ENABLE
250mV
AV = 5
500mV
FB5*
B5*
*MAX1865 ONLY
OUT
0.9VREF
FB1
0.9VREF
FB_100mV
OUT
COMP
FB1
SOFT-
START
VREF
1.236V
CLK
SLOPE
COMP
AV = 5
ENABLE
MAX1864
MAX1865
Figure 2. Functional Diagram
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
Current-Limit CircuitThe current-limit circuit employs a unique “valley” cur-
rent-limiting algorithm that uses the low-side MOSFET’s
on-resistance as a sensing element (Figure 3). If the
voltage across the low-side MOSFET (RDS(ON)✕IIN-
DUCTOR) exceeds the current-limit threshold at the
beginning of a new oscillator cycle, the MAX1864/
MAX1865 will not turn on the high-side MOSFET. The
actual peak current is greater than the current-limit
threshold by an amount equal to the inductor ripple
current. Therefore, the exact current-limit characteristic
and maximum load capability are a function of the low-
side MOSFET on-resistance, inductor value, input volt-
age, and output voltage. The reward for this uncertainty
is robust, loss-less overcurrent limiting.
In adjustable mode, the current-limit threshold voltage
is 1/5th the voltage seen at ILIM (IVALLEY= 0.2 ✕VILIM).
Adjust the current-limit threshold by connecting a resis-
tive-divider from VL to ILIM to GND. The current-limit
threshold can be set from 106mV to 530mV, which cor-
responds to ILIM input voltages of 500mV to 2.5V. This
adjustable current limit accommodates MOSFETs with
a wide range of on-resistance characteristics (see
Design Procedure). The current-limit threshold defaults
to 250mV when ILIM is connected to VL. The logic
threshold for switchover to the 250mV default value is
approximately VL - 1V.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors don’t corrupt the cur-
rent-sense signals seen by LX and GND. The IC must
be mounted close to the low-side MOSFET with short
(less than 5mm), direct traces making a Kelvin sense
connection.
Synchronous Rectifier Driver (DL)Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal Schottky catch
diode with a low-resistance MOSFET switch. The
MAX1864/MAX1865 also use the synchronous rectifier
to ensure proper startup of the boost gate-driver circuit
and to provide the current-limit signal.
The DL low-side drive waveform is always the comple-
ment of the DH high-side drive waveform (with con-
trolled dead time to prevent cross-conduction or
“shoot-through”). A dead-time circuit monitors the DL
output and prevents the high-side FET from turning on
until DL is fully off. For the dead-time circuit to work
properly, there must be a low-resistance, low-induc-
tance path from the DL driver to the MOSFET gate.
Otherwise, the sense circuitry in the MAX1864/
MAX1865 will interpret the MOSFET gate as “off” when
gate charge actually remains. Use very short, wide
traces (50mil to 100mil wide if the MOSFET is 1 inch
from the device). The dead time at the other edge (DH
turning off) is determined by a fixed internal delay.
High-Side Gate-Drive Supply (BST)Gate-drive voltage for the high-side N-channel switch is
generated by a flying-capacitor boost circuit (Figure 1).
The capacitor between BST and LX is alternately
charged from the VL supply and placed parallel to the
high-side MOSFET’s gate-source terminals.
On startup, the synchronous rectifier (low-side MOS-
FET) forces LX to ground and charges the boost
capacitor to 5V. On the second half-cycle, the switch-
mode power supply turns on the high-side MOSFET by
closing an internal switch between BST and DH. This
provides the necessary gate-to-source voltage to turn
on the high-side switch, an action that boosts the 5V
gate-drive signal above the battery voltage.
Internal 5V Linear Regulator (VL)All MAX1864/MAX1865 functions, except the current-
sense amplifier, are internally powered from the on-
chip, low-dropout 5V regulator. The maximum regulator
input voltage (VIN) is 28V. Bypass the regulator’s output
(VL) with at least a 1µF ceramic capacitor to GND. The
VIN-to-VL dropout voltage is typically 200mV, so when
VINis less than 5.2V, VL is typically VIN- 200mV.
The internal linear regulator can source up to 20mA to
supply the IC, power the low-side gate driver, charge
the external boost capacitor, and supply small external
loads. When driving particularly large FETs, little or no
regulator current may be available for external loads.
For example, when switched at 200kHz, a large FET
with 40nC total gate charge requires 40nC x 200kHz,
or 8mA.
INDUCTOR CURRENT
IVALLEY
ILOAD()]
TIME
-IPEAK
VOUT
VINfOSC
(VIN - VOUT)IPEAK = IVALLEY +
Figure 3. “Valley” Current-Limit Threshold Point
