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19-3312; Rev 0; 7/04
T AVAILABLE EVALUATION KI
Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications
General Description
The MAX5074 isolated PWM power IC features integrated switching power MOSFETs connected in a voltageclamped, two-transistor, power-circuit configuration. This device can be used in both forward and flyback configurations with a wide input voltage range from 11V to 76V and up to 15W of output power. The voltage-clamped power topology enables full recovery of stored magnetizing and leakage inductive energy for enhanced efficiency and reliability. A lookahead signal for driving secondary-side synchronous rectifiers can be used to increase efficiency. A wide array of protection features includes UVLO, overtemperature shutdown, and short-circuit protection with hiccup current limit for enhanced performance and reliability. Operation up to 500kHz allows smaller external magnetics and capacitors. The MAX5074 is rated for operation over the -40C to +125C temperature range and is available in a 20-pin TSSOP package. Warning: The MAX5074 is designed to work with high voltages. Exercise caution. No Reset Winding Required Up to 15W Output Power Bias Voltage Regulator with Automatic HighVoltage Supply Turn-Off 11V to 76V Wide Input Voltage Range Integrated High-Voltage 0.4 Power MOSFETs Feed-Forward Voltage-Mode Control For Fast Input Transient Rejection Programmable Brownout Undervoltage Lockout Internal Overtemperature Shutdown Indefinite Short-Circuit Protection With Programmable Fault Integration Integrated Look-Ahead Signal for Secondary-Side Synchronous Rectification >90% Efficiency with Synchronous Rectification Up to 500kHz Switching Frequency High-Power (1.74W), Small-Footprint 20-Pin Thermally Enhanced TSSOP Package
Features
Clamped, Two-Switch Power IC for High Efficiency
MAX5074
Applications
IEEE 802.3af PD Power Supplies Isolated IP Phone Power Supplies High-Efficiency Telecom/Datacom Power Supplies 48V Input, Isolated Power-Supply Modules WLAN Access-Point Power Supplies ADSL Line Cards ADSL Line-Driver Power Supplies Distributed Power Systems with 48V Bus
Ordering Information
PART MAX5074AUP TEMP RANGE -40C to +125C PIN-PACKAGE 20-TSSOP-EP*
*EP = Exposed pad.
Pin Configuration
TOP VIEW
REGOUT 1 RTCT 2 FLTINT 3 RCFF 4 RAMP 5 OPTO 6 CSS 7 PPWM 8 GND 9 CS 10 20 INBIAS 19 HVIN 18 UVLO 17 BST
Simplified Application Circuit
VIN CIN DRNH D1 T1 XFRMRH D3 VOUT COUT
DRVH
QH
MAX5074
16 DRNH 15 XFRMRH 14 DRVIN 13 XFRMRL 12 SRC 11 PGND DRVL
MAX5074
XFRMRL QL D2
SRC
TSSOP
________________________________________________________________ Maxim Integrated Products
1
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
ABSOLUTE MAXIMUM RATINGS
HVIN, INBIAS, DRNH, XFRMRH, XFRMRL to GND.................................................-0.3V to +80V BST to GND ............................................................-0.3V to +95V BST to XFRMRH .....................................................-0.3V to +12V PGND to GND .......................................................-0.3V to +0.3V UVLO, RAMP, CSS, OPTO, FLTINT, RCFF, RTCT to GND......................................................-0.3V to +12V SRC, CS to GND.......................................................-0.3V to +6V REGOUT, DRVIN to GND .......................................-0.3V to +12V REGOUT to HVIN ...................................................-80V to +0.3V REGOUT to INBIAS ................................................-80V to +0.3V REGOUT Current ................................................................50mA PPWM to GND....................................-0.3V to (REGOUT + 0.3V) PPWM Current .................................................................20mA DRNH, XFRMRH, XRFMRL, SRC Continuous Current (Average) TJ = +125C......................................................................0.9A TJ = +150C......................................................................0.6A Continuous Power Dissipation (TA = +70C) 20-Pin TSSOP-EP (derate 21.7mW/C above +70C) ....1.739W 20-Pin TSSOP-EP (JA) ................................................46C/W Operating Temperature Range .........................-40C to +125C Maximum Junction Temperature .....................................+150C Storage Temperature Range .............................-60C to +150C Lead Temperature (soldering, 10s) .................................+300C
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
(VHVIN = 12V, CINBIAS = 1F, CREGOUT = 2.2F, RRTCT = 25k, CRTCT = 100pF, CBST = 0.22F, VCSS = VCS = 0V, VRAMP = VUVLO = 3V, TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Input Supply Range OSCILLATOR (RTCT) PWM Frequency Maximum PWM Duty Cycle Maximum RTCT Frequency RTCT Peak Trip Level RTCT Valley Trip Level RTCT Input Bias Current RTCT Discharge MOSFET RDS(ON) RTCT Discharge Pulse Width LOOK-AHEAD LOGIC (PPWM) PPWM to XFRMRL Output Propagation Delay PPWM Output High PPWM Output Low Common-Mode Range Input Offset Voltage Input Bias Current RAMP to XFRMRL Propagation Delay Minimum OPTO Voltage Minimum RCFF Voltage From RAMP (50mV overdrive) rising to XFRMRL rising VCSS = 0V, OPTO sinking 2mA RCFF sinking 2mA -2 100 1.47 2.18 tPPWM VOH VOL VCM-PWM PPWM rising to XFRMRL falling Sourcing 2mA Sinking 2mA 0 10 +2 7.0 110 11.0 0.4 5.5 ns V V V mV A ns V V Sinking 20mA fS DMAX fRTCTMAX VTH RRTCT = 25k, CRTCT = 100pF RRTCT = 25k, CRTCT = 100pF (Note 2) 256 47 1 0.51 x VREGOUT 0.04 x VREGOUT 1 30 50 60 kHz % MHz V V A ns SYMBOL VHVIN CONDITIONS MIN 11 TYP MAX 76 UNITS V
PWM COMPARATOR (OPTO, RAMP, RCFF)
2
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications
ELECTRICAL CHARACTERISTICS (continued)
(VHVIN = 12V, CINBIAS = 1F, CREGOUT = 2.2F, RRTCT = 25k, CRTCT = 100pF, CBST = 0.22F, VCSS = VCS = 0V, VRAMP = VUVLO = 3V, TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER REGOUT LDO (REGOUT) REGOUT Voltage Set Point VREGOUT INBIAS floating, VHVIN = 11V to 76V VINBIAS = VHVIN = 11V to 76V INBIAS floating, VHVIN = 15V, IREGOUT = 0 to 30mA REGOUT Load Regulation VINBIAS = VHVIN = 15V, IREGOUT = 0 to 30mA REGOUT Dropout Voltage REGOUT Undervoltage Lockout Threshold REGOUT Undervoltage Lockout Threshold Hysteresis SOFT-START (CSS) Soft-Start Current FLTINT Source Current FLTINT Trip Point FLTINT Hysteresis INTERNAL POWER MOSFETs (See Figure 1, QH and QL) On-Resistance Off-State Leakage Current Total Gate Charge Per FET HIGH-SIDE DRIVER Low-to-High Delay High-to-Low Delay Driver Output Voltage LOW-SIDE DRIVER Low-to-High Delay High-to-Low Delay CURRENT-LIMIT COMPARATOR (CS) Current-Limit Threshold Voltage Current-Limit Input Bias Current VILIM IBILIM 0 < VCS < 0.3V 140 -2 156 172 +2 mV A Driver delay until FET VGS reaches 0.9 x VDRVIN and is fully on Driver delay until FET VGS reaches 0.1 x VDRVIN and is fully off 80 40 ns ns Driver delay until FET VGS reaches 0.9 x (VBST - VXFRMRH) and is fully on Driver delay until FET VGS reaches 0.1 x (VBST - VXFRMRH) and is fully off BST to XFRMRH with high side on 80 40 8 ns ns V RDS(ON) VDRVIN = VBST = 9V, VXFRMRH = VSRC = 0V, IDS = 50mA -5 15 0.4 0.8 +5 A nC ICSS IFLTINT FLTINT rising 33 80 2.7 0.8 A A V V INTEGRATING FAULT PROTECTION (FLTINT) INBIAS floating, IREGOUT = 30mA VINBIAS = VHVIN, IREGOUT = 30mA REGOUT rising REGOUT falling 6.6 0.7 0.25 1.25 1.25 7.4 V V V 8.3 9.5 9.2 11.0 0.25 V V SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX5074
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
ELECTRICAL CHARACTERISTICS (continued)
(VHVIN = 12V, CINBIAS = 1F, CREGOUT = 2.2F, RRTCT = 25k, CRTCT = 100pF, CBST = 0.22F, VCSS = VCS = 0V, VRAMP = VUVLO = 3V, TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Propagation Delay to XFRMRL SYMBOL tdILIM CONDITIONS From CS rising (10mV overdrive) to XFRMRL rising MIN TYP 160 MAX UNITS ns
BOOST VOLTAGE CIRCUIT (See Figure 1, QB) Driver Output Delay One-Shot Pulse Width QB RDS(ON) THERMAL SHUTDOWN Shutdown Temperature Thermal Hysteresis UVLO Threshold UVLO Hysteresis UVLO Input Bias Current SUPPLY CURRENT From VHVIN = 11V to 76V, VCSS = 0V, VINBIAS = 11V Supply Current From VINBIAS = 11V to 76V, VCSS = 0V, VHVIN = 76V From VHVIN = 76V Standby Supply Current VUVLO = 0V 0.7 4.4 7 1 mA 2 6.0 mA UNDERVOLTAGE LOCKOUT (UVLO) VUVLO VHYST IBUVLO VUVLO = 3V -100 VUVLO rising 1.14 140 +100 1.38 V mV nA TSH THYST Temperature rising +160 15 C C tPPWMD tPWQB Sinking 20mA 200 300 30 60 ns ns
Note 1: All limits at -40C are guaranteed by design and not production tested. Note 2: Output switching frequency is half of oscillator frequency.
Typical Operating Characteristics
(VHVIN = 48V, VINBIAS = 15V, CINBIAS = 1F, CREGOUT = 2.2F, RRTCT = 25k, CRTCT = 100pF, CBST = 0.22F, VCSS = VCS = 0V, VRAMP = VUVLO = 3V, TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C, unless otherwise noted.)
UNDERVOLTAGE LOCKOUT THRESHOLD vs. TEMPERATURE
MAX5074 toc01
STANDBY CURRENT vs. TEMPERATURE
MAX5074 toc02
HVIN INPUT CURRENT vs. TEMPERATURE
4.7 4.6 IHVIN (mA) 4.5 4.4 4.3 4.2 4.1 4.0 VHVIN = 76V INBIAS FLOATING REGOUT = DRVIN
MAX5074 toc03
1.2600 UVLO RISING 1.2575 1.2550 VUVLO (V) 1.2525 1.2500 1.2475 1.2450 1.2425 1.2400
350 315 STANDBY CURRENT (A) 280 245 210 175 140 105 70 35 0 IHVIN VUVLO = 0V
4.8
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
4
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications
Typical Operating Characteristics (continued)
(VHVIN = 48V, VINBIAS = 15V, CINBIAS = 1F, CREGOUT = 2.2F, RRTCT = 25k, CRTCT = 100pF, CBST = 0.22F, VCSS = VCS = 0V, VRAMP = VUVLO = 3V, TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C, unless otherwise noted.)
REGOUT VOLTAGE vs. TEMPERATURE
MAX5074 toc04
MAX5074
REGOUT VOLTAGE vs. INPUT VOLTAGE
MAX5074 toc05
REGOUT UVLO VOLTAGE vs. TEMPERATURE
MAX5074 toc06 MAX5074 toc09
8.800 8.775 8.750 VREGOUT (V) VHVIN = 76V INBIAS FLOATING
8.75 INBIAS FLOATING 8.73 VREGOUT (V)
7.50 7.25 REGOUT UVLO VOLTAGE (V) 7.00 6.75 6.50 6.25 6.00 FALLING RISING
8.725 8.700 8.675 8.650 8.625 8.600 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
8.71
8.69
8.67
8.65 11 24 37 50 63 76 VHVIN (V)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
REGOUT VOLTAGE vs. LOAD CURRENT
MAX5074 toc07
HVIN AND INBIAS INPUT CURRENT vs. TEMPERATURE
MAX5074 toc08
REGOUT VOLTAGE vs. TEMPERATURE
10.60 10.59 10.58 10.57 VREGOUT (V) 10.56 10.55 10.54 10.53 10.52 10.51 10.50 VHVIN = VINBIAS = 76V
8.75 VHVIN = 15V INBIAS FLOATING 8.70 VREGOUT (V)
5.0 4.5 4.0 3.5 IHVIN (mA) 3.0 2.5 2.0 1.5 1.0 0.5 IHVIN VHVIN = VINBIAS = 76V IINBIAS VHVIN = VINBIAS = 76V
8.65
8.60
8.55 0 5 10 15 IREGOUT (mA) 20 25 30
0 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
REGOUT VOLTAGE vs. INPUT VOLTAGE
MAX5074 toc10
REGOUT VOLTAGE vs. LOAD CURRENT
VHVIN = VINBIAS = 15V 10.55 VREGOUT (V)
MAX5074 toc11
10.60 HVIN = INBIAS 10.58 VREGOUT (V)
10.60
10.56
10.50
10.54
10.52
10.45
10.50 11 24 37 50 63 76 VHVIN (V)
10.40 0 5 10 15 IREGOUT (mA) 20 25 30
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
Typical Operating Characteristics (continued)
(VHVIN = 48V, VINBIAS = 15V, CINBIAS = 1F, CREGOUT = 2.2F, RRTCT = 25k, CRTCT = 100pF, CBST = 0.22F, VCSS = VCS = 0V, VRAMP = VUVLO = 3V, TA = TJ = -40C to +125C, unless otherwise noted. Typical values are at TA = +25C, unless otherwise noted.)
OPERATING FREQUENCY vs. TEMPERATURE
MAX5074 toc12
SOFT-START CURRENT vs. TEMPERATURE
MAX5074 toc13
MINIMUM RCFF AND OPTO LEVELS vs. TEMPERATURE
2.75 RCFF 2.50 VRCFF (V), VOPTO (V) 2.25 2.00 1.75 1.50 1.25 1.00 OPTO
MAX5074 toc14 MAX5074 toc17
600 550 OPERATING FREQUENCY (kHz) 500 450 400 350 300 250 200 RRTCT = 25k CRTCT = 100pF RRTCT = 12k CRTCT = 100pF
33.00 32.75 SOFT-START CURRENT (A) 32.50 32.25 32.00 31.75 31.50 31.25 31.00
3.00
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
CURRENT-LIMIT COMPARATOR THRESHOLD vs. TEMPERATURE
159 158 157 VREGOUT (mV) 156 155 154 153 152 151 150 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C) HVIN RISING
MAX5074 toc15
PPWM TO XFRMRL SKEW vs. TEMPERATURE
114 PPWM TO XFRML SKEW (ns) 113 112 111 110 109 108 107 106 105 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C) IFLTINT (A)
MAX5074 toc16
FLTINT CURRENT vs. TEMPERATURE
85 84 83 82 81 80 79 78 77 76 75 -40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
160
115
FLTINT SHUTDOWN VOLTAGE vs. TEMPERATURE
MAX5074 toc18
POWER MOSFETS RDS(ON) vs. TEMPERATURE
0.65 0.60 0.55 RDS(ON) () 0.50 0.45 0.40 0.35 0.30 0.25 0.20
MAX5074 toc19
2.8 2.7 2.6 2.5 VFLTINT (V) 2.4 2.3 2.2 2.1 2.0 1.9 1.8 FALLING RISING
0.70
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
-40 -25 -10 5 20 35 50 65 80 95 110 125 TEMPERATURE (C)
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications
Pin Description
PIN 1 2 NAME REGOUT RTCT FUNCTION Regulator Output. Always present as long as HVIN is powered with a voltage above UVLO threshold. Bypass REGOUT to GND with a minimum 2.2F ceramic capacitor. Oscillator Frequency Set Input. Connect a resistor from RTCT to REGOUT and a capacitor from RTCT to GND to set the oscillator frequency. Fault Integration Input. During persistent current-limit faults, a capacitor connected to FLTINT is charged with an internal 80A current source. Switching is terminated when VFLTINT reaches 2.7V. An external resistor connected in parallel discharges the capacitor. Switching resumes when VFLTINT drops to 1.9V. Feed-Forward Input. To generate the PWM ramp, connect a resistor from RCFF to HVIN and a capacitor from RCFF to GND. PWM Ramp Sense Input. Connect RAMP to RCFF. PWM Comparator Inverting Input. Connect the collector of the optotransistor to OPTO and a pullup resistor to REGOUT. Soft-Start and Reference. Connect a 10nF or greater capacitor from CSS to GND. PWM Pulse Output. PPWM leads the internal power MOSFET pulse by approximately 100ns. Signal Ground. Connect GND to PGND. Current-Sense Input. The current-limit threshold is internally set to 156mV relative to PGND. The device has an internal noise filter. If necessary, connect an external RC filter for additional filtering. Power Ground. Connect PGND to GND. Internal Low-Side Power MOSFET Source. Connect SRC to PGND with a low-value resistor for current limiting. Low-Side Connection for the Isolation Transformer MOSFET Gate-Driver Supply Input. Bypass DRVIN with at least 0.1F to PGND. Connect DRVIN to REGOUT. High-Side Connection for the Isolation Transformer Drain Connection of the Internal High-Side PWM Power MOSFET. Connect DRNH to the most positive rail of the input supply. Bypass DRNH appropriately to handle the heavy switching current through the transformer. Boost Input. BST is the boost connection point for the high-side MOSFET driver. Connect a minimum 0.1F capacitor from BST to XFRMRH with short and wide PC board traces. Undervoltage Lockout Input. Connect a resistive divider from HVIN to UVLO and from UVLO to GND to set the UVLO threshold. High-Voltage Input. Connect HVIN to the most positive input supply rail. Input from the Rectified Bias Winding. INBIAS is an input to the internal linear voltage regulator (REGOUT). Exposed Paddle. EP is internally connected to GND. Connect the exposed paddle to a copper pad to improve power dissipation.
MAX5074
3
FLTINT
4 5 6 7 8 9 10 11 12 13 14 15 16
RCFF RAMP OPTO CSS PPWM GND CS PGND SRC XFRMRL DRVIN XFRMRH DRNH
17 18 19 20 --
BST UVLO HVIN INBIAS EP
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
Detailed Description
The MAX5074 PWM power IC is the primary-side controller for voltage-mode, isolated forward or flyback power converters. This device provides a high degree of integration aimed at reducing the cost and printed circuit board area of isolated output power supplies. Use the MAX5074 primarily in 48V power bus applications. The MAX5074 is a complete power IC capable of delivering up to 15W of output power. This device contains PWM circuitry and integrated power MOSFETs. Figure 1 shows the MAX5074's block diagram. The MAX5074 includes undervoltage lockout, overtemperature shutdown, and short-circuit protection for enhanced performance and reliability. Operation up to 500kHz allows the use of small external magnetics and capacitors.
REGOK 1 REGOUT REG OVT REFOK 4 RCFF 1.25V 5V IFLT 80A Q D T R 3 FLTINT 5 RAMP 2.7V/1.9V OVRLD R Q GND 80ns DELAY LEVEL SHIFT 0.4 5V REF (1.25V)
INBIAS 20 HVIN 19 UVLO 18 UVLO CUVLO
PPWM 8 7.5V 50 BST 17 DRNH 16 QH XFRMRH 15 S LEADINGEDGE DELAY CLK Q R T-FF T SHDN OSC 0.4 THERMAL SHUTDOWN OVT QL SRC 12 PGND 11 ONE SHOT 30 QB DRVIN 14 XFRMRL 13
CPWM 6 OPTO 5V
33A 7 CSS
9 GND
50
OVT UVLO REFOK REGOK OVRLD GND ILIM 10MHz
RTCT 2 CS 10
MAX5074
150mV PGND
Figure 1. Block Diagram 8 _______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications
Power Topology
The two-switch forward converter topology offers outstanding robustness against faults and transformer saturation while affording efficient use of the integrated 0.4 power MOSFETs. Voltage-mode control with feed-forward compensation allows the rejection of input supply disturbances within a single cycle similar to that of current-mode controlled topologies. The two-switch power topology recovers energy stored in both the magnetizing and the parasitic leakage inductances of the transformer. The Typical Application Circuit, forward converter (Figure 3) shows the schematic diagram of a 48V input and 5V, 3A output isolated power supply. Figure 4 shows the schematic diagram of a flyback converter using the MAX5074.
MAX5074
U1
MAX5074
R PPWM PGND PS9715 OR EQUIVALENT HIGH-SPEED OPTOCOUPLER U2
5V
C
Undervoltage Lockout (UVLO)
The UVLO block monitors the input voltage HVIN through an external resistive divider (R24 and R25) connected to UVLO (see Figure 3). Use the following equation to calculate R24 and R25: R24 VUVLOIN = VUVLO x 1 + R25 where V UVLOIN is the desired input voltage lockout level and VUVLO is the undervoltage lockout threshold (1.25V, typ).
Figure 2. Secondary-Side Synchronous Rectifier Driver Using a High-Speed Optocoupler
Secondary-Side Synchronization
The MAX5074 provides convenient synchronization for optional secondary-side synchronous rectifiers. Figure 2 shows the connection diagram with a high-speed optocoupler. Choose an optocoupler with a propagation delay of less than 80ns. The synchronizing pulse is generated approximately 110ns ahead of the main pulse that drives the two power MOSFETs.
Internal Regulators
As soon as power is provided to HVIN, internal power supplies power the UVLO detection circuitry. REGOUT is used to drive the internal power MOSFETs. Bypass REGOUT with a minimum 2.2F ceramic capacitor. The HVIN LDO steps down VHVIN to a nominal output voltage (REGOUT) of 8.75V. A second parallel LDO powers REGOUT from INBIAS. A tertiary winding connected through a diode to INBIAS powers up REGOUT once switching commences. This will bring REGOUT to 10.5V (typ) and shut off the current flowing from HVIN to REGOUT. This results in a lower on-chip power dissipation and higher efficiency.
Voltage-Mode Control and the PWM Ramp
For voltage-mode control, the feed-forward PWM ramp is generated at RCFF. From RCFF, connect a capacitor to GND and a resistor to HVIN. The ramp generated is applied to the noninverting input of the PWM comparator at RAMP and has a minimum voltage of approximately 2V. The slope of the ramp is determined by the voltage at HVIN and affects the overall loop gain. The ramp peak must remain below the dynamic range of RCFF of 5.5V. Assuming the maximum duty cycle approaches 50% at a minimum input voltage (PWM UVLO turn-on threshold), use the following formula to calculate the minimum value of either the ramp capacitor or resistor: RRCFFCRCFF VUVLOIN 2fS VR(P - P)
Soft-Start
Program the MAX5074 soft-start with an external capacitor between CSS and GND. When the device turns on, the soft-start capacitor (CCSS) charges with a constant current of 33A, ramping up to 7.3V. During this time, the feedback pin (OPTO) is clamped to VCSS + 0.6V. This initially holds the duty cycle lower than the value the regulator tries to impose, thus preventing voltage overshoot at the output. When the MAX5074 turns off, the soft-start capacitor internally discharges to GND.
where f S is the switching frequency, V R(P-P) is the peak-to-peak ramp voltage (2V, typ).
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9
Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
Maximize the signal-to-noise ratio by setting the ramp peak as high as possible. Calculate the low-frequency, small-signal gain of the power stage (the gain from the inverting input of the PWM comparator to the output) using the following formula: GPS = NSP x RRCFF x CRCFF x fS where NSP is the secondary to primary power transformer turns ratio. FLTINT for one clock every time the current-limit comparator ILIM (Figure 1) trips. Use the following formula to calculate the approximate capacitor needed for the desired shutdown time: I t CFLTINT FLTINT SH 1.4 where IFLTINT is typically 80A, and tSH is the desired ignore time during which current-limit events from the current-limit comparator are ignored. This is an approximate formula; some testing may be required to fine tune the actual value of the capacitor. Calculate the approximate bleed resistor needed for the desired recovery time using the following formula: RFLTINT t RT 2.7 CFLTINT ln 1.9
Oscillator
The MAX5074 oscillator is externally programmable through a resistor connected from RTCT to REGOUT and a capacitor connected from RTCT to GND. The PWM frequency will be 1/2 the frequency at RTCT with a 50% duty cycle. Use the following formula to calculate the oscillator components: RRTCT 1 VREGOUT 2fS (CRTCT + CPCB )ln VREGOUT - VTH
where CPCB is the stray capacitance on the PC board (14pF, typ), VTH is the RTCT peak trip level, and fS is the switching frequency.
Integrating Fault Protection
The integrating fault protection feature allows the MAX5074 to ignore transient overcurrent conditions for a programmable amount of time, giving the power supply time to behave like a current source to the load. This can happen, for example, under load-current transients when the control loop requests maximum current to keep the output voltage from going out of regulation. Program the ignore time externally by connecting a capacitor to FLTINT. Under sustained overcurrent faults, the voltage across this capacitor ramps up toward the FLTINT shutdown threshold (typically 2.7V). When FLTINT reaches the threshold, the power supply shuts down. A highvalue bleed resistor connected in parallel with the FLTINT capacitor allows the capacitor to discharge toward the restart threshold (typically 1.9V). Crossing the restart threshold soft-starts the supply again. The ILIM comparator provides cycle-by-cycle current limiting with a typical threshold of 156mV. The fault integration circuit works by forcing an 80A current into
where tRT is the desired recovery time. Choose at least tRT = 10 x tSH. Typical values for tSH range from a few hundred microseconds to a few milliseconds.
Shutdown
Shut down the MAX5074 by driving UVLO to GND using an open-collector or open-drain transistor connected to GND. The IC will be internally shut down if REGOUT is below its UVLO level. The MAX5074 also features internal thermal shutdown using a temperature sensor that monitors the high-power area. A thermal fault arises from excessive dissipation in the power MOSFETs or in the regulator. When the temperature limit is reached (+160C), the temperature sensor terminates switching and shuts down the regulator. The integration of thermal shutdown and the power MOSFETs results in a very robust power circuit.
Applications Information
Isolated Telecom Power Supply
Figure 3 shows a typical application circuit of an isolated power supply with a 30V to 60V input. This power supply is fully protected and can sustain a continuous short circuit at its output terminals.
10
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Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
SGND C18 VOUT
C17
R1
R2 LED FB C15 COMP U2 FOD2712 C8 PGND R22 R6 R21 PGND C E
L1
R3
D4
D3
D1
T1
D2
C6
R9
D5
C7
C10
BST
XFRMRH
XFRMRL
INBIAS
SRC
CS
R23
PPWM
MAX5074
U1
DRNH
GND
OPTO
PGND
HVIN
CSS
REGOUT
FLTINT
DRVIN
RAMP
UVLO
RCFF
RTCT
R24
R25
R15
C12
R12
Figure 3. Typical Application Circuit (48V Power Supply, Evaluation Kit Available) ______________________________________________________________________________________ 11
VIN+
C1
C9
C13
R13
C14
C11
C20
R10
C19
GND
Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications MAX5074
C11 SGND VOUT
C17
R1
R2 LED FB C15 COMP U2 FOD2712 C8 PGND R22 R6 R21 PGND C E
R3
D1
T1
D4
D2
C6
R9
D3
C7
BST
XFRMRH
XFRMRL
INBIAS
SRC
CS
R23
PPWM
MAX5074
U1
DRNH
GND
OPTO
PGND
HVIN
CSS
REGOUT
FLTINT
DRVIN
RAMP
UVLO
RCFF
RTCT
R24
R25
R15
C12
R12
Figure 4. For lower power applications, the MAX5074 can be used in a flyback converter configuration. This eliminates the need for an output inductor and simplifies the design of multiple output power supplies.
VIN+
C1
C9
C13
R13
C14
C11
C20
R10
C19
GND
Chip Information
TRANSISTOR COUNT: 7043 PROCESS: BiCMOS
12 ______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE 802.3af PD and Telecom Power-Supply Applications
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
MAX5074
PACKAGE OUTLINE, TSSOP, 4.40 MM BODY EXPOSED PAD
21-0108
D
1
1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
TSSOP 4.4mm BODY.EPS

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