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
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 -40°C to
+125°C temperature range and is available in a 20-pin
TSSOP package.
Warning: The MAX5074 is designed to work with
high voltages. Exercise caution.
Applications
IEEE 802.3af PD Power Supplies
Isolated IP Phone Power Supplies
Features
♦ Clamped, Two-Switch Power IC for High Efficiency
♦ 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 0.4Ω 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
High-Efficiency Telecom/Datacom Power Supplies
Ordering Information
48V Input, Isolated Power-Supply Modules
WLAN Access-Point Power Supplies
PART
MAX5074AUP
ADSL Line Cards
ADSL Line-Driver Power Supplies
Distributed Power Systems with 48V Bus
Pin Configuration
TEMP RANGE
PIN-PACKAGE
-40°C to +125°C
20-TSSOP-EP*
*EP = Exposed pad.
Simplified Application Circuit
VIN
TOP VIEW
RTCT 2
19 HVIN
FLTINT 3
18 UVLO
DRNH
QH
DRVH
D1
T1
17 BST
RCFF 4
RAMP 5
CIN
20 INBIAS
REGOUT 1
MAX5074
16 DRNH
15 XFRMRH
OPTO 6
XFRMRH
D3
VOUT
COUT
MAX5074
XFRMRL
14 DRVIN
CSS 7
PPWM 8
13 XFRMRL
GND 9
QL
DRVL
D2
12 SRC
CS 10
11 PGND
SRC
TSSOP
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX5074
General Description
MAX5074
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
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 = +125°C......................................................................0.9A
TJ = +150°C......................................................................0.6A
Continuous Power Dissipation (TA = +70°C)
20-Pin TSSOP-EP (derate 21.7mW/°C above +70°C) ....1.739W
20-Pin TSSOP-EP (θJA) ................................................46°C/W
Operating Temperature Range .........................-40°C to +125°C
Maximum Junction Temperature .....................................+150°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
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 = 1µF, CREGOUT = 2.2µF, RRTCT = 25kΩ, CRTCT = 100pF, CBST = 0.22µF, VCSS = VCS = 0V, VRAMP = VUVLO = 3V,
TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
Input Supply Range
SYMBOL
CONDITIONS
VHVIN
MIN
TYP
11
MAX
UNITS
76
V
OSCILLATOR (RTCT)
PWM Frequency
fS
RRTCT = 25kΩ, CRTCT = 100pF
256
Maximum PWM Duty Cycle
DMAX
RRTCT = 25kΩ, CRTCT = 100pF
47
%
Maximum RTCT Frequency
fRTCTMAX
(Note 2)
1
MHz
0.51 x
VREGOUT
V
0.04 x
VREGOUT
V
±1
µA
RTCT Peak Trip Level
VTH
RTCT Valley Trip Level
RTCT Input Bias Current
RTCT Discharge MOSFET
RDS(ON)
Sinking 20mA
30
RTCT Discharge Pulse Width
kHz
60
Ω
50
ns
110
ns
LOOK-AHEAD LOGIC (PPWM)
PPWM to XFRMRL Output
Propagation Delay
tPPWM
PPWM rising to XFRMRL falling
PPWM Output High
VOH
Sourcing 2mA
PPWM Output Low
VOL
Sinking 2mA
7.0
11.0
V
0.4
V
5.5
V
PWM COMPARATOR (OPTO, RAMP, RCFF)
Common-Mode Range
VCM-PWM
0
Input Offset Voltage
10
Input Bias Current
-2
mV
+2
µA
RAMP to XFRMRL Propagation
Delay
From RAMP (50mV overdrive) rising to
XFRMRL rising
100
ns
Minimum OPTO Voltage
VCSS = 0V, OPTO sinking 2mA
1.47
V
Minimum RCFF Voltage
RCFF sinking 2mA
2.18
V
2
_______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
(VHVIN = 12V, CINBIAS = 1µF, CREGOUT = 2.2µF, RRTCT = 25kΩ, CRTCT = 100pF, CBST = 0.22µF, VCSS = VCS = 0V, VRAMP = VUVLO = 3V,
TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
REGOUT LDO (REGOUT)
REGOUT Voltage Set Point
VREGOUT
INBIAS floating, VHVIN = 11V to 76V
8.3
9.2
VINBIAS = VHVIN = 11V to 76V
9.5
11.0
INBIAS floating, VHVIN = 15V, IREGOUT = 0
to 30mA
V
0.25
REGOUT Load Regulation
V
VINBIAS = VHVIN = 15V, IREGOUT = 0 to
30mA
REGOUT Dropout Voltage
0.25
INBIAS floating, IREGOUT = 30mA
1.25
VINBIAS = VHVIN, IREGOUT = 30mA
1.25
REGOUT Undervoltage Lockout
Threshold
REGOUT rising
REGOUT Undervoltage Lockout
Threshold Hysteresis
REGOUT falling
6.6
7.4
V
V
0.7
V
33
µA
80
µA
SOFT-START (CSS)
Soft-Start Current
ICSS
INTEGRATING FAULT PROTECTION (FLTINT)
FLTINT Source Current
IFLTINT
FLTINT Trip Point
FLTINT rising
FLTINT Hysteresis
2.7
V
0.8
V
INTERNAL POWER MOSFETs (See Figure 1, QH and QL)
On-Resistance
RDS(ON)
VDRVIN = VBST = 9V,
VXFRMRH = VSRC = 0V, IDS = 50mA
Off-State Leakage Current
0.4
-5
Total Gate Charge Per FET
0.8
+5
Ω
µA
15
nC
HIGH-SIDE DRIVER
Low-to-High Delay
Driver delay until FET VGS reaches 0.9 x
(VBST - VXFRMRH) and is fully on
80
ns
High-to-Low Delay
Driver delay until FET VGS reaches 0.1 x
(VBST - VXFRMRH) and is fully off
40
ns
Driver Output Voltage
BST to XFRMRH with high side on
8
V
Low-to-High Delay
Driver delay until FET VGS reaches 0.9 x
VDRVIN and is fully on
80
ns
High-to-Low Delay
Driver delay until FET VGS reaches 0.1 x
VDRVIN and is fully off
40
ns
LOW-SIDE DRIVER
CURRENT-LIMIT COMPARATOR (CS)
Current-Limit Threshold Voltage
VILIM
Current-Limit Input Bias Current
IBILIM
140
0 < VCS < 0.3V
-2
156
172
mV
+2
µA
_______________________________________________________________________________________
3
MAX5074
ELECTRICAL CHARACTERISTICS (continued)
ELECTRICAL CHARACTERISTICS (continued)
(VHVIN = 12V, CINBIAS = 1µF, CREGOUT = 2.2µF, RRTCT = 25kΩ, CRTCT = 100pF, CBST = 0.22µF, VCSS = VCS = 0V, VRAMP = VUVLO = 3V,
TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1)
PARAMETER
Propagation Delay to XFRMRL
SYMBOL
CONDITIONS
MIN
From CS rising (10mV overdrive) to
XFRMRL rising
tdILIM
TYP
MAX
UNITS
160
ns
tPPWMD
200
ns
tPWQB
300
ns
BOOST VOLTAGE CIRCUIT (See Figure 1, QB)
Driver Output Delay
One-Shot Pulse Width
QB RDS(ON)
Sinking 20mA
30
60
Ω
THERMAL SHUTDOWN
Shutdown Temperature
Thermal Hysteresis
TSH
Temperature rising
THYST
+160
°C
15
°C
UNDERVOLTAGE LOCKOUT (UVLO)
UVLO Threshold
VUVLO
UVLO Hysteresis
VHYST
UVLO Input Bias Current
IBUVLO
VUVLO rising
1.14
1.38
140
VUVLO = 3V
-100
V
mV
+100
nA
SUPPLY CURRENT
Supply Current
From VHVIN = 11V to 76V, VCSS = 0V,
VINBIAS = 11V
0.7
2
From VINBIAS = 11V to 76V, VCSS = 0V,
VHVIN = 76V
4.4
6.0
From VHVIN = 76V
Standby Supply Current
mA
7
VUVLO = 0V
1
mA
Note 1: All limits at -40°C 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 = 1µF, CREGOUT = 2.2µF, RRTCT = 25kΩ, CRTCT = 100pF, CBST = 0.22µF, VCSS = VCS = 0V,
VRAMP = VUVLO = 3V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.)
1.2525
1.2500
1.2475
1.2450
280
245
210
4.8
IHVIN
VUVLO = 0V
175
140
4.7
VHVIN = 76V
INBIAS FLOATING
REGOUT = DRVIN
4.6
IHVIN (mA)
1.2550
4
315
STANDBY CURRENT (µA)
1.2575
350
MAX5074 toc02
UVLO RISING
MAX5074 toc01
1.2600
HVIN INPUT CURRENT
vs. TEMPERATURE
STANDBY CURRENT vs. TEMPERATURE
MAX5074 toc03
UNDERVOLTAGE LOCKOUT THRESHOLD
vs. TEMPERATURE
VUVLO (V)
MAX5074
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
4.5
4.4
4.3
105
4.2
70
1.2425
35
1.2400
0
4.1
4.0
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
_______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
VREGOUT (V)
VREGOUT (V)
8.725
8.700
8.675
8.650
8.71
8.69
7.00
6.75
8.67
6.25
8.65
6.00
11
24
37
76
63
TEMPERATURE (°C)
VHVIN (V)
REGOUT VOLTAGE vs. LOAD CURRENT
HVIN AND INBIAS INPUT CURRENT
vs. TEMPERATURE
4.5
4.0
IINBIAS
VHVIN = VINBIAS = 76V
IHVIN (mA)
3.5
8.65
3.0
8.60
10.58
10.57
2.5
2.0
IHVIN
VHVIN = VINBIAS = 76V
1.5
1.0
15
20
25
30
10.54
10.51
10.50
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
TEMPERATURE (°C)
IREGOUT (mA)
REGOUT VOLTAGE vs. INPUT VOLTAGE
10.60
REGOUT VOLTAGE vs. LOAD CURRENT
10.60
MAX5074 toc10
HVIN = INBIAS
10.58
VHVIN = VINBIAS = 15V
10.55
VREGOUT (V)
10
VREGOUT (V)
5
10.55
10.52
0
0
10.56
10.53
0.5
8.55
VHVIN = VINBIAS = 76V
10.59
MAX5074 toc11
8.70
REGOUT VOLTAGE vs. TEMPERATURE
10.60
MAX5074 toc08
VHVIN = 15V
INBIAS FLOATING
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
5.0
MAX5074 toc07
8.75
VREGOUT (V)
50
VREGOUT (V)
-40 -25 -10 5 20 35 50 65 80 95 110 125
FALLING
6.50
8.625
8.600
RISING
7.25
REGOUT UVLO VOLTAGE (V)
8.73
MAX5074 toc06
INBIAS FLOATING
8.750
7.50
MAX5074 toc05
VHVIN = 76V
INBIAS FLOATING
8.775
8.75
MAX5074 toc04
8.800
REGOUT UVLO VOLTAGE
vs. TEMPERATURE
REGOUT VOLTAGE vs. INPUT VOLTAGE
MAX5074 toc09
REGOUT VOLTAGE vs. TEMPERATURE
10.56
10.54
10.50
10.45
10.52
10.50
10.40
11
24
37
50
VHVIN (V)
63
76
0
5
10
15
20
25
30
IREGOUT (mA)
_______________________________________________________________________________________
5
MAX5074
Typical Operating Characteristics (continued)
(VHVIN = 48V, VINBIAS = 15V, CINBIAS = 1µF, CREGOUT = 2.2µF, RRTCT = 25kΩ, CRTCT = 100pF, CBST = 0.22µF, VCSS = VCS = 0V,
VRAMP = VUVLO = 3V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.)
Typical Operating Characteristics (continued)
(VHVIN = 48V, VINBIAS = 15V, CINBIAS = 1µF, CREGOUT = 2.2µF, RRTCT = 25kΩ, CRTCT = 100pF, CBST = 0.22µF, VCSS = VCS = 0V,
VRAMP = VUVLO = 3V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.)
SOFT-START CURRENT
vs. TEMPERATURE
450
400
250
200
2.50
32.25
32.00
31.75
2.00
OPTO
1.75
1.50
31.25
1.25
1.00
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
CURRENT-LIMIT COMPARATOR
THRESHOLD vs. TEMPERATURE
PPWM TO XFRMRL SKEW
vs. TEMPERATURE
157
156
155
154
115
FLTINT CURRENT vs. TEMPERATURE
85
114
113
84
83
112
82
111
110
109
81
80
79
108
78
152
107
77
151
106
76
150
105
153
MAX5074 toc17
HVIN RISING
158
75
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
FLTINT SHUTDOWN
VOLTAGE vs. TEMPERATURE
2.6
0.60
0.55
RDS(ON) (Ω)
VFLTINT (V)
0.65
RISING
2.4
2.3
2.2
2.1
0.70
FALLING
0.50
0.45
0.40
0.35
2.0
0.30
1.9
0.25
1.8
MAX5074 toc19
2.7
2.5
POWER MOSFETS RDS(ON)
vs. TEMPERATURE
MAX5074 toc18
2.8
6
2.25
31.50
31.00
PPWM TO XFRML SKEW (ns)
159
RCFF
32.50
-40 -25 -10 5 20 35 50 65 80 95 110 125
MAX5074 toc15
160
2.75
IFLTINT (µA)
300
RRTCT = 25kΩ
CRTCT = 100pF
32.75
MAX5074 toc16
350
3.00
VRCFF (V), VOPTO (V)
500
33.00
MAX5074 toc13
RRTCT = 12kΩ
CRTCT = 100pF
SOFT-START CURRENT (µA)
OPERATING FREQUENCY (kHz)
550
MAX5074 toc12
600
MINIMUM RCFF AND OPTO LEVELS
vs. TEMPERATURE
MAX5074 toc14
OPERATING FREQUENCY
vs. TEMPERATURE
VREGOUT (mV)
MAX5074
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
0.20
-40 -25 -10 5 20 35 50 65 80 95 110 125
-40 -25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
TEMPERATURE (°C)
_______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
PIN
NAME
FUNCTION
1
REGOUT
2
RTCT
Oscillator Frequency Set Input. Connect a resistor from RTCT to REGOUT and a capacitor from RTCT to
GND to set the oscillator frequency.
3
FLTINT
Fault Integration Input. During persistent current-limit faults, a capacitor connected to FLTINT is charged
with an internal 80µA 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.
4
RCFF
Feed-Forward Input. To generate the PWM ramp, connect a resistor from RCFF to HVIN and a capacitor
from RCFF to GND.
5
RAMP
PWM Ramp Sense Input. Connect RAMP to RCFF.
6
OPTO
PWM Comparator Inverting Input. Connect the collector of the optotransistor to OPTO and a pullup resistor
to REGOUT.
7
CSS
8
PPWM
9
GND
10
CS
11
PGND
12
SRC
13
XFRMRL
14
DRVIN
15
XFRMRH
16
DRNH
17
BST
18
UVLO
Undervoltage Lockout Input. Connect a resistive divider from HVIN to UVLO and from UVLO to GND to set
the UVLO threshold.
19
HVIN
High-Voltage Input. Connect HVIN to the most positive input supply rail.
20
INBIAS
—
EP
Regulator Output. Always present as long as HVIN is powered with a voltage above UVLO threshold.
Bypass REGOUT to GND with a minimum 2.2µF ceramic capacitor.
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 0.1µF 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
0.1µF capacitor from BST to XFRMRH with short and wide PC board traces.
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.
_______________________________________________________________________________________
7
MAX5074
Pin Description
MAX5074
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
Detailed Description
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.
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.
INBIAS 20
REGOK
1 REGOUT
HVIN 19
REG
UVLO 18
OVT
REF
(1.25V)
REFOK
UVLO
CUVLO
4 RCFF
1.25V
5V
5V
IFLT
80µA
Q
PPWM 8
7.5V
D
50Ω
T
R
3 FLTINT
5 RAMP
BST 17
GND
DRNH 16
OVRLD
2.7V/1.9V
R
80ns
DELAY
Q
LEVEL
SHIFT
0.4Ω
QH
XFRMRH 15
CPWM
S
6 OPTO
LEADINGEDGE
DELAY
5V
ONE
SHOT
30Ω
QB
DRVIN 14
33µA
CLK
Q
T-FF
7 CSS
SHDN
R
T
XFRMRL 13
OSC
0.4Ω
THERMAL
SHUTDOWN
OVT
QL
SRC 12
PGND 11
OVT
UVLO
REFOK
REGOK
OVRLD
50Ω
9 GND
MAX5074
GND
RTCT 2
CS 10
ILIM
10MHz
150mV
PGND
Figure 1. Block Diagram
8
_______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
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.
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 × ⎜1 +
⎟
⎝
R25 ⎠
where V UVLOIN is the desired input voltage lockout
level and VUVLO is the undervoltage lockout threshold
(1.25V, typ).
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 2.2µF 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.
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 33µA, 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.
MAX5074
Power Topology
The two-switch forward converter topology offers outstanding robustness against faults and transformer
saturation while affording efficient use of the integrated
0.4Ω 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.
U1
MAX5074
5V
R
U2
PPWM
PGND
PS9715
OR EQUIVALENT
HIGH-SPEED
OPTOCOUPLER
C
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.
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)
where f S is the switching frequency, V R(P-P) is the
peak-to-peak ramp voltage (2V, typ).
_______________________________________________________________________________________
9
MAX5074
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
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.
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 80µA current into
10
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 80µA, 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 ⎠
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 (+160°C), 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.
______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
GND
COMP
LED
FB
E
C10
T1
R10
C7
OPTO
R22
C14
R25
R13
C13
PGND
VIN+
C1
C9
R12
C12
R24
R15
R21
RTCT
UVLO
FLTINT
DRVIN
REGOUT
RCFF
RAMP
CSS
HVIN
C11
DRNH
GND
PPWM
U1
MAX5074
PGND
C20
CS
R23
PGND
SRC
INBIAS
XFRMRL
XFRMRH
BST
C6
R9
R6
C8
D5
D2
D1
C
D3
C19
U2
FOD2712
D4
L1
R3
C15
C17
R1
R2
C18
VOUT
SGND
MAX5074
Figure 3. Typical Application Circuit (48V Power Supply, Evaluation Kit Available)
______________________________________________________________________________________
11
SGND
GND
COMP
R10
C7
OPTO
R22
C14
R25
R13
C13
PGND
VIN+
C1
C9
R12
C12
R24
R15
R21
RTCT
UVLO
FLTINT
DRVIN
REGOUT
RCFF
RAMP
CSS
HVIN
C11
DRNH
GND
PPWM
U1
MAX5074
PGND
C20
CS
R23
PGND
SRC
INBIAS
XFRMRL
XFRMRH
BST
C6
R9
R6
C8
D3
D2
D1
T1
E
C
D4
C19
U2
FOD2712
LED
FB
R3
C15
C17
R1
R2
C11
VOUT
MAX5074
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
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.
Chip Information
TRANSISTOR COUNT: 7043
PROCESS: BiCMOS
12
______________________________________________________________________________________
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
TSSOP 4.4mm BODY.EPS
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
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX5074
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.)
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