Rainbow Electronics MAX5074 User Manual

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 -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

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

Features

♦ Clamped, Two-Switch Power IC for High Efficiency

♦ No Reset Winding Required

♦ Up to 15W Output Power

♦ Bias Voltage Regulator with Automatic High-

Voltage 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

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

________________________________________________________________ Maxim Integrated Products 1

Pin Configuration

Ordering Information

MAX5074

DRNH

SRC

DRVL

DRVH

QH

QL

D2

D1

D3

T1

V

IN

C

IN

C

OUT

V

OUT

XFRMRH

XFRMRL

Simplified Application Circuit

19-3312; Rev 0; 7/04

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.

*EP = Exposed pad.

EVALUATION KIT AVAILABLE

20

INBIAS

19

HVIN

18

UVLO

17

BSTRCFF

MAX5074

TSSOP

DRNH

16

XFRMRH

15

14

DRVIN

XFRMRLPPWM

13

12

SRC

11

PGNDCS

TOP VIEW

1

REGOUT

2

RTCT

3

FLTINT

4

RAMP

5

OPTO

6

CSS

7

8

9

GND

10

PART TEMP RANGE PIN-PACKAGE

MAX5074AUP -40°C to +125°C 20-TSSOP-EP*

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(V

HVIN

= 12V, C

INBIAS

= 1µF, C

REGOUT

= 2.2µF, R

RTCT

= 25kΩ, C

RTCT

= 100pF, C

BST

= 0.22µF, V

CSS

= VCS= 0V, V

RAMP

= V

UVLO

= 3V,

T

A

= TJ= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)

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.

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)

T

J

= +125°C......................................................................0.9A

T

J

= +150°C......................................................................0.6A

Continuous Power Dissipation (T

A

= +70°C)

20-Pin TSSOP-EP (derate 21.7mW/°C above +70°C) ....1.739W

20-Pin TSSOP-EP (θ

JA

) ................................................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

Input Supply Range V

OSCILLATOR (RTCT)

PWM Frequency f

Maximum PWM Duty Cycle D

Maximum RTCT Frequency f

RTCT Peak Trip Level V

RTCT Valley Trip Level

RTCT Input Bias Current ±1 µA

RTCT Discharge MOSFET
R

DS(ON)

RTCT Discharge Pulse Width 50 ns

LOOK-AHEAD LOGIC (PPWM)

PPWM to XFRMRL Output
Propagation Delay

PPWM Output High V

PPWM Output Low V

PWM COMPARATOR (OPTO, RAMP, RCFF)

Common-Mode Range V

Input Offset Voltage 10 mV

Input Bias Current -2 +2 µA

RAMP to XFRMRL Propagation
Delay

Minimum OPTO Voltage V

Minimum RCFF Voltage RCFF sinking 2mA 2.18 V

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

HVIN

S

MAX

RTCTMAX

TH

t

PPWM

OH

OL

CM-PWM

R

= 25kΩ, C

RTCT

R

= 25kΩ, C

RTCT

(Note 2) 1 MHz

Sinking 20mA 30 60 Ω

PPWM rising to XFRMRL falling 110 ns

Sourcing 2mA 7.0 11.0 V

Sinking 2mA 0.4 V

From RAMP (50mV overdrive) rising to
XFRMRL rising

= 0V, OPTO sinking 2mA 1.47 V

CSS

RTCT

RTCT

= 100pF 256 kHz

= 100pF 47 %

11 76 V

0.51 x

V

REGOUT

0.04 x

V

REGOUT

0 5.5 V

100 ns

V

V

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(V

HVIN

= 12V, C

INBIAS

= 1µF, C

REGOUT

= 2.2µF, R

RTCT

= 25kΩ, C

RTCT

= 100pF, C

BST

= 0.22µF, V

CSS

= VCS= 0V, V

RAMP

= V

UVLO

= 3V,

T

A

= 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 V

REGOUT Load Regulation

REGOUT Dropout Voltage

REGOUT Undervoltage Lockout
Threshold

REGOUT Undervoltage Lockout
Threshold Hysteresis

SOFT-START (CSS)

Soft-Start Current I

INTEGRATING FAULT PROTECTION (FLTINT)

FLTINT Source Current I

FLTINT Trip Point FLTINT rising 2.7 V

FLTINT Hysteresis 0.8 V

INTERNAL POWER MOSFETs (See Figure 1, QH and QL)

On-Resistance R

Off-State Leakage Current -5 +5 µA

Total Gate Charge Per FET 15 nC

HIGH-SIDE DRIVER

Low-to-High Delay

High-to-Low Delay

Driver Output Voltage BST to XFRMRH with high side on 8 V

LOW-SIDE DRIVER

Low-to-High Delay

High-to-Low Delay

CURRENT-LIMIT COMPARATOR (CS)

Current-Limit Threshold Voltage V

Current-Limit Input Bias Current I

REGOUT

CSS

FLTINT

DS(ON

ILIM

BILIM

INBIAS floating, V

V

= V

INBIAS

INBIAS floating, V
to 30mA

V

= V

INBIAS

30mA

INBIAS floating, I

= V

V

INBIAS

= 11V to 76V 8.3 9.2

HVIN

= 11V to 76V 9.5 11.0

HVIN

= 15V, I

HVIN

= 15V, I

HVIN

HVIN

, I

REGOUT

= 30mA 1.25

REGOUT

= 30mA 1.25

REGOUT

REGOUT

= 0 to

= 0

0.25

0.25

REGOUT rising 6.6 7.4 V

REGOUT falling 0.7 V

33 µA

80 µA

V

= V

DRVIN

V

XFRMRH

Driver delay until FET V

- V

(V

BST

Driver delay until FET V

- V

(V

BST

Driver delay until FET V
V

DRVIN

Driver delay until FET V
V

DRVIN

= 9V,

BST

= V

= 0V, IDS = 50mA

SRC

) and is fully on

XFRMRH

) and is fully off

XFRMRH

and is fully on

and is fully off

reaches 0.9 x

GS

reaches 0.1 x

GS

reaches 0.9 x

GS

reaches 0.1 x

GS

0.4 0.8 Ω

80 ns

40 ns

80 ns

40 ns

140 156 172 mV

0 < VCS < 0.3V -2 +2 µA

V

V

V

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS (continued)

(V

HVIN

= 12V, C

INBIAS

= 1µF, C

REGOUT

= 2.2µF, R

RTCT

= 25kΩ, C

RTCT

= 100pF, C

BST

= 0.22µF, V

CSS

= VCS= 0V, V

RAMP

= V

UVLO

= 3V,

T

A

= TJ= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C, unless otherwise noted.) (Note 1)

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.

UNDERVOLTAGE LOCKOUT THRESHOLD

vs. TEMPERATURE

MAX5074 toc01

TEMPERATURE (°C)

V

UVLO

(V)

11095-25 -10 5 35 50 6520 80

1.2425

1.2450

1.2475

1.2500

1.2525

1.2550

1.2575

1.2600

1.2400

-40 125

UVLO RISING

STANDBY CURRENT vs. TEMPERATURE

MAX5074 toc02

TEMPERATURE (°C)

STANDBY CURRENT (µA)

1109565 80-10 5 20 35 50-25

35

70

105

140

175

210

245

280

315

350

0

-40 125

I

HVIN

V

UVLO =

0V

HVIN INPUT CURRENT

vs. TEMPERATURE

MAX5074 toc03

TEMPERATURE (°C)

I

HVIN

(mA)

11095-25 -10 5 35 50 6520 80

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.0

-40 125

V

HVIN

= 76V
INBIAS FLOATING
REGOUT = DRVIN

Typical Operating Characteristics

(V

HVIN

= 48V, V

INBIAS

= 15V, C

INBIAS

= 1µF, C

REGOUT

= 2.2µF, R

RTCT

= 25kΩ, C

RTCT

= 100pF, C

BST

= 0.22µF, V

CSS

= VCS= 0V,

V

RAMP

= V

UVLO

= 3V, TA = TJ= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C, unless otherwise noted.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Propagation Delay to XFRMRL t

BOOST VOLTAGE CIRCUIT (See Figure 1, QB)

Driver Output Delay t

One-Shot Pulse Width t

QB R

DS(ON)

THERMAL SHUTDOWN

Shutdown Temperature T

Thermal Hysteresis T

UNDERVOLTAGE LOCKOUT (UVLO)

UVLO Threshold V

UVLO Hysteresis V

UVLO Input Bias Current I

SUPPLY CURRENT

Supply Current

Standby Supply Current V

dILIM

PPWMD

PWQB

SH

HYST

UVLO

HYST

BUVLO

From CS rising (10mV overdrive) to
XFRMRL rising

160 ns

200 ns

300 ns

Sinking 20mA 30 60 Ω

Temperature rising +160 °C

15 °C

V

rising 1.14 1.38 V

UVLO

140 mV

V

= 3V -100 +100 nA

UVLO

From V
V

INBIAS

From V
V

HVIN

From V

UVLO

= 11V to 76V, V

HVIN

= 11V

= 11V to 76V, V

INBIAS

= 76V

= 76V 7

HVIN

CSS

CSS

= 0V,

= 0V,

0.7 2

4.4 6.0

= 0V 1 mA

mA

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

_______________________________________________________________________________________ 5

Typical Operating Characteristics (continued)

(V

HVIN

= 48V, V

INBIAS

= 15V, C

INBIAS

= 1µF, C

REGOUT

= 2.2µF, R

RTCT

= 25kΩ, C

RTCT

= 100pF, C

BST

= 0.22µF, V

CSS

= VCS= 0V,

V

RAMP

= V

UVLO

= 3V, TA = TJ= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C, unless otherwise noted.)

REGOUT VOLTAGE vs. TEMPERATURE

8.800
V

= 76V

HVIN

INBIAS FLOATING

8.775

8.750

8.725

(V)

8.700

REGOUT

V

8.675

8.650

8.625

8.600

-40 125

REGOUT VOLTAGE vs. LOAD CURRENT

8.75
V

= 15V

HVIN

INBIAS FLOATING

8.70

(V)

8.65

REGOUT

V

8.60

8.55

030

REGOUT VOLTAGE vs. INPUT VOLTAGE

8.75
INBIAS FLOATING

MAX5074 toc04

8.73

8.71

(V)

REGOUT

V

8.69

8.67

8.65

11095-25 -10 5 35 50 6520 80

TEMPERATURE (°C)

11 76

HVIN AND INBIAS INPUT CURRENT

5.0

4.5

MAX5074 toc07

4.0

3.5

3.0

(mA)

2.5

HVIN

I

2.0

1.5

1.0

0.5

0

-40 125

I

REGOUT

252015105

(mA)

vs. TEMPERATURE

TEMPERATURE (°C)

V

HVIN

I

HVIN

V

HVIN

I
V

(V)

INBIAS

= V

HVIN

= V

INBIAS

INBIAS

63503724

= 76V

= 76V

1109565 80-10 5 20 35 50-25

7.50

7.25

MAX5074 toc05

7.00

6.75

6.50

REGOUT UVLO VOLTAGE (V)

6.25

6.00

-40 125

REGOUT VOLTAGE vs. TEMPERATURE

10.60

10.59

MAX5074 toc08

10.58

10.57

10.56

(V)

10.55

REGOUT

V

10.54

10.53

10.52

10.51

10.50

-40 125

REGOUT UVLO VOLTAGE

vs. TEMPERATURE

RISING

FALLING

TEMPERATURE (°C)

V

= V

HVIN

= 76V

INBIAS

TEMPERATURE (°C)

MAX5074 toc06

1109580655035205-10-25

MAX5074 toc09

1109565 80-10 5 20 35 50-25

REGOUT VOLTAGE vs. INPUT VOLTAGE

10.60

HVIN = INBIAS

10.58

10.56

(V)

REGOUT

V

10.54

10.52

10.50

11 76

V

(V)

HVIN

MAX5074 toc10

63503724

REGOUT VOLTAGE vs. LOAD CURRENT

10.60

V

= V

INBIAS

= 15V

I

REGOUT

MAX5074 toc11

252015105

(mA)

HVIN

10.55

(V)

10.50

REGOUT

V

10.45

10.40

030

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(V

HVIN

= 48V, V

INBIAS

= 15V, C

INBIAS

= 1µF, C

REGOUT

= 2.2µF, R

RTCT

= 25kΩ, C

RTCT

= 100pF, C

BST

= 0.22µF, V

CSS

= VCS= 0V,

V

RAMP

= V

UVLO

= 3V, TA = TJ= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C, unless otherwise noted.)

OPERATING FREQUENCY

vs. TEMPERATURE

MAX5074 toc12

TEMPERATURE (°C)

OPERATING FREQUENCY (kHz)

11095-25 -10 5 35 50 6520 80

250

300

350

400

450

500

550

600

200

-40 125

R

RTCT

= 12kΩ

C

RTCT

= 100pF

R

RTCT

= 25kΩ

C

RTCT

= 100pF

SOFT-START CURRENT

vs. TEMPERATURE

MAX5074 toc13

TEMPERATURE (°C)

SOFT-START CURRENT (µA)

11095-25 -10 5 35 50 6520 80

31.25

31.50

31.75

32.00

32.25

32.50

32.75

33.00

31.00

-40 125

MINIMUM RCFF AND OPTO LEVELS

vs. TEMPERATURE

MAX5074 toc14

TEMPERATURE (°C)

V

RCFF

(V), V

OPTO

(V)

11095-25 -10 5 35 50 6520 80

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

1.00

-40 125

OPTO

RCFF

CURRENT-LIMIT COMPARATOR
THRESHOLD vs. TEMPERATURE

MAX5074 toc15

TEMPERATURE (°C)

V

REGOUT

(mV)

1109565 80-10 5 20 35 50-25

151

152

153

154

155

156

157

158

159

160

150

-40 125

HVIN RISING

PPWM TO XFRMRL SKEW

vs. TEMPERATURE

MAX5074 toc16

TEMPERATURE (°C)

PPWM TO XFRML SKEW (ns)

1109565 80-10 5 20 35 50-25

106

107

108

109

110

111

112

113

114

115

105

-40 125

FLTINT CURRENT vs. TEMPERATURE

MAX5074 toc17

TEMPERATURE (°C)

I

FLTINT

(µA)

1109565 80-10 5 20 35 50-25

76

77

78

79

80

81

82

83

84

85

75

-40 125

FLTINT SHUTDOWN

VOLTAGE vs. TEMPERATURE

MAX5074 toc18

TEMPERATURE (°C)

V

FLTINT

(V)

1109565 80-10 5 20 35 50-25

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

1.8

-40 125

FALLING

RISING

POWER MOSFETS R

DS(ON)

vs. TEMPERATURE

MAX5074 toc19

TEMPERATURE (°C)

R

DS(ON)

(Ω)

1109565 80-10 5 20 35 50-25

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.20

-40 125

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

_______________________________________________________________________________________ 7

Pin Description

PIN NAME FUNCTION

1 REGOUT

2 RTCT

3 FLTINT

4 RCFF

5 RAMP PWM Ramp Sense Input. Connect RAMP to RCFF.

6 OPTO

7 CSS Soft-Start and Reference. Connect a 10nF or greater capacitor from CSS to GND.

8 PPWM PWM Pulse Output. PPWM leads the internal power MOSFET pulse by approximately 100ns.

9 GND Signal Ground. Connect GND to PGND.

10 CS

11 PGND Power Ground. Connect PGND to GND.

12 SRC

13 XFRMRL Low-Side Connection for the Isolation Transformer

14 DRVIN

15 XFRMRH High-Side Connection for the Isolation Transformer

16 DRNH

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.

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 80µA current source. Switching is terminated when V
resistor connected in parallel discharges the capacitor. Switching resumes when V

Feed-Forward Input. To generate the PWM ramp, connect a resistor from RCFF to HVIN and a capacitor
from RCFF to GND.

PWM Comparator Inverting Input. Connect the collector of the optotransistor to OPTO and a pullup resistor
to REGOUT.

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.

Internal Low-Side Power MOSFET Source. Connect SRC to PGND with a low-value resistor for current
limiting.

MOSFET Gate-Driver Supply Input. Bypass DRVIN with at least 0.1µF to PGND. Connect DRVIN to
REGOUT.

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.

reaches 2.7V. An external

FLTINT

FLTINT

drops to 1.9V.

17 BST

18 UVLO

19 HVIN High-Voltage Input. Connect HVIN to the most positive input supply rail.

20 INBIAS Input from the Rectified Bias Winding. INBIAS is an input to the internal linear voltage regulator (REGOUT).

—EP

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.

Undervoltage Lockout Input. Connect a resistive divider from HVIN to UVLO and from UVLO to GND to set
the UVLO threshold.

Exposed Paddle. EP is internally connected to GND. Connect the exposed paddle to a copper pad to
improve power dissipation.

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.

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

8 _______________________________________________________________________________________

Figure 1. Block Diagram

IFLT

50Ω

REGOK

CPWM

OVRLD

GND

REG

OVT

REFOK

5V

7.5V

D

Q

T

R

CLK

OVT
UVLO
REFOK
REGOK
OVRLD

GND

50Ω

Q

R

T-FF

14REGOUT

RCFF

80µA

3

FLTINT

5

RAMP

6

OPTO

33µA

79CSS

GND

5V

2.7V/1.9V

5V

MAX5074

REF

(1.25V)

R

S

T

Q

ILIM

SHDN

OSC

UVLO

CUVLO

THERMAL

SHUTDOWN

80ns

DELAY

10MHz

OVT

1.25V

LEVEL
SHIFT

LEADING-

EDGE

DELAY

ONE

SHOT

30Ω

0.4Ω

QB

0.4Ω

QH

QL

INBIAS

HVIN

UVLO

PPWM

BST

DRNH

XFRMRH

DRVIN

XFRMRL

SRC

PGND

RTCT

20

19

18

8

17

16

15

14

13

12

11

2

CS

10

150mV

PGND

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.

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:

where V

UVLOIN

is the desired input voltage lockout

level and V

UVLO

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 V

HVIN

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 (C

CSS

) charges with a constant
current of 33µA, ramping up to 7.3V. During this time,
the feedback pin (OPTO) is clamped to V

CSS

+ 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.

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:

where fSis the switching frequency, V

R(P-P)

is the

peak-to-peak ramp voltage (2V, typ).

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

_______________________________________________________________________________________ 9

Figure 2. Secondary-Side Synchronous Rectifier Driver Using a
High-Speed Optocoupler

R

24

⎞
⎟

⎠

R

25

VV

UVLOIN UVLO

=×+

⎛

1

⎜
⎝

U1

MAX5074

R

PPWM

PGND

PS9715
OR EQUIVALENT
HIGH-SPEED
OPTOCOUPLER

5V

U2

C

RC

RCFF RCFF

V

≥

fV

SRPP

UVLOIN

−2()

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= N

SP xRRCFF xCRCFF xfS

where NSPis 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:

where C

PCB

is the stray capacitance on the PC board
(14pF, typ), VTHis the RTCT peak trip level, and fSis
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

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:

where I

FLTINT

is typically 80µA, and tSHis 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:

where tRTis the desired recovery time.

Choose at least tRT= 10 x tSH. Typical values for t

SH

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

10 ______________________________________________________________________________________

R

RTCT

≅

2( )ln

1

⎛

V

fC C

S RTCT PCB

+

REGOUT

⎜

VV

⎝

REGOUT TH

−

C

FLTINT

⎞
⎟

⎠

R

FLTINT

It

FLTINT SH

≅

≅

C

FLTINT

14.

t

RT

..27

⎛

⎞

ln

⎜

⎟

⎝

⎠

19

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

______________________________________________________________________________________ 11

Figure 3. Typical Application Circuit (48V Power Supply, Evaluation Kit Available)

OUT

V

C18

SGND

C17

L1

D4

D3

T1

D1

C6

BST

XFRMRH

D2

D5

R9

INBIAS

XFRMRL

U1

R3 R1

C19

C10

C7

R10

R23

C20

R2

C15

FB

LED

E

C8

PGND

U2

R6

COMP

FOD2712

GND

C

MAX5074

R22

HVIN DRNH PPWM

C14 C11

RTCT SRCCSS GND OPTO PGND CS

UVLO

FLTINT

DRVIN

REGOUT

RCFF

RAMP

R15

R24

R12

VIN+

R25

C12

C9

C1

R13

C13

R21

PGND

MAX5074

Chip Information

TRANSISTOR COUNT: 7043

PROCESS: BiCMOS

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

12 ______________________________________________________________________________________

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.

OUT

V

D4

T1

D1

C6

BST

XFRMRH

U1

XFRMRL

MAX5074

SGND

C11

C17

R3 R1

C19

D2

D3

R9

C7

INBIAS

R10

R23

C20

R2

C15

FB

LED

E

C8

PGND

R22

U2

FOD2712

R6

COMP

GND

C

HVIN DRNH PPWM

VIN+

R24

R12

RAMP

RCFF

REGOUT

DRVIN

FLTINT

UVLO

C1

C14 C11

RTCT SRCCSS GND OPTO PGND CS

R15

R25

C12

C9

R13

C13

PGND

R21

MAX5074

Power IC with Integrated MOSFETs for Isolated IEEE

802.3af PD and Telecom Power-Supply Applications

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.

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

.)

TSSOP 4.4mm BODY.EPS

PACKAGE OUTLINE, TSSOP, 4.40 MM BODY
EXPOSED PAD

21-0108

1

D

1