MAX5015EVKIT+ - Maxim Integrated

General Description

The MAX5014/MAX5015 integrate all the building

blocks necessary for implementing DC-DC fixed-fre-

quency isolated power supplies. These devices are

current-mode controllers with an integrated high-volt-

age startup circuit suitable for isolated telecom/industri-

al voltage range power supplies. Current-mode control

with leading-edge blanking simplifies control-loop

design and internal ramp compensation circuitry stabi-

lizes the current loop when operating at duty cycles

above 50% (MAX5014). The MAX5014 allows 85%

operating duty cycle and could be used to implement

flyback converters, whereas the MAX5015 limits the

operating duty cycle to less than 50% and can be used

in single-ended forward converters. A high-voltage

startup circuit allows these devices to draw power

directly from the 18V to 110V input supply during start-

up. The switching frequency is internally trimmed to

275kHz ±10%, thus reducing magnetics and filter com-

ponent costs.

The MAX5014/MAX5015 are available in 8-pin SO

packages. An evaluation kit (MAX5015EVKIT) is also

available.

Warning: The MAX5014/MAX5015 are designed to

operate with high voltages. Exercise caution.

Applications

Telecom Power Supplies

Industrial Power Supplies

Networking Power Supplies

Isolated Power Supplies

Features

♦Wide Input Range: (18V to 110V) or (13V to 36V)

♦Current-Mode Control

♦Leading-Edge Blanking

♦Internally Trimmed 275kHz ±10% Oscillator

♦Low External Component Count

♦Soft-Start

♦High-Voltage Startup Circuit

♦Pulse-by-Pulse Current Limiting

♦Thermal Shutdown

♦SO-8 Package

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

________________________________________________________________ Maxim Integrated Products 1

GND

VCC

NDRVVDD

OPTO

V+

8-SO

TOP VIEW

MAX5014/

MAX5015

SS_SHDN

Pin Configuration

Ordering Information

MAX5015

VDD V+

SS_SHDN

NDRV

GND

VCC

OPTO

VIN VOUT

OPTOCOUPLER

Typical Operating Circuit

19-2082; Rev 0; 7/01

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.

EVALUATION KIT

AVAILABLE

PART TEMP. RANGE PIN-PACKAGE

MAX5014CSA* 0°C to +70°C8-SO

MAX5014ESA* -40°C to +85°C8-SO

MAX5015CSA* 0°C to +70°C8-SO

MAX5015ESA* -40°C to +85°C8-SO

*See Selector Guide at end of data sheet.

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected to SS_SHDN, NDRV = open cir-

cuit, OPTO = GND, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°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.

V+ to GND……………………………………...……-0.3V to +120V

VDD to GND.………………………………….……….-0.3V to +40V

VCC to GND.………………….……………………-0.3V to +12.5V

OPTO, NDRV, SS_SHDN, CS to GND .……-0.3V to VCC + 0.3V

VDD and VCC Current …………………...…………………..20mA

NDRV Current Continuous...………………………………….25mA

NDRV Current for Less than 1µs..………….…………….……±1A

Continuous Power Dissipation (TA= +70°C)

8-Pin SO (derate 5.88mW/°C above +70°C) .………....471mW

Operating Temperature Range…………..……...-40°C to +85°C

Storage Temperature Range……………..…….-65°C to +150°C

Lead Temperature (soldering, 10s) ………………………+300°C

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

SUPPLY CURRENT

IV+

(

)

VDD = 0, V+ = 110V, driver not switching 0.85 1.3

V+ Supply Current IV+(S) V+ = 110V, VDD = 0, VOPTO = 4V,

driver switching 1.4 2.6 mA

V+ Supply Current After Startup V+ = 110V, VDD = 13V, VOPTO = 4V 11 µA

IVDD

(

)

VDD = 36V, driver not switching 0.9 1.3

D D

S up p l y C ur r ent IVDD

(

)

VDD = 36V, driver switching, VOPTO = 4V 1.9 2.7 mA

V+ Shutdown Current VSS_SHDN = 0, V+ = 110V 190 290 µA

VDD Shutdown Current VSS_SHDN = 0 8 20 µA

PREREGULATOR/STARTUP

V+ Input Voltage 18 110 V

VDD Supply Voltage 13 36 V

INTERNAL REGULATORS (VCC)

Powered from V+, ICC = 7.5mA, VDD = 0 7.5 9.8 12 V

VCC Output Voltage Powered from VDD, ICC = 7.5mA 9.0 10.0 11.0 V

VCC Undervoltage Lockout VCC_UVLO VCC falling 6.6 V

OUTPUT DRIVER

Peak Source Current VCC = 11V, (externally forced) 570 mA

Peak Sink Current VCC = 11V, (externally forced) 1000 mA

NRDV High-Side Driver

Resistance ROH VCC = 11V, externally forced,

NDRV sourcing 50mA 412Ω

NDRV Low Side Driver

Resistance ROL VCC = 11V, externally forced,

NDRV sinking 50mA 1.6 4 Ω

PWM COMPARATOR

OPTO Input Bias C ur r ent VOPTO = VSS_SHDN -1.00 1.00 µA

OPTO Control Range 23V

Slope Compensation VSCOMP MAX5014 26 mV/µs

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected to SS_SHDN, NDRV = open cir-

cuit, OPTO = GND, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

THERMAL SHUTDOWN

Thermal Shutdown Temperature 150 °C

Thermal Hysteresis 25 °C

CURRENT LIMIT

CS Threshold Voltage VILIM VOPTO = 4V 419 465 510 mV

CS Input Bias Current 0 ≤ VCS ≤ 2V, VOPTO = 4V -1 1 µA

Current Limit Comparator

Propagation Delay 25mV overdrive on CS, VOPTO = 4V 180 ns

CS Blanking Time VOPTO = 4V 70 ns

OSCILLATOR

Clock Frequency Range VOPTO = 4V 247 275 302 kHz

MAX5014, VOPTO = 4V 75 85

Max Duty Cycle MAX5015, VOPTO = 4V 44 50 %

SOFT-START

SS Source Current ISSO VSS_SHDN = 0 2.0 4.6 6.5 µA

SS Sink Current 1.0 mA

Peak Soft-Start Voltage Clamp No external load 2.331 2.420 2.500 V

VSS_SHDN falling 0.25 0.37 0.41

Shutdown Threshold VSS_SHDN rising 0.53 0.59 0.65 V

Typical Operating Characteristics

(V+ = 48V, VDD = 13V, NRDV is open circuit, TA= +25°C, unless otherwise noted.)

0.999

1.000

1.001

1.002

1.003

-40 0-20 20 40 60 80

VSS_SHDN vs. TEMPERATURE

(AT THE END OF SOFT-START)

MAX5014 toc01

TEMPERATURE (°C)

OPTO = CS = GND

VSS_SHDN (V) (NORMALIZED TO VREF = 2.4V)

273

274

276

275

277

278

-40 0-20 20 40 60 80

NDRV FREQUENCY vs.

TEMPERATURE

MAX5014 toc02

TEMPERATURE (°C)

NDRV FREQUENCY (kHz)

VOPTO = 4V, CS = GND

80.4

80.6

80.5

80.8

80.7

80.9

81.0

-40 20 40-20 0 60 80

MAX5014 MAXIMUM DUTY CYCLE

vs. TEMPERATURE

MAX5014 toc03

TEMPERATURE (°C)

MAXIMUM DUTY CYCLE (%)

VOPTO = 4V, CS = GND

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

4 _______________________________________________________________________________________

46.8

47.2

47.0

47.6

47.4

47.8

48.0

-40 20 40-20 0 60 80

MAX5015 MAXIMUM DUTY CYCLE

vs. TEMPERATURE

MAX5014 toc04

TEMPERATURE (°C)

MAX DUTY CYCLE (%)

VOPTO = 4V, CS = GND

V+ SUPPLY CURRENT

vs. TEMPERATURE

MAX5014 toc05

1.38

1.39

1.41

1.40

1.44

1.45

1.43

1.42

1.46

V+ INPUT CURRENT (mA)

-40 0 20-20 40 60 80

TEMPERATURE (°C)

VOPTO = 4V, VDD = CS = GND

4.50

4.65

4.60

4.55

4.75

4.70

4.95

4.90

4.85

4.80

5.00

-40-200 20406080

SOFT-START SOURCE CURRENT

vs. TEMPERATURE

MAX5014 toc06

TEMPERATURE (°C)

SOFT-START SOURCE CURRENT (µA)

VDD = 0, V+ = 110V, OPTO =

CS = SS_SHDN = GND

11.00

11.05

11.15

11.10

11.20

11.25

-40 0-20 20 40 60 80

V+ INPUT CURRENT vs.

TEMPERATURE (AFTER STARTUP)

MAX5014 toc07

TEMPERATURE (°C)

V+ INPUT CURRENT (µA)

V+ = 110V, VOPTO = 4V,

CS = GND, VDD = 13V

185

188

187

186

190

189

194

193

192

191

195

-40 -20 0 20 40 60 80

V+ SHUTDOWN CURRENT

vs. TEMPERATURE

MAX5014 toc08

TEMPERATURE (°C)

V+ SHUTDOWN CURRENT (µA)

V+ = 110V, OPTO = SS_SHDN =

CS = GND, VDD = 13V

0.483

0.484

0.486

0.485

0.487

0.488

-40 0-20 20 40 60 80

CS THRESHOLD VOLTAGE

vs. TEMPERATURE

MAX5014 toc09

TEMPERATURE (°C)

CS THRESHOLD VOLTAGE (V)

VOPTO = 4V, V+ = 110V

NDRV RESISTANCE

vs. TEMPERATURE

MAX5014 toc10

1.0

1.5

2.5

2.0

4.0

4.5

3.5

3.0

5.0

NDRV RESISTANCE (Ω)

-40 0 20-20 40 60 80

TEMPERATURE (°C)

HIGH-SIDE DRIVER

LOW-SIDE DRIVER

170

176

174

172

180

178

188

186

184

182

190

-40 -20 0 20 40 60 80

CURRENT-LIMIT DELAY

vs. TEMPERATURE

MAX5014 toc11

TEMPERATURE (°C)

CURRENT LIMIT DELAY (ns)

VOPTO = 4V, 100mV OVERDRIVE ON CS

2.400

2.402

2.406

2.404

2.408

2.410

010155 2025303540

VSS_SHDN vs. VDD

MAX5014 toc12

VDD (V)

VSS_SHDN (V)

Typical Operating Characteristics (continued)

(V+ = 48V, VDD = 13V, NRDV is open circuit, TA= +25°C, unless otherwise noted.)

_______________________________________________________________________________________ 5

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

Typical Operating Characteristics (continued)

(V+ = 48V, VDD = 13V, NRDV is open circuit, TA= +25°C, unless otherwise noted.)

267.0

268.0

267.5

269.0

268.5

269.5

270.0

270.5

271.0

010155 2025303540

NDRV FREQUENCY vs. VDD

MAX5014 toc13

VDD (V)

NDRV FREQUENCY (kHz)

VOPTO = 4V, CS = GND

47.0

47.2

47.1

47.4

47.3

47.6

47.5

47.7

47.9

47.8

48.0

010155 2025303540

MAX5015 MAXIMUM DUTY

CYCLE vs. VDD

MAX5014 toc14

VDD (V)

MAXIMUM DUTY CYCLE (%)

VOPTO = 4V, CS = GND

DRIVER POWERED

FROM V+

DRIVER POWERED

FROM VDD

9.5

9.6

9.8

9.7

10.0

10.1

9.9

10.2

010155 2025303540

VCC vs. VDD

MAX5014 toc15

VDD (V)

VCC (V)

DEVICE POWERED FROM VDD

DEVICE POWERED

FROM V+

1.31

1.33

1.32

1.36

1.35

1.34

1.39

1.38

1.37

1.40

04020 60 80 100

V+ SUPPLY CURRENT vs.

V+ VOLTAGE

MAX5014 toc16

V+ VOLTAGE (V)

V+ SUPPLY CURRENT (mA)

VOPTO = 4V, CS = GND, VDD = 0

0403010 20 50 60 70 80 90 110

V+ INPUT CURRENT vs. VOLTAGE

(AFTER STARTUP)

MAX5014 toc17

V+ VOLTAGE (V)

V+ INPUT CURRENT (µA)

100

VOPTO = 4V, CS = GND, VDD = 13V

9.0

9.4

9.2

9.8

9.6

10.2

10.0

10.4

VCC VOLTAGE vs. VCC CURRENT

MAX5014 toc18

VCC CURRENT (mA)

VCC VOLTAGE (V)

0 5.0 10.0 15.0 20.0

V+ = 110V, OPTO = CS = GND

VDD = 36V

VDD = 13V

9.0

9.3

9.2

9.1

9.4

9.5

9.6

9.7

9.8

9.9

10.0

0 5.0 10.0 15.0 20.0

VCC VOLTAGE vs. VCC CURRENT

MAX5014 toc19

VCC CURRENT (mA)

VCC VOLTAGE (V)

VDD = OPTO = CS = GND

V+ = 110V

V+ = 90V

V+ = 72V

V+ = 48V

V+ = 36V

V+ = 24V

MAX5014/MAX5015

Detailed Description

Use the MAX5014/MAX5015 PWM current-mode con-

trollers to design flyback- or forward-mode power sup-

plies. Current-mode operation simplifies control-loop

design while enhancing loop stability. An internal high-

voltage startup regulator allows the device to connect

directly to the input supply without an external startup

resistor. Current from the internal regulator starts the

controller. Once the tertiary winding voltage is estab-

lished the internal regulator is switched off and bias

current for running the IC is derived from the tertiary

winding. The internal oscillator is set to 275kHz and

trimmed to ±10%. This permits the use of small mag-

netic components to minimize board space. Both the

MAX5014 and MAX5015 can be used in power sup-

plies providing multiple output voltages. A functional

diagram of the IC is shown in Figure 1. Typical applica-

tions circuits for forward and flyback topologies are

shown in Figure 2 and Figure 3, respectively.

Current-Mode Control

The MAX5014/MAX5015 offer current-mode control

operation with added features such as leading-edge

blanking with dual internal path that only blanks the

sensed current signal applied to the input of the PWM

comparator. The current limit comparator monitors the

CS pin at all times and provides cycle-by-cycle current

limit without being blanked. The leading-edge blanking

of the CS signal prevents the PWM comparator from

prematurely terminating the on cycle. The CS signal

contains a leading-edge spike that is the result of the

MOSFET gate charge current, capacitive and diode

reverse recovery current of the power circuit. Since this

leading-edge spike is normally lower than the current

limit comparator threshold, current limiting is not

blanked and cycle-by-cycle current limiting is provided

under all conditions.

Use the MAX5014 in discontinuous flyback applications

where wide line voltage and load current variation is

expected. Use the MAX5015 for single transistor for-

ward converters where the maximum duty cycle must

be limited to less than 50%.

Under certain conditions it may be advantageous to

use a forward converter with greater than 50% duty

cycle. For those cases use the MAX5014. The large

duty cycle results in much lower operating primary

RMS currents through the MOSFET switch and in most

cases a smaller output filter inductor. The major disad-

vantage to this is that the MOSFET voltage rating must

be higher and that slope compensation must be provid-

ed to stabilize the inner current loop. The MAX5014

provides internal slope compensation.

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

6 _______________________________________________________________________________________

Pin Description

PIN NAME FUNCTION

1V+

High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects

internally to a high-voltage linear regulator that generates VCC during startup.

VDD is the Input of the Linear Regulator that Generates VCC. For supply voltages less than 36V, VDD

and V+ can both be connected to the supply. For supply voltages greater than 36V, VDD receives

its power from the tertiary winding of the transformer and accepts voltages from 13V to 36V. Bypass

to GND with a 4.7µF capacitor.

3 OPTO Optocoupler Input. The control voltage range on this input is 2V to 3V.

4 SS_SHDN

Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 0.45ms/nF.

This pin is also the reference voltage output. Bypass with a minimum 10nF capacitor to GND. The

device goes into shutdown when VSS_SHDN is pulled below 0.25V.

5CS

Current Sense Input. Turns power switch off if VCS rises above 465mV for cycle-by-cycle current

limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM

comparator through a leading edge blanking circuit.

6 GND Ground

7 NDRV Gate Drive. Drives a high-voltage external N-channel power MOSFET.

Regulated IC Supply. Provides power for the entire IC. VCC is regulated from VDD during normal

operation and from V+ during startup. Bypass VCC with a 10µF tantalum capacitor in parallel with

0.1µF ceramic capacitor to GND.

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

_______________________________________________________________________________________ 7

HIGH-

VOLTAGE

REGULATOR

EN OUT

BIAS

WINDING

REGULATOR

EN OUT

SLOPE

COMPENSATION

26mV/µs

275kHz

OSCILLATOR

70ns

BLANKING

80%/50%

DUTY CYCLE

CLAMP

ILIM

BUF

UVLO

∑

GND

V+

VDD

OPTO

SS_SHDN

PWM

VDD-OK

VCC

NDRV

4µA

5kΩ

2.4V

6.6V

0.7V

125mV

0.4V

26mV/µs

MAX5014 ONLY

Figure 1. Functional Diagram

Optocoupled Feedback

Isolated voltage feedback is achieved by using an opto-

coupler and a shunt regulator as shown in Figure 2. The

output voltage set point accuracy is a function of the

accuracy of the shunt regulator and feedback resistor-

divider tolerance.

Internal Regulators

The internal regulators of the MAX5014/MAX5015

enable initial startup without a lossy startup resistor and

regulate the voltage at the output of a tertiary (bias)

winding to provide power for the IC. At startup V+ is

regulated down to VCC to provide bias for the device.

The VDD regulator then regulates from the output of the

tertiary winding to VCC. This architecture allows the ter-

tiary winding to only have a small filter capacitor at its

output thus eliminating the additional cost of a filter

inductor.

When designing the tertiary winding calculate the num-

ber of turns so the minimum reflected voltage is always

higher than 12.7V. The maximum reflected voltage

must be less than 36V.

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

8 _______________________________________________________________________________________

MAX5015

VDD V+

SS_SHDN

NDRV

GND

VCC

OPTO

VIN

(36V TO 72V) VOUT

OPTOCOUPLER

CDD

47µF

CSS

0.1µF

3kΩ

4.75kΩ

CCC

10µF

100Ω

RSENSE

100mΩ

20Ω

25.5kΩ

8.25kΩ

COUT

3 × 560µF0.1µF

CIN

3 × 0.47µF

4.7µH

0.1µF

240kΩ

NTNR

NPNS

IRF640N

4.7nF

250VAC

TLV431

1N4148

CMHD2003

14 51nF

SBL204OCT

5V/10A

220Ω

Figure 2. Forward Converter

To reduce power dissipation the high-voltage regulator

is disabled when the VDD voltage reaches 12.7V. This

greatly reduces power dissipation and improves effi-

ciency. If VCC falls below the undervoltage lockout

threshold (VCC = 6.6V), the low-voltage regulator is dis-

abled, and soft-start is reinitiated. In undervoltage lock-

out the MOSFET driver output (NDRV) is held low.

If the input voltage range is between 13V and 36V, V+

and VDD may be connected to the line voltage provid-

ed that the maximum power dissipation is not exceed-

ed. This eliminates the need for a tertiary winding.

Undervoltage Lockout (UVLO), Soft-Start,

and Shutdown

The soft-start feature of the MAX5014/MAX5015 allows

the load voltage to ramp up in a controlled manner,

thus eliminating output voltage overshoot.

While the part is in UVLO, the capacitor connected to

the SS_SHDN pin is discharged. Upon coming out of

UVLO an internal current source starts charging the

capacitor to initiate the soft-start cycle. Use the follow-

ing equation to calculate total soft-start time:

_______________________________________________________________________________________ 9

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

MAX5014

VDD V+

SS_SHDN

NDRV

GND

VCC

OPTO

VIN

VOUT

OPTOCOUPLER

CDD

CSS CCC

100Ω

RSENSE

COUT

CIN

NPNS

4.7nF

250VAC

TLV431

220Ω

Figure 3. Flyback Converter

MAX5014/MAX5015

where CSS is the soft-start capacitor as shown in Figure 2.

Operation begins when VSS_SHDN ramps above 0.6V.

When soft-start has completed, VSS_SHDN is regulated

to 2.4V, the internal voltage reference. Pull VSS_SHDN

below 0.25V to disable the controller.

Undervoltage lockout shuts down the controller when

VCC is less than 6.6V. The regulators for V+ and the ref-

erence remain on during shutdown.

Current-Sense Comparator

The current-sense (CS) comparator and its associated

logic limit the peak current through the MOSFET.

Current is sensed at CS as a voltage across a sense

resistor between the source of the MOSFET and GND.

To reduce switching noise, connect CS to the external

MOSFET source through a 100Ωresistor or an RC low-

pass filter (Figures 2, 3). Select the current-sense resis-

tor, RSENSE according to the following equation:

where ILimPrimary is the maximum peak primary-side

current.

When VCS > 465mV, the power MOSFET switches off.

The propagation delay from the time the switch current

reaches the trip level to the driver turn-off time is 170ns.

PWM Comparator and Slope Compensation

An internal 275kHz oscillator determines the switching

frequency of the controller. At the beginning of each

cycle, NDRV switches the N-channel MOSFET on.

NDRV switches the external MOSFET off after the maxi-

mum duty cycle has been reached, regardless of the

feedback.

The MAX5014 uses an internal ramp generator for

slope compensation. The internal ramp signal is reset

at the beginning of each cycle and slews at 26mV/µs.

The PWM comparator uses the instantaneous current,

the error voltage, the internal reference, and the slope

compensation (MAX5014 only) to determine when to

switch the N-channel MOSFET off. In normal operation

the N-channel MOSFET turns off when:

where IPRIMARY is the current through the N-channel

MOSFET, VREF is the 2.4V internal reference and

VSCOMP is a ramp function starting at 0 and slewing at

26mV/µs (MAX5014 only). When using the MAX5014 in

a forward-converter configuration the following condi-

tion must be met to avoid control-loop subharmonic

oscillations:

where k = 0.75 to 1, and NSand NPare the number of

turns on the secondary and primary side of the trans-

former, respectively. L is the output filter inductor. This

makes the output inductor current downslope as refer-

enced across RSENSE equal to the slope compensa-

tion. The controller responds to transients within one

cycle when this condition is met.

N-Channel MOSFET Gate Driver

NDRV drives an N-channel MOSFET. NDRV sources

and sinks large transient currents to charge and dis-

charge the MOSFET gate. To support such switching

transients, bypass VCC with a ceramic capacitor. The

average current as a result of switching the MOSFET is

the product of the total gate charge and the operating

frequency. It is this current plus the DC quiescent cur-

rent that determines the total operating current.

Applications Information

Design Example

The following is a general procedure for designing a

forward converter (Figure 2) using the MAX5015.

1) Determine the requirements.

2) Set the output voltage.

3) Calculate the transformer primary to secondary

winding turns ratio.

4) Calculate the reset to primary winding turns ratio.

5) Calculate the tertiary to primary winding turns

ratio.

6) Calculate the current-sense resistor value.

7) Calculate the output inductor value.

8) Select the output capacitor.

The circuit in Figure 2 was designed as follows:

1) 36V ≤VIN ≤72V, VOUT = 5V, IOUT = 10A, VRIPPLE ≤

50mV

2) To set the output voltage calculate the values of

resistors R1 and R2 according to the following

equation:

kR V

SENSE OUT

×××

=µ

LmV s26 /

IRV-V-VS

PRIMARY SENSE OPTO REF COMP

×>

SENSE LimPrimary

=0 465./V

tms C

startup ss

=×045.nF

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

10 ______________________________________________________________________________________

where VREF is the reference voltage of the shunt

regulator, and R1and R2are the resistors shown in

Figures 2 and 3.

3) The turns ratio of the transformer is calculated based

on the minimum input voltage and the lower limit of

the maximum duty cycle for the MAX5015 (44%). To

enable the use of MOSFETs with drain-source

breakdown voltages of less than 200V use the

MAX5015 with the 50% maximum duty cycle.

Calculate the turns ratio according to the following

equation:

where:

NS/NP= Turns ratio (NSis the number of secondary

turns and NPis the number of primary turns).

VOUT = Output voltage (5V).

VD1 = Voltage drop across D1 (typically 0.5V for

power Schottky diodes).

DMAX = Minimum value of maximum operating duty

cycle (44%).

VIN_MIN = Minimum Input voltage (36V).

In this example:

Choose NPbased on core losses and DC resis-

tance. Use the turns ratio to calculate NS, rounding

up to the nearest integer. In this example NP= 14

and NS= 5.

For a forward converter choose a transformer with a

magnetizing inductance in the neighborhood of

200µH. Energy stored in the magnetizing inductance

of a forward converter is not delivered to the load

and must be returned back to the input; this is

accomplished with the reset winding.

The transformer primary to secondary leakage

inductance should be less than 1µH. Note that all

leakage energy will be dissipated across the MOS-

FET. Snubber circuits may be used to direct some or

all of the leakage energy to be dissipated across a

resistor.

To calculate the minimum duty cycle (DMIN) use the

following equation:

where VIN_MAX is the maximum input voltage (72V).

4) The reset winding turns ratio (NR/NP) needs to be

low enough to guarantee that the entire energy in

the transformer is returned to V+ within the off cycle

at the maximum duty cycle. Use the following equa-

tion to determine the reset winding turns ratio:

where:

NR/NP= Reset winding turns ratio.

DMAX’ = Maximum value of Maximum Duty Cycle.

Round NRto the nearest smallest integer.

The turns ratio of the reset winding (NR/NP) will

determine the peak voltage across the N-channel

MOSFET.

Use the following equation to determine the maxi-

mum drain-source voltage across the N-channel

MOSFET:

VDSMAX = Maximum MOSFET drain-source voltage.

VIN_MAX = Maximum input voltage.

Choose MOSFETs with appropriate avalanche

power ratings to absorb any leakage energy.

5) Choose the tertiary winding turns ratio (NT/NP) so

that the minimum input voltage provides the mini-

mum operating voltage at VDD (13V). Use the follow-

V71 + 14

DSMAX ≥×









=2 144VV

VV 1 + N

DSMAX IN_MAX P

≥×











N11- 0.5

0.5

R≤× =414

NN1-D

RP MAX

MAX

≤× ′

′

N-V

MIN OUT

IN_MAX S

=19 8.

5V+ 0.5V 0.44

≥×

()

×=

044 36 0 330

VVD

OUT D1 MAX

MAX IN_MIN

≥+×

()

REF

OUT

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

______________________________________________________________________________________ 11

MAX5014/MAX5015

ing equation to calculate the tertiary winding turns

ratio:

where:

VDDMIN is the minimum VDD supply voltage (13V).

VDDMAX is the maximum VDD supply voltage (36V).

VIN_MIN is the minimum input voltage (36V).

VIN_MAX is the maximum input voltage (72V in this

design example).

NPis the number of turns of the primary winding.

NTis the number of turns of the tertiary winding.

Choose NT= 6.

6) Choose RSENSE according to the following equation:

where:

VILim is the current-sense comparator trip threshold

voltage (0.465V).

NS/NPis the secondary side turns ratio (5/14 in this

example).

IOUTMAX is the maximum DC output current (10A in

this example).

7) Choose the inductor value so that the peak ripple

current (LIR) in the inductor is between 10% and

20% of the maximum output current.

where VDis the output Schottky diode forward volt-

age drop (0.5V) and LIR is the ratio of inductor rip-

ple current to DC output current.

8) The size and ESR of the output filter capacitor deter-

mine the output ripple. Choose a capacitor with a

low ESR to yield the required ripple voltage.

Use the following equations to calculate the peak-to-

peak output ripple:

where:

VRIPPLE is the combined RMS output ripple due to

VRIPPLE,ESR, the ESR ripple, and VRIPPLE,C, the

capacitive ripple. Calculate the ESR ripple and

capacitive ripple as follows:

VRIPPLE,ESR = IRIPPLE x ESR

VRIPPLE,C = IRIPPLE/(2 x πx 275kHz x COUT)

Layout Recommendations

All connections carrying pulsed currents must be very

short, be as wide as possible, and have a ground plane

as a return path. The inductance of these connections

must be kept to a minimum due to the high di/dt of the

currents in high-frequency switching power converters.

Current loops must be analyzed in any layout pro-

posed, and the internal area kept to a minimum to

reduce radiated EMI. Ground planes must be kept as

intact as possible.

Chip Information

TRANSISTOR COUNT: 589

PROCESS: BiCMOS

VV V

RIPPLE RIPPLEESR RIPPLE C

L-

≥

()

××

=µ

5 5 1 0 198

0 4 275 10 401

kHz A H

LV-

OUT

≥+

()

×× ×

LIR kHz I

DMIN

OUTMAX

2 275

RSENSE ≤

××

=Ω

0 465

14 12 10

109

SENSE ILIM

≤

××12.I

OUTMAX

13 7 436 7 14

533 714

36 1N 72

×≤ ≤ ×

≤≤

VNN

DDMIN

IN_MIN

DDMAX

IN_MAX

+×≤≤

+×

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

12 ______________________________________________________________________________________

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

______________________________________________________________________________________ 13

International Rectifier www.irf.com

Fairchild www.fairchildsemi.com

Power FETS

Vishay-Siliconix www.vishay.com/brands/siliconix/main.html

Dale-Vishay www.vishay.com/brands/dale/main.html

Current-Sense Resistors IRC www.irctt.com/pages/index.cfm

On Semi www.onsemi.com

General Semiconductor www.gensemi.comDiodes

Central Semiconductor www.centralsemi.com

Sanyo www.sanyo.com

Taiyo Yuden www.t-yuden.com

Capacitors

AVX www.avxcorp.com

Coiltronics www.cooperet.com

Coilcraft www.coilcraft.comMagnetics

Pulse Engineering www.pulseeng.com

Table 1. Component Manufacturers

PART MAXIMUM

DUTY CYCLE

SLOPE

COMPENSATION

MAX5014CSA 85% Yes

MAX5014ESA 85% Yes

MAX5015CSA 50% No

MAX5015ESA 50% No

Selector Guide

MAX5014/MAX5015

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

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.

14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

Package Information

SOICN.EPS