MAXIM MAX5019, MAX5020 User Manual

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General Description

The MAX5019/MAX5020 integrate all the building
blocks necessary for implementing DC-DC fixed-fre­quency power supplies. Either primary- or secondary­side regulation may be used to implement isolated or
nonisolated power supplies. These devices are current­mode controllers with an integrated high-voltage start­up circuit suitable for telecom/industrial voltage range
power supplies. Current-mode control with leading­edge blanking simplifies control-loop design and inter­nal ramp compensation circuitry stabilizes the current
loop when operating at duty cycles above 50%
(MAX5019). The MAX5019 allows 85% operating duty
cycle and can be used to implement flyback converters
whereas the MAX5020 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 startup. The switching fre­quency is internally trimmed to 275kHz ±10%, thus
reducing magnetics and filter component costs.

The MAX5019/MAX5020 are available in 8-pin SO
packages.

Warning: The MAX5019/MAX5020 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)

♦ Isolated (without optocoupler) or Nonisolated

Power Supply

♦ 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

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

________________________________________________________________ Maxim Integrated Products 1

Pin Configuration

Ordering Information

NDRV

V

IN

V

OUT

V

DD

V

CC

CS

FB

MAX5020

V+

SS_SHDN

GND

Typical Operating Circuit

19-2115; 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.

*See Selector Guide at end of data sheet.

PART TEMP. RANGE PIN-PACKAGE

MAX5019CSA* 0°C to +70°C8-SO

MAX5019ESA* -40°C to +85°C8-SO
MAX5020CSA* 0°C to +70°C8-SO
MAX5020ESA* -40°C to +85°C8-SO

TOP VIEW

V+

1

2

DD

FB

SS_SHDN

3

4

MAX5019/

MAX5020

8-SO

87V

6

5

CC

NDRVV

GND

CS

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated
Startup Circuit

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VDD= 13V, a 10µF capacitor connects VCCto GND, VCS= 0, V+ = 48V, 0.1µF capacitor connected from SS_SHDN to GND, NDRV
= open circuit, V

FB

= 3V, 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
V

DD

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

V

CC

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

FB, NDRV, SS_SHDN, CS to GND .……-0.3V to V

CC

+ 0.3V

V

DD

and VCCCurrent …………………...…………………..20mA
NDRV Current Continuous...………………………………….25mA
NDRV Current for Less than 1µs..………….…………….……±1A

Continuous Power Dissipation (T

A

= +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

V+ Supply Current

V+ Supply Current After Startup V+ = 110V, VDD = 13V, FB = GND 14 µA

V

S up p l y C ur r ent

D D

V+ Shutdown Current V

VDD Shutdown Current V

PREREGULATOR/STARTUP

V+ Input Voltage 18 110 V

V

Supply Voltage 13 36 V

DD

INTERNAL REGULATORS (VCC)

VCC Output Voltage

VCC Undervoltage Lockout 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

I

V+(NS)

I

V+(S)

I

VDD(NS

I

VDD(S)

CC_UVLOVCC

R

OH

VDD = 0, V+ = 110V, driver not switching 0.8 1.6

V+ = 110V, VDD = 0, FB = GND,
driver switching

VDD = 36V, driver not switching 0.9 1.6

VDD = 36V, driver switching, FB = GND 2.1 3.0

SS_SHDN

SS_SHDN

Powered from V+, ICC = 7.5mA, VDD = 0 7.5 9.8 12.0 V

Powered from V

VCC = 11V, externally forced,
NDRV sourcing 50mA

= 0, V+ = 110V 180 290 µA

= 0 4 20 µ A

, ICC = 7.5mA 9.0 10.0 11.0 V

DD

falling 6.6 V

1.6 3.0

412Ω

mA

mA

NDRV Low-Side Driver
Resistance

ERROR AMPLIFIER

FB Input Resistance R

FB Input Bias Current I

Error Amplifier Gain (Inverting) A

Closed-Loop 3dB Bandwidth 200 kHz

FB Input Voltage Range 23V

R

OL

IN

FB

VCL

VCC = 11V, externally forced,
NDRV sinking 50mA

V

FB = VSS_SHDN

1.6 4 Ω

50 kΩ
±1 µA

-20 V/V

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(VDD= 13V, a 10µF capacitor connects VCCto GND, VCS= 0, V+ = 48V, 0.1µF capacitor connected from SS_SHDN to GND, NDRV
= open circuit, V

FB

= 3V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

Typical Operating Characteristics

(V+ = 48V, VDD= 13V, CS = GND, 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

V

SS_SHDN

vs. TEMPERATURE

(AT THE END OF SOFT-START)

MAX5019 toc01

TEMPERATURE (°C)

VFB = 4V

V

SS_SHDN

(V) (NORMALIZED TO V

REF

= 2.4V)

273

274

276

275

277

278

-40 0-20 20 40 60 80

NDRV FREQUENCY

vs. TEMPERATURE

MAX5019 toc02

TEMPERATURE (°C)

NDRV FREQUENCY (kHz)

FB = GND

80.4

80.6

80.5

80.8

80.7

80.9

81.0

-40 20 40-20 0 60 80

MAX5019

MAXIMUM DUTY CYCLE

vs. TEMPERATURE

MAX5019 toc03

TEMPERATURE (°C)

MAXIMUM DUTY CYCLE (%)

FB = GND

SLOPE COMPENSATION

Slope Compensation
(MAX5019 only)

THERMAL SHUTDOWN

Thermal Shutdown Temperature 150 °C
Thermal Hysteresis 25 °C

CURRENT LIMIT

CS Threshold Voltage V
CS Input Bias Current 0 ≤ VCS ≤ 2V, FB = GND -1 1 µA

Current Limit Comparator
Propagation Delay

CS Blanking Time FB = GND, only PWM comparator is blanked 70 ns

OSCILLATOR

Clock Frequency Range FB = GND 247 275 302 kHz

Max Duty Cycle

SOFT-START

SS Source Current I

SS Sink Current 1.0 mA

Steady State Reference Voltage
at SS_SHDN

Shutdown Threshold

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

V

SCOMP

ILIM

FB = GND 419 465 510 mV

26 mV/µs

50mV overdrive on CS, FB = GND 180 ns

MAX5019, FB = GND 75 85

MAX5020, FB = GND 44 50

SSO

V

_SHDN No external load 2.331 2.420 2.500 V

SS

V

SS_SHDN

V

SS_SHDN

V

SS_SHDN

= 0 2.0 4.5 6.5 µA

falling 0.25 0.37 0.41

rising 0.53 0.59 0.65

%

V

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated
Startup Circuit

4 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

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

MAX5020

MAXIMUM DUTY CYCLE

vs. TEMPERATURE

48.0

47.8

47.6

47.4

47.2

MAXIMUM DUTY CYCLE (%)

47.0

46.8

-40 20 40-20 0 60 80

FB = GND

TEMPERATURE (°C)

MAX5019 toc04

V+ SUPPLY CURRENT

vs. TEMPERATURE

1.64

1.63

1.62

1.61

1.60

1.59

1.58

V+ SUPPLY CURRENT (mA)

1.57

1.56

1.55

-40 0 20-20

FB = VDD = GND

TEMPERATURE (°C)

MAX5019 toc05

SOFT-START SOURCE CURRENT (µA)

60 80

40

SOFT-START SOURCE CURRENT

vs. TEMPERATURE

4.50

4.49

4.48

4.47

4.46

4.45

4.44

4.43

4.42

4.41

4.40

-40 -20 0 20 40 60 80

VDD = FB = SS_SHDN = GND

V+ = 110V

TEMPERATURE (°C)

MAX5019 toc06

V+ INPUT CURRENT vs.

TEMPERATURE (AFTER STARTUP)

13.80

13.75

V+ = 110V, VDD = 13V, FB = GND

-40 0-20 20 40 60 80
TEMPERATURE (°C)

V+ INPUT CURRENT (µA)

13.70

13.65

13.60

13.55

13.50

NDRV RESISTANCE
vs. TEMPERATURE

5.0

4.5

NDRV RESISTANCE (Ω)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

-40 0 20-20

HIGH-SIDE DRIVER

LOW-SIDE DRIVER

TEMPERATURE (°C)

MAX5019 toc07

V+ SHUTDOWN CURRENT

vs. TEMPERATURE

182.5

V+ SHUTDOWN CURRENT (µA)

182.0

181.5

181.0

180.5

180.0

179.5

179.0

V+ = 110V, FB = SS_SHDN = GND

-40 -20 0 20 40 60 80
TEMPERATURE (°C)

0.488

MAX5019 toc08

0.487

0.486

0.485

CS THRESHOLD VOLTAGE (V)

0.484

0.483

CS THRESHOLD VOLTAGE

vs. TEMPERATURE

MAX5019 toc09

FB = GND

-40 0-20 20 40 60 80
TEMPERATURE (°C)

CURRENT-LIMIT DELAY

V

vs. TEMPERATURE

MAX5019 toc11

2.410

2.408

(V)

2.406

SS_SHDN

V

2.404

2.402

2.400
010155 2025303540

210

208

MAX5019 toc10

206

FB = GND, 100mV OVERDRIVE ON CS

204

202

200

198

196

194

CURRENT-LIMIT DELAY (ns)

192

190

60 80

40

188

-40 -20 0 20 40 60 80

TEMPERATURE (°C)

SS_SHDN

VDD (V)

vs. V

DD

MAX5019 toc12

_______________________________________________________________________________________ 5

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

Typical Operating Characteristics (continued)

(V+ = 48V, VDD= 13V, CS = GND, 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. V

DD

MAX5019 toc13

VDD (V)

NDRV FREQUENCY (kHz)

FB = 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

MAX5020

MAXIMUM DUTY CYCLE vs. V

DD

MAX5019 toc14

VDD (V)

MAXIMUM DUTY CYCLE (%)

VFB = 4V, CS = GND

DEVICE POWERED
FROM V+

DEVICE POWERED
FROM V

DD

9.5

9.6

9.8

9.7

10.0

10.1

9.9

10.2

010155 2025303540

VCC vs. V

DD

MAX5019 toc15

VDD (V)

V

CC

(V)

DEVICE POWERED FROM V

DD

DEVICE POWERED
FROM V+

FB = GND

1.51

1.53

1.52

1.56

1.55

1.54

1.59

1.58

1.57

1.60

04020 60 80 100

V+ SUPPLY CURRENT vs.

V+ VOLTAGE

MAX5019 toc16

V+ VOLTAGE (V)

V+ SUPPLY CURRENT (mA)

VFB = VDD = GND

0

2

14

6

4

8

10

12

16

0403010 20 50 60 70 80 90 110

V+ SUPPLY CURRENT vs. V+ VOLTAGE

(AFTER STARTUP)

MAX5019 toc17

V+ VOLTAGE (V)

V+ LEAKAGE CURRENT (µA)

100

VDD = 13V, FB = GND

9.0

9.4

9.2

9.8

9.6

10.2

10.0

10.4

VCC VOLTAGE vs. VCC CURRENT

MAX5019 toc18

VCC CURRENT (mA)

V

CC

VOLTAGE (V)

0 5.0 10.0 15.0 20.0

V+ = 110V, VFB = 4V

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

MAX5019 toc19

VCC CURRENT (mA)

V

CC

VOLTAGE (V)

VDD = GND, VFB = 4V

V+ = 110V

V+ = 90V

V+ = 72V

V+ = 48V

V+ = 36V

V+ = 24V

MAX5019/MAX5020

Detailed Description

Use the MAX5019/MAX5020 PWM current-mode con­trollers to design flyback- or forward-mode power sup­plies. Current-mode operation simplifies control-loop
design while enhancing loop stability. An internal high­voltage 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 estab­lished 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 mag­netic components to minimize board space. Both the
MAX5019 and MAX5020 can be used in power sup­plies providing multiple output voltages. A functional
diagram of the IC is shown in Figure 1. Typical applica­tions circuits for forward and flyback topologies are
shown in Figure 2 and Figure 3, respectively. For isolat­ed flyback power supplies use the circuit of Figure 4.

Current-Mode Control

The MAX5019/MAX5020 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 MAX5019 in discontinuous flyback applications
where wide line voltage and load current variation is
expected. Use the MAX5020 for single transistor for­ward 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 MAX5019. 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-

Current-Mode PWM Controllers with Integrated
Startup Circuit

6 _______________________________________________________________________________________

Pin Description

PIN NAME FUNCTION

1V+

2V

3FB

4 SS_SHDN

DD

High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects
internally to a high-voltage linear regulator that generates V

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

Input of the Fixed-Gain Inverting Amplifier. Connect a voltage-divider from the regulated output to
this pin. The noninverting input of the amplifier is referenced to 2.4V.

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 SS_SHDN is pulled below 0.25V.

during startup.

CC

DD

Current Sense Input. Turns power switch off if V

5CS

6 GND Ground

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

8V

CC

limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM
comparator through a leading-edge blanking circuit.

Regulated IC Supply. Provides power for the entire IC. V
operation and from V+ during startup. Bypass V

0.1µF ceramic capacitor to GND.

rises above 465mV for cycle-by-cycle current

CS

is regulated from VDD during normal

CC

with a 10µF tantalum capacitor in parallel with

CC

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

_______________________________________________________________________________________ 7

Figure 1. Functional Diagram

V

DD

V

DD-OK

V+

GND

IN

EN

HIGH-

VOLTAGE

REGULATOR

OUT

EN

BIAS

WINDING

REGULATOR

OUT

IN

0.7V

V

CC

FB

SS_SHDN

50kΩ

MAX5019 ONLY

26mV/µs

1MΩ

ERROR
AMP

2.4V

SLOPE

COMPENSATION

26mV/µs

∑

DUTY CYCLE

PWM

OSCILLATOR

80%/50%

CLAMP

275kHz

4µA

BUF

UVLO

6.6V

R

Q

S

ILIM

V

CC

70ns

BLANKING

3R

125mV

5kΩ

R

V

CC

NDRV

CS

0.25V

MAX5019/MAX5020

vantage to this is that the MOSFET voltage rating must
be higher and that slope compensation must be provid­ed to stabilize the inner current loop. The MAX5019
provides internal slope compensation.

Internal Regulators

The internal regulators of the MAX5019/MAX5020
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 VCCto provide bias for the device.
The VDDregulator then regulates from the output of the
tertiary winding to VCC. This architecture allows the ter­tiary 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 num­ber of turns so the minimum reflected voltage is always
higher than 12.7V. The maximum reflected voltage
must be less than 36V.

To reduce power dissipation the high-voltage regulator
is disabled when the VDDvoltage reaches 12.7V. This
greatly reduces power dissipation and improves effi­ciency. If VCCfalls below the undervoltage lockout
threshold (VCC= 6.6V), the low-voltage regulator is dis-

abled, and soft-start is reinitiated. In undervoltage lock­out the MOSFET driver output (NDRV) is held low.

If the input voltage range is between 13V and 36V, V+
and VDDmay be connected to the line voltage provid­ed that the maximum power dissipation is not exceed­ed. This eliminates the need for a tertiary winding.

Undervoltage Lockout (UVLO), Soft-Start,

and Shutdown

The soft-start feature of the MAX5019/MAX5020 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 follow­ing equation to calculate total soft-start time:

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

Operation begins when V

SS_SHDN

ramps above 0.6V.

When soft-start has completed, V

SS_SHDN

is regulated

Current-Mode PWM Controllers with Integrated
Startup Circuit

8 _______________________________________________________________________________________

Figure 2. Forward Converter

1N4148

V

IN

(36V TO 72V)

V

C

4.7µF

C

10µF

C

0.1µF

DD

CC

SS

DD

V

CC

SS_SHDN

6

MAX5020

N

T

NDRV

GND

V+

CS

FB

N

R

14
CMHD2003

C

3

0.47µF

IRF640N

IN

✕

M1

100Ω

N

P

14 5

R

SENSE

100mΩ

N

S

SBL204OCT

20Ω

1nF

C

FB

(OPTIONAL)

L1

4.7µH

R
2kΩ

R
2kΩ

1

2

5V/10A

C

OUT

✕

3
560µF

V

OUT

0.1µF

=×045.

ms

nF

C

t

startup ss

to 2.4V, the internal voltage reference. Pull V

SS_SHDN

below 0.25V to disable the controller.

Undervoltage lockout shuts down the controller when
VCCis less than 6.6V. The regulators for V+ and the ref­erence 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 100Ω resistor or an RC low-

_______________________________________________________________________________________ 9

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

Figure 3. Nonisolated Flyback Converter

Figure 4. Isolated Flyback Converter

N

V

IN

T

C

V

C

DD

C

CC

C

SS

DD

V

CC

SS_SHDN

MAX5019
MAX5020

V+

NDRV

CS

GND

FB

IN

M1

100Ω

N

N

P

S

R

SENSE

V

OUT

C

OUT

R

1

R

2

V

IN

V

C

DD

R

1

R

2

C

CC

C

SS

DD

FB

V

CC

SS_SHDN

MAX5019
MAX5020

N

NDRV

GND

T

C

V+

CS

IN

M1

100Ω

N

N

P

S

R

SENSE

V

OUT

C

OUT

MAX5019/MAX5020

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

SENSE

according to the following equation:

where I

LimPrimary

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 180ns.

Internal Error Amplifier

The MAX5019/MAX5020 include an internal error ampli­fier that can be used to regulate the output voltage in
the case of a nonisolated power supply (see Figure 2).
Calculate the output voltage using the following equa­tion:

where V

REF

= 2.4V.

Choose R1//R2<< RIN, where RIN, ≅ 50kΩ is the input
resistance of FB. The gain of the error amplifier is inter­nally configured for -20 (see Figure 1).

The error amplifier may also be used to regulate the out­put of the tertiary winding for implementing a primary­side regulated isolated power supply (see Figure 4).
Calculate the output voltage using the following equation:

where NSis the number of secondary turns and NTis
the number of tertiary winding turns.

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 maxi­mum duty cycle has been reached, regardless of the
feedback.

The MAX5019 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 (MAX5019 only) to determine when to

switch the N-channel MOSFET off. In normal operation
the N-channel MOSFET turns off when:

where I

PRIMARY

is the current through the N-channel

MOSFET, V

REF

is the 2.4V internal reference, VEAis the

output voltage of the internal amplifier, and V

SCOMP

is
a ramp function starting at 0 and slewing at 26mV/µs
(MAX5019 only). When using the MAX5019 in a for­ward-converter configuration the following condition
must be met to avoid control-loop subharmonic oscilla­tions:

where k = 0.75 to 1, and N

S

and NPare the number of
turns on the secondary and primary side of the trans­former, respectively. L is the output filter inductor. This
makes the output inductor current downslope as refer­enced across R

SENSE

equal to the slope compensa­tion. 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 dis­charge the MOSFET gate. To support such switching
transients, bypass VCCwith 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 cur­rent that determines the total operating current.

Applications Information

Design Example

The following is a general procedure for designing a
forward converter using the MAX5020.

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:

Current-Mode PWM Controllers with Integrated
Startup Circuit

10 ______________________________________________________________________________________

RI

= 0 465./V

SENSE LimPrimary

V

OUT



R

=+



R





1

×1

V

REF




2



N

V

OUT

S

=+

N

T



R

1

V

×1



R



REF




2

IRV-V-V

PRIMARY SENSE EA REF SCOMP

×>

NNkR V

××

S

P

×

SENSE OUT

L

=µ

mV s26 /

1) 36V ≤ VIN≤ 72V, V

OUT

= 5V, I

OUT

= 10A, V

RIPPLE

≤

50mV

2) To set the output voltage calculate the values of
resistors R1 and R2 according to the following
equation:

where V

REF

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 MAX5020 (44%). To
enable the use of MOSFETs with drain-source
breakdown voltages of less than 200V use the
MAX5020 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).

V

OUT

= Output voltage (5V).

VD1= Voltage drop across D1 (typically 0.5V for
power Schottky diodes).

D

MAX

= Minimum value of maximum operating duty

cycle (44%).

V

IN_MIN

= Minimum Input voltage (36V).

In this example:

Choose NPbased on core losses and DC resis­tance. 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 MOS­FET. 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 (D

MIN

) use the

following equation:

where V

IN_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 equa­tion to determine the reset winding turns ratio:

where:

NR/NP= Reset winding turns ratio.

D

MAX

’ = Maximum value of Maximum Duty Cycle.

Round N

R

to 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 maxi­mum drain-source voltage across the N-channel
MOSFET:

V

DSMAX

= Maximum MOSFET drain-source voltage.

V

IN_MAX

= Maximum input voltage.

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

______________________________________________________________________________________ 11

VV

OUT REF

RR k//

12

VV V

REF SS_



≅+




<<

50

=≅

SHDN



R

1

1



R



2

Ω

24

.

VVD

+×

N

S

≥

N

P

()

OUT D1 MAX

×

DV

MAX IN_MIN

5V+ 0.5V 0.44

N

S

≥

N

P

×

()

×

044 36

=

V

0 330..

D

=

MIN






V

IN_MAX

V

OUT

×



N

S

-V

D1



N



P

NN

≤×

RP

1-D

D

MAX

MAX

′

′

1- 0.5

≤× =414

0.5

N1

R

VV 1 +

≥×

DSMAX IN_MAX








N

P



N



R

MAX5019/MAX5020

Choose MOSFETs with appropriate avalanche
power ratings.

5) Choose the tertiary winding turns ratio (NT/NP) so
that the minimum input voltage provides the mini­mum operating voltage at V

DD

(13V). Use the follow­ing equation to calculate the tertiary winding turns
ratio:

where:

V

DDMIN

is the minimum VDDsupply voltage (13V).

V

DDMAX

is the maximum VDDsupply voltage (36V).

V

IN_MIN

is the minimum input supply voltage (36V).

V

IN_MAX

is the maximum input supply 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 R

SENSE

according to the following equation:

where:

V

ILim

is the current-sense comparator trip threshold

voltage (0.465V).

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

I

OUTMAX

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 V

D

is the output Schottky diode forward volt-

age drop (0.5V).

8) The size and ESR of the output filter capacitor deter­mine the output ripple. Choose a capacitor with a
low ESR to yield the required ripple voltage.

Use the following equations to calculate the peak-to­peak output ripple:

where:

V

RIPPLE

is the combined RMS output ripple due to

VRIPPLE,ESR, the ESR ripple, and V

RIPPLE,C

, the
capacitive ripple. Calculate the ESR ripple and
capacitive ripple as follows:

V

RIPPLE,ESR

= I

RIPPLE

x ESR

V

RIPPLE,C

= I

RIPPLE

/(2 x π x 275kHz x C

OUT

)

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 pro­posed, 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

Current-Mode PWM Controllers with Integrated
Startup Circuit

12 ______________________________________________________________________________________

14

V71 +

DSMAX

≥×








=2 144VV




14

V

IN_MIN

V

IN_MAX

0707.

+

×≤≤

+

.

×

NN

PT

N

P

V

DDMIN

V

DDMAX

13 7

..

4

1N

×≤ ≤ ×

36

533 714

..

N

≤≤

36 7

72

14

T

V

R

SENSE

≤

N
N

ILIM

S

××12.I

P

OUTMAX

0 465

R

SENSE

≤

14

..V

5

××

12 10

=Ω

109

m

VD

×

1

()

DMIN

OUTMAX

V-

+

()

L

OUT

≥

×× ×

LIR kHz I

2 275

×

5 5 1 0 198

..

()

L-≥

0 4 275 10

()

××

.

kHz A

=µ

401

.

H

VV V

RIPPLE RIPPLEESR RIPPLE C

22

=+

,,

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated

Startup Circuit

______________________________________________________________________________________ 13

Table 1. Component Manufacturers

Selector Guide

International Rectifier www.irf.com

Power FETS

Current-Sense Resistors

Capacitors

Fairchild www.fairchildsemi.com

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

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

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

AVX www.avxcorp.com

Coiltronics www.cooperet.com

Coilcraft www.coilcraft.comMagnetics

Pulse Engineering www.pulseeng.com

PART

MAX5019CSA 85% Yes

MAX5019ESA 85% Yes

MAX5020CSA 50% No

MAX5020ESA 50% No

MAXIMUM

DUTY CYCLE

SLOPE

COMPENSATION

MAX5019/MAX5020

Current-Mode PWM Controllers with Integrated
Startup Circuit

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

© 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

Package Information

SOICN.EPS