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19-2066; Rev 1; 9/01
BLE IT AVAILA LUATION K EVA
Current-Mode PWM Controllers for Isolated Power Supplies
General Description
The MAX5021/MAX5022 current-mode PWM controllers contain all the control circuitry required for the design of wide input voltage range isolated power supplies. These devices are well suited for use in universal input (85VAC to 265VAC) off-line or telecom (-36VDC to -72VDC) power supplies. An undervoltage lockout (UVLO) circuit with large hysteresis coupled with low startup and operating current reduce power dissipation in the startup resistor and allow use of ceramic bypass capacitors. The 262kHz switching frequency is internally trimmed to 12% accuracy; this allows the optimization of the magnetic and filter components resulting in compact, cost-effective power supplies. The MAX5021 with 50% maximum duty cycle and MAX5022 with 75% maximum duty cycle are recommended for forward converters and flyback converters, respectively. The MAX5021/MAX5022 are available in 6-pin SOT23, 8-pin MAX, and 8-pin DIP packages and are rated for operation over the -40C to +85C temperature range. o 50A Typical Startup Current o 1.2mA Typical Operating Current o Large UVLO Hysteresis of 14V o Fixed Switching Frequency of 262kHz 12% o 50% Maximum Duty Cycle Limit (MAX5021) o 75% Maximum Duty Cycle Limit (MAX5022) o 60ns Cycle-by-Cycle Current-Limit Response Time
Features
o Available in a Tiny 6-Pin SOT23 Package
MAX5021/MAX5022
Ordering Information
PART MAX5021EUT MAX5021EUA MAX5021EPA MAX5022EUT MAX5022EUA MAX5022EPA MAX DUTY CYCLE 50% 50% 50% 75% 75% 75% TEMP. RANGE PINTOP PACKAGE MARK
Applications
Universal Off-Line Power Supplies Standby Power Supplies Isolated Power Supplies Isolated Telecom Power Supplies Mobile Phone Chargers
-40C to +85C 6 SOT23-6 AASQ -40C to +85C 8 MAX -40C to +85C 8 PDIP -40C to +85C 6 SOT23-6 -40C to +85C 8 MAX -40C to +85C 8 PDIP -- -- AASR -- --
WARNING: The MAX5021/MAX5022 are designed to work with high voltages. Exercise caution!
Typical Operating Circuit
VSUPPLY
Pin Configuration
TOP VIEW
CS 1 6 OPTO OPTO 1 VIN 2 5 VIN VCC 3 8 CS GND NDRV N.C.
VOUT
GND 2
VCC VIN
MAX5021 MAX5022
MAX5021 MAX5022
7 6 5
MAX5021 MAX5022 OPTO NDRV
NDRV 3
4
VCC
N.C. 4
SOT23
GND CS
PDIP/MAX
________________________________________________________________ Maxim Integrated Products
1
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Current-Mode PWM Controllers for Isolated Power Supplies MAX5021/MAX5022
ABSOLUTE MAXIMUM RATINGS
VIN to GND .............................................................-0.3V to +30V VCC to GND ............................................................-0.3V to +13V NDRV to GND.............................................-0.3V to (VCC + 0.3V) CS, OPTO to GND ....................................................-0.3V to +6V NDRV Short-Circuit to GND........................................Continuous Continuous Power Dissipation (TA = +70C) 6-Pin SOT23 (derate 8.7mW/C above +70C).............696mW 8-Pin MAX (derate 4.5mW/C above +70C) ..............362mW 8-Pin PDIP (derate 9.1mW/C above +70C)................727mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-55C to +150C Lead Temperature (soldering 10s) ..................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = +11V to +28V, VCS = 0, OPTO is unconnected, 10nF bypass capacitors at VIN and VCC, NDRV unconnected, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +12V, TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Undervoltage Lockout Wakeup Level Undervoltage Lockout Shutdown Level VIN Supply Current at Startup VIN Range Undervoltage Lockout Propagation Delay INTERNAL SUPPLY VCC Regulator Set Point VIN Supply Current after Startup GATE DRIVER Driver Output Impedance Driver Peak Sink Current Driver Peak Source Current PWM COMPARATOR Comparator Offset Voltage CS Input Bias Current Propagation Delay from Comparator Input to NDRV Minimum On-Time CURRENT-LIMIT COMPARATOR Current-Limit Trip Threshold Current-Limit Propagation Delay from Comparator Input to NDRV VCS TCL 25mV overdrive 540 600 60 660 mV ns VOPWM ICS TPWM TON(MIN) 25mV overdrive VOPTO - VCS 600 -2 60 150 750 900 +2 mV A ns ns RON(LOW) ISINK ISOURCE Measured at NDRV sinking 5mA 10 20 250 150 20 40 RON(HIGH) Measured at NDRV sourcing 5mA mA mA VCCSP IIN VIN = +11V to +28V, sourcing 1A to 5mA from VCC VIN = +28V, OPTO connected to GND VIN = +28V, OPTO unconnected (Note 2) 7.0 0.9 0.4 10.5 2.43 V mA SYMBOL CONDITIONS MIN TYP MAX UNITS
UNDERVOLTAGE LOCKOUT/STARTUP VUVR VUVF ISTART VIN TUVR TUVF VIN steps up from +9V to +26V VIN steps down from +26V to +9V VIN rising VIN falling VIN = +22V 11 5 1 22 9.3 24 10 50 26 10.9 85 28 V V A V s
2
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Current-Mode PWM Controllers for Isolated Power Supplies
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +11V to +28V, VCS = 0, OPTO is unconnected, 10nF bypass capacitors at VIN and VCC, NDRV unconnected, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +12V, TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER OSCILLATOR Switching Frequency Maximum Duty Cycle OPTO INPUT OPTO Pullup Voltage OPTO Pullup Resistance VOPTO ROPTO OPTO sourcing 10A 4.5 6.2 5.5 7.9 V k fSW DMAX MAX5021 MAX5022 230 262 50 75 290 51 76 kHz % SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX5021/MAX5022
Note 1: All devices are 100% tested at TA = +25C. All limits over temperature are guaranteed by characterization. Note 2: This minimum current after startup is a safeguard that prevents the VIN pin voltage from rising in the event that OPTO and NDRV become unconnected.
Typical Operating Characteristics
(VIN = 15V, TA = +25C, unless otherwise noted.)
UNDERVOLTAGE LOCKOUT vs. TEMPERATURE
MAX5021/22 toc01
UNDERVOLTAGE LOCKOUT vs. TEMPERATURE
MAX5021/22 toc02
STARTUP CURRENT vs. TEMPERATURE
MAX5021/22 toc03
24.3 VIN RISING UNDERVOLTAGE LOCKOUT (V) 24.2
10.2 VIN FALLING 10.1
53 52 STARTUP CURRENT (A) 51 50 49 48 VIN = 23.0V
24.1
UNDERVOLTAGE LOCKOUT (V)
10.0
24.0
9.9
23.9 -40 -20 0 20 40 60 80 TEMPERATURE (C)
9.8 -40 -20 0 20 40 60 80 TEMPERATURE (C)
47 -40 -20 0 20 40 60 80 TEMPERATURE (C)
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Current-Mode PWM Controllers for Isolated Power Supplies MAX5021/MAX5022
Typical Operating Characteristics (continued)
(VIN = 15V, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX5021/22 toc04
MAXIMUM VCC vs. TEMPERATURE
MAX5021/22 toc05
MINIMUM VCC vs. TEMPERATURE
VIN = 10.8V 5mA LOAD ON VCC VCS = 0 OPTO = UNCONNECTED
MAX5021/22 toc06
1.60 VIN = 28.0V VOPTO = VCS = 0 SUPPLY CURRENT (mA) 1.55
9.15
8.30 8.20 MINIMUM VCC (V) 8.10 8.00 7.90 7.80 7.70
9.12 MAXIMUM VCC (V)
9.09
1.50
9.06 VIN = 28.0V VCS = 0 OPTO = UNCONNECTED -40 -20 0 20 40 60 80
1.45
9.03
1.40 -40 -20 0 20 40 60 80 TEMPERATURE (C)
9.00 TEMPERATURE (C)
-40
-20
0
20
40
60
80
TEMPERATURE (C)
CURRENT SENSE THRESHOLD vs. TEMPERATURE
MAX5021/22 toc07
CURRENT SENSE THRESHOLD
MAX5021/22 toc08
OSCILLATOR FREQUENCY vs. TEMPERATURE
TOTAL NUMBER OF DEVICES = 50 +3 MEAN
MAX5021/22 toc09
640 CURRENT SENSE THRESHOLD (mV) 630 620 610 600 590 580 570 -40 -20 0 20 40 60 80 TEMPERATURE (C) -3 TOTAL NUMBER OF DEVICES = 50 +3 MEAN
25 TOTAL NUMBER OF DEVICES = 200
280 OSCILLATOR FREQUENCY (kHz) 275 270 265 260 255 250 -3
20 FREQUENCY (%)
15
10
5
0 540 560 580 600 620 640 660 CURRENT SENSE THRESHOLD (mV)
245 -40 -20 0 20 40 60 80 TEMPERATURE (C)
OSCILLATOR FREQUENCY
MAX5021/22 toc10
CURRENT SENSE DELAY vs. TEMPERATURE
MAX5021/22 toc11
UNDERVOLTAGE LOCKOUT DELAY vs. TEMPERATURE
UNDERVOLTAGE LOCKOUT DELAY (s)
MAX5021/22 toc12
25 TOTAL NUMBER OF DEVICES = 200 20 FREQUENCY (%)
75
6 5 VIN RISING 4 3 2 VIN FALLING 1 0
15
CURRENT SENSE DELAY (ns)
70
65
10
60
5
55
0 230 240 250 260 270 280 290 OSCILLATOR FREQUENCY (kHz)
50 -40 -20 0 20 40 60 80 TEMPERATURE (C)
-40
-20
0
20
40
60
80
TEMPERATURE (C)
4
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Current-Mode PWM Controllers for Isolated Power Supplies
Pin Description
PIN SOT23 1 2 3 4 PDIP MAX 8 7 6 3 NAME FUNCTION Current Sense Connection for PWM Regulation and Overcurrent Protection. The current-limit comparator threshold is internally set to 0.6V. Power-Supply Ground External N-Channel MOSFET Gate Connection Gate Drive Supply. Internally regulated down from VIN. Decouple with a 10nF or larger capacitor to GND. IC Supply. Decouple with a 10nF or larger capacitor to GND. Connect a startup resistor (Rs) from the input supply line to VIN. Connect to bias winding through diode rectifier. See Typical Operating Circuit. Optocoupler Transistor Collector Connection. Connect emitter of optocoupler to GND. The OPTO has an internal pullup resistor with a typical value of 6.2k. No Connection. Do not make connections to these pins.
MAX5021/MAX5022
CS GND NDRV VCC
5
2
VIN
6 --
1 4, 5
OPTO N.C.
Detailed Description
The MAX5021/MAX5022 are current-mode PWM controllers that have been specifically designed for use in isolated power supplies. An undervoltage lockout circuit (UVLO) with a large hysteresis (14V) along with very low startup and operating current result in highefficiency, universal input power supplies. Both devices can be used in power supplies capable of operating from a universal 85VAC to 265VAC line or the telecom voltage range of -36VDC to -72VDC. Power supplies designed with these devices use a high-value startup resistor, RS, (series combination of R1 and R2) that charges a reservoir capacitor, C2 (see Figure 1). During this initial period while the voltage is less than the UVLO start threshold, the IC typically consumes only 50A of quiescent current. This low startup current and the large UVLO hysteresis combined with the use of a ceramic capacitor C2 keeps the power dissipation in RS to less than 1/4W even at the high end of the universal AC input voltage (265VAC). The MAX5021/MAX5022 include a cycle-by-cycle current limit which turns off the gate drive to the external MOSFET during an overcurrent condition. If the output on the secondary side of transformer T1 is shorted, the tertiary winding voltage will drop below the 10V threshold causing the UVLO circuit to turn off the gate drive to the external power MOSFET, thus re-initiating the startup sequence.
Startup
Figure 2 shows the voltages on VIN and VCC during startup. Initially, both VIN and VCC are 0V. After the line voltage is applied, C2 charges through the startup resistor, RS, to an intermediate voltage at which point the internal reference and regulator begin charging C3 (see Figure 1). The bias current consumed by the device during this period is only 50A; the remaining input current charges C2 and C3. Charging of C3 stops when the V CC voltage reaches approximately 9.5V, while the voltage across C2 continues rising until it reaches the wakeup level of 24V. Once VIN exceeds the UVLO threshold, NDRV begins switching the MOSFET, transferring energy to the secondary and tertiary outputs. If the voltage on the tertiary output builds to higher than 10V (UVLO lower threshold), then startup has been accomplished and sustained operation will commence. If VIN drops below 10V before startup is complete, then the IC goes back into UVLO. In this case, increase the value of C2 and/or use a MOSFET with a lower gatecharge requirement.
Startup Time Considerations
The VIN bypass capacitor C2 supplies current immediately after wakeup. The size of C2 will determine the number of cycles available for startup. Large values for C2 will increase the startup time, but will also supply more gate charge, allowing for more cycles after wakeup. If the value of C2 is too small, VIN will drop below 10V because
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Current-Mode PWM Controllers for Isolated Power Supplies MAX5021/MAX5022
L 85VAC TO N 265VAC IN G AC C1 10F 400V R8 1.2k C9 10F 400V CENTRAL SEMICONDUCTOR CBR1-D100S AC L2 470H R1 360k D1 RS = R1 + R2 R2 360k C2 0.22F R11 10 D2 CTX03-15256 3A, 40V T1 ON SEMICONDUCTOR MBRS340T3 480H, 60T 3T 8T C4 150F 6.3V OPTO C7 1000pF TEXAS INSTRUMENTS TLV431AIDBV R3 1k 1% R4 24.9k 1% R10 10 C5 0.01F R5 8.06k 1% C6 0.1F
+5V OUT
D1 250mA, 75V CENTRAL SEMICONDUCTOR CMPD914 VCC VIN
C3 0.22F OPTO FAIRCHILD CNY17-3
U1 MAX5022
OPTO
NDRV
N1 INTERNATIONAL RECTIFIER IRFRC20
NOTE: ALL RESISTORS ARE 5% UNLESS OTHERWISE SPECIFIED. GND R9 240k CS C8 8200pF R6 10 RCS 1.78 1%
Figure 1. Universal 5W Off-Line Standby Power Supply
NDRV did not switch enough times to build up sufficient voltage across the tertiary output to power the device. The device will go back into UVLO and will not start. Use a low-leakage ceramic or film capacitor for C2 and C3. As a rule of thumb, off-line power supplies keep typical startup times to less than 500ms even in low-line conditions (85VAC input). Size the startup resistor, RS, to supply the maximum startup bias of the IC (85A) plus the additional current required for charging the capacitors C2 and C3 in less than 500ms. This resistor dissipates continuous power in normal operation, despite the fact that it is only used during the startup sequence. Therefore it must be chosen to provide enough current for the low-line condition as well as have an appropriate power rating for the high-line condition (265VAC). In most cases, split the value into two resistors connected in series for the required voltage of approximately 400VDC. The typical value for C2 and C3 is 220nF. The startup resistor, RS, provides both the maximum quiescent current of 85A and the charging current for C2 and C3. Bypass capacitor C3 charges to 9.5V and C2 charges
6
25 IC COMES OUT OF UVLO (WAKEUP) 20 VIN SUPPLIED BY C2 VIN 10 VIN SUPPLIED BY TERTIARY WINDING (NORMAL OPERATION) VCC BYPASS CAPACITOR FULLY CHARGED VCC 0 0 50 100 TIME (ms) 150 200 VCC DROPS SLIGHTLY WHEN NDRV BEGINS SWITCHING
VIN, VCC (V)
15
5
Figure 2. VIN and VCC During Startup
to 24V all within the desired time period of 500ms, for an overall average charging current of 15A. Hence, the startup resistor must provide a total of at least 100A. Developing 100A from an input voltage of
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Current-Mode PWM Controllers for Isolated Power Supplies
85VAC (corresponding to 120VDC) to the 24V wakeup level results in a resistor value of about 1M. If we assume RS values between 750k and 1M, then at the high-line voltage of 265VAC (corresponding to 374VDC) power dissipation will be between 140mW to 190mW. A single 1/4W resistor or a series combination of two 1/4W resistors is adequate.
Optocoupler Feedback
The MAX5021/MAX5022 do not include an internal error amplifier and are recommended for use in optocoupler feedback power supplies. Isolated voltage feedback is achieved by using an optocoupler and a shunt regulator as shown in the Typical Operating Circuit. The output voltage set point accuracy is a function of the accuracy of the shunt regulator and resistor divider. When a TLV431 shunt regulator is used for output voltage regulation, the output voltage is set by the ratio of R4 and R5 (Figure 1). Output voltage is given by the following equation: R4 VOUT = VREF x 1 + R5 where VREF = 1.24V for the TLV431. During normal operation, the optocoupler feedback pin (OPTO) is pulled up through a 6.2k resistor to the internal supply voltage of 5.25V. When the device is in UVLO, OPTO is disconnected from the 5.25V regulator and connected to ground (Functional Diagram). This helps initial startup by reducing the current consumption of the device.
MAX5021/MAX5022
Undervoltage Lockout (UVLO)
The device will attempt to start when VIN exceeds the UVLO threshold of 24V. During startup, the UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver shut down to reduce current consumption (Functional Diagram). Once V IN reaches 24V, the UVLO circuit turns on both the CPWM and ILIM comparators, as well as the oscillator, and allows the output driver to switch. If VIN drops below 10V, the UVLO circuit will shut down the CPWM comparator, ILIM comparator, oscillator, and output driver returning the MAX5021/MAX5022 to the startup mode.
N-Channel MOSFET Switch Driver
The NDRV pin drives an external N-channel MOSFET. The NDRV output is supplied by the internal regulator (VCC), which is internally set to approximately 9V. For the universal input voltage range, the MOSFET used must be able to withstand the DC level of the high-line input voltage plus the reflected voltage at the primary of the transformer. For most applications that use the discontinuous flyback topology, this requires a MOSFET rated at 600V. NDRV can source/sink 150mA/ 250mA peak current, thus select a MOSFET that will yield acceptable conduction and switching losses.
Current Limit
The current limit is set by a current sense resistor, RCS, connected between the source of the MOSFET and ground. The CS input has a voltage trip level (VCS) of 600mV. Use the following equation to calculate the value of RCS: V RCS = CS IPRI where IPRI is the peak current in the primary that flows through the MOSFET. When the voltage produced by this current through the current sense resistor exceeds the current-limit comparator threshold, the MOSFET driver (NDRV) will quickly terminate the current ON-cycle, typically within 60ns. In most cases a small RC filter will be required to filter out the leading-edge spike on the sense waveform. Set the corner frequency at a few MHz.
Internal Oscillator
The internal oscillator switches at 1.048MHz and is divided down to 262kHz by two D flip-flops. The MAX5021 inverts the Q output of the last D flip-flop to provide a duty cycle of 50% (Figure 3). The MAX5022 performs a logic NAND operation on the Q outputs of both D flip-flops to provide a duty cycle of 75%.
D OSCILLATOR 1.048MHz
Q
D
Q
262kHz WITH 50% (MAX5021) 262kHz WITH 75% (MAX5022)
Applications Information
Universal Off-Line Power Supply
Figure 1 shows the design of a 5V/1A isolated power supply capable of operating from a line voltage of 85VAC to 265VAC. This circuit is implemented in the MAX5022EVKIT.
Q
Q
Figure 3. Internal Oscillator
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Current-Mode PWM Controllers for Isolated Power Supplies MAX5021/MAX5022
T1 RS VOUT
C2
VCC C3 MAX5022 OPTO
VIN
NDRV
GND
CS
-36VDC TO -72VDC IN
Figure 4. -48VDC Input to +5V Output
WARNING! DANGEROUS AND LETHAL VOLTAGES ARE PRESENT IN OFF-LINE CIRCUITS! USE EXTREME CAUTION IN THE CONSTRUCTION, TESTING, AND USE OF OFF-LINE CIRCUITS.
Isolated Telecom Power Supply
Figure 4 shows a -48VDC telecom power supply capable of generating an isolated +5V output.
To achieve best performance, a star ground connection is recommended to avoid ground loops. For example, the ground returns for the power-line input filter, power MOSFET switch, and sense resistor should be routed separately through wide copper traces to meet at a single system ground connection.
Layout Recommendations
All printed circuit board traces carrying switching currents must be kept as short as possible, and the current loops they form must be minimized. The pins of the SOT23 package have been placed to allow simple interfacing to the external MOSFET. The order of these pins directly corresponds to the order of a TO-220 or similar package MOSFET. For universal AC input design all applicable safety regulations must be followed. Off-line power supplies may require UL, VDE, and other similar agency approvals. These agencies can be contacted for the latest layout and component rules. Typically there are two sources of noise emission in a switching power supply: high di/dt loops and high dv/dt surfaces. For example, traces that carry the drain current often form high di/dt loops. Similarly the heatsink of the MOSFET presents a dv/dt source, thus the surface area of the heatsink must be minimized as much as possible. TRANSISTOR COUNT: 519 PROCESS: BiCMOS
Chip Information
8
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Current-Mode PWM Controllers for Isolated Power Supplies
Functional Diagram
MAX5021/MAX5022
VIN
IN
VCC REGULATOR
VCC
IN REFERENCE 1.25V GND 24V 10V
UVLO
REG_OK
VL
(INTERNAL 5.25V SUPPLY)
6.2k DRIVER S Q OPTO CPWM 0.75V CS VOPWM VCS 0.6V *MAX5021: 50% MAXIMUM DUTY CYCLE MAX5022: 75% MAXIMUM DUTY CYCLE ILIM GND OSCILLATOR 262kHz* R NDRV
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Current-Mode PWM Controllers for Isolated Power Supplies MAX5021/MAX5022
Package Information
6LSOT.EPS
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 8L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0036
J
1 1
10
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Current-Mode PWM Controllers for Isolated Power Supplies
Package Information (continued)
PDIPN.EPS
MAX5021/MAX5022
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 ____________________ 11 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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