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19-2590; Rev 1; 11/03
KIT ATION EVALU ABLE AVAIL
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies
General Description
The MAX5052/MAX5053 current-mode PWM controllers contain all the control circuitry required for the design of wide-input-voltage isolated and nonisolated power supplies. The MAX5052 is well suited for universal input (rectified 85VAC to 265VAC) or telecom (-36VDC to -72VDC) power supplies. The MAX5053 is well suited for low-input-voltage (10.8VDC to 24VDC) power supplies. The MAX5052/MAX5053 contain an internal error amplifier that regulates the tertiary winding output voltage. This implements a primary-side regulated, isolated power supply, eliminating the need for an optocoupler. An input undervoltage lockout (UVLO) is provided for programming the input-supply start voltage and to ensure proper operation during brownout conditions. The input-supply start voltage is externally programmable with a voltage-divider. To shutdown the device, the UVLO pin is pulled low. Internal digital soft-start reduces output voltage overshoot. The internal thermal shutdown circuit protects the device in the event the junction temperature exceeds +130C. The MAX5052 has an internal bootstrap UVLO with large hysteresis that requires a minimum voltage of 23.6V for startup. The MAX5053 does not have the internal bootstrap UVLO and can be biased directly from a minimum voltage of 10.8V. 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 MAX5052A/ MAX5053A are offered with a 50% maximum duty-cycle limit. The MAX5052B/MAX5053B are offered with a 75% maximum duty-cycle limit. These devices are available in 8-pin MAX packages and operate over the -40C to +85C temperature range.
Features
Available in a Tiny 8-Pin MAX Package Current-Mode Control 50W Output Power Universal Offline Input Voltage Range Rectified 85VAC to 265VAC (MAX5052) VIN Directly Driven from 10.8V to 24V Input (MAX5053) Digital Soft-Start Programmable Input Startup Voltage Internal Bootstrap UVLO with Large Hysteresis (MAX5052) Internal Error Amplifier with 1% Accurate Reference Thermal Shutdown 45A (typ) Startup Supply Current 1.4mA (typ) Operating Supply Current Fixed Switching Frequency of 262kHz 12% 50% Maximum Duty-Cycle Limit (MAX5052A/MAX5053A) 75% Maximum Duty-Cycle Limit (MAX5052B/MAX5053B) 60ns Cycle-by-Cycle Current-Limit Response Time
MAX5052/MAX5053
Ordering Information
PART MAX5052AEUA MAX5052BEUA MAX5053AEUA MAX5053BEUA TEMP RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 8 MAX 8 MAX 8 MAX 8 MAX
Applications
Universal Input AC Power Supplies Isolated Telecom Power Supplies Networking Systems Computer Systems/ Servers Industrial Power Conversion Isolated Keep-Alive Circuits 12V Boost Regulators 12V SEPIC Regulators
Warning: The MAX5052/MAX5053 are designed to work with high voltages. Exercise caution.
Pin Configuration
TOP VIEW
UVLO/EN 1 FB 2 COMP 3 8 7 VIN VCC NDRV GND
MAX5052 MAX5053
6 5
CS 4
Functional Diagram/Typical Operating Circuit/Selector Guide appear at end of data sheet.
MAX 1
________________________________________________________________ Maxim Integrated Products
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Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
ABSOLUTE MAXIMUM RATINGS
VIN to GND .............................................................-0.3V to +30V VCC to GND ............................................................-0.3V to +13V FB, COMP, UVLO, CS to GND .................................-0.3V to +6V NDRV to GND.............................................-0.3V to (VCC + 0.3V) Continuous Power Dissipation (TA = +70C) 8-Pin MAX (derate 4.5mW/C above +70C) ..............362mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range ............................-65C to +150C Junction Temperature ......................................................+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 = +12V (for MAX5052, VIN must first be brought up to 23.6V for startup), 10nF bypass capacitors at VIN and VCC, CNDRV = 0, VUVLO = +1.4V, VFB = +1.0V, VCOMP = floating, VCS = 0V, typical values are measured at TA = +25C, TA = -40C to + 85C, unless otherwise noted.) (Note 1)
PARAMETER Bootstrap UVLO Wake-Up Level Bootstrap UVLO Shutdown Level UVLO/EN Wake-Up Threshold UVLO/EN Shutdown Threshold UVLO/EN Input Current UVLO/EN Hysteresis VIN Supply Current In Undervoltage Lockout VIN Range UVLO/EN Propagation Delay Bootstrap UVLO Propagation Delay INTERNAL SUPPLY VCC Regulator Set Point VIN Supply Current After Startup Shutdown Supply Current GATE DRIVER Driver Output Impedance Driver Peak Sink Current Driver Peak Source Current PWM COMPARATOR Comparator Offset Voltage CS Input Bias Current Comparator Propagation Delay Minimum On-Time VOPWM ICS tPWM tON(MIN) VCOMP - VCS VCS = 0V VCS = +0.1V 1.15 -2 60 150 1.38 1.70 +2 V A ns ns RON(LOW) Measured at NDRV sinking, 100mA 2 4 1 0.65 4 12 RON(HIGH) Measured at NDRV sourcing, 20mA A A VCCSP IIN VIN = +10.8V to +24V, sinking 1A to 20mA from VCC VIN = +24V UVLO/EN = low 7 1.4 10.5 2.5 90 V mA A ISTART VIN tEXTR tEXTF tBUVR tBUVF UVLO/EN steps up from +1.1V to +1.4V UVLO/EN steps down from +1.4V to +1.1V VIN steps up from +9V to +24V VIN steps down from +24V to +9V VIN = +19V, for MAX5052 only when in bootstrap UVLO 10.8 12 1.8 5 1 SYMBOL VSUVR VSUVF VULR2 VULF2 IUVLO CONDITIONS VIN rising (MAX5052 only) VIN falling (MAX5052 only) UVLO/EN rising UVLO/EN falling TJ = +125C MIN 19.68 9.05 1.188 1.168 TYP 21.6 9.74 1.28 1.23 25 50 45 90 24 MAX 23.60 10.43 1.371 1.291 UNITS V V V V nA mV A V s s
UNDERVOLTAGE LOCKOUT/STARTUP
2
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Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +12V (for MAX5052, VIN must first be brought up to 23.6V for startup), 10nF bypass capacitors at VIN and VCC, CNDRV = 0, VUVLO = +1.4V, VFB = +1.0V, VCOMP = floating, VCS = 0V, typical values are measured at TA = +25C, TA = -40C to + 85C, unless otherwise noted.) (Note 1)
PARAMETER CURRENT-LIMIT COMPARATOR Current-Limit Trip Threshold CS Input Bias Current Propagation Delay From Comparator Input to NDRV Switching Frequency Maximum Duty Cycle VIN CLAMP VOLTAGE VIN Clamp Voltage ERROR AMPLIFIER Voltage Gain Unity-Gain Bandwidth Phase Margin FB Input Offset Voltage COMP Pin Clamp Voltage Source Current Sink Current Reference Voltage Input Bias Current COMP Short-Circuit Current THERMAL SHUTDOWN Thermal-Shutdown Temperature Thermal Hysteresis DIGITAL SOFT-START Soft-Start Duration Reference Voltage Steps During Soft-Start Reference Voltage Step 15,872 31 40 clock cycles steps mV 130 25 C C 8 VREF (Note 2) High Low 2.2 0.4 0.5 0.5 1.218 1.230 1.242 50 RLOAD = 100k RLOAD = 100k, CLOAD = 200pF RLOAD = 100k, CLOAD = 200pF 80 2 65 3 3.5 1.1 dB MHz degrees mV V mA mA V nA mA VINC 2mA sink current, MAX5052 only (Note 3) 24.1 26.1 29.0 V VCS ICS tPWM fSW DMAX MAX505_A MAX505_B VCS = 0V 50mV overdrive 230 262 -2 60 262 50 75 290 50.5 76 291 320 +2 mV A ns kHz % SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX5052/MAX5053
Note 1: All devices are 100% tested at TA = +85C. All limits over temperature are guaranteed by characterization. Note 2: VREF is measured with FB connected to the COMP pin (see Functional Diagram). Note 3: The MAX5052 is intended for use in universal input power supplies. The internal clamp circuit is used to prevent the bootstrap capacitor (C1 in Figure 1) from charging to a voltage beyond the absolute maximum rating of the device when EN/UVLO is low. The maximum current to VIN (hence to clamp) when UVLO is low (device in shutdown) must be externally limited to 2mA, max. Clamp currents higher than 2mA may result in clamp voltage higher than 30V, thus exceeding the absolute maximum rating for VIN. For the MAX5053, do not exceed the 24V maximum operating voltage of the device.
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3
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
Typical Operating Characteristics
(UVLO = +1.4V, VFB = +1V, VCOMP = floating, VCS = 0V, TA = +25C, unless otherwise noted.)
BOOTSTRAP UVLO WAKE-UP LEVEL vs. TEMPERATURE
MAX5052 toc01
BOOTSTRAP UVLO SHUTDOWN LEVEL vs. TEMPERATURE
MAX5052 VIN FALLING 10.0 UVLO/EN (V)
MAX5052 toc02
UVLO/EN WAKE-UP THRESHOLD vs. TEMPERATURE
UVLO/EN RISING 1.275 1.270 1.265 1.260
MAX5052 toc03 MAX5052 toc09 MAX5052 toc06
21.60 MAX5052 VIN RISING 21.55 21.50 VIN (V) 21.45 21.40
10.1
1.280
VIN (V)
9.9
9.8 21.35 21.30 -40 -20 0 20 40 60 80 TEMPERATURE (C) 9.7 -40 -20 0 20 40 60 80 TEMPERATURE (C) 1.255 1.250 -40 -20 0 20 40 60 80 TEMPERATURE (C)
UVLO/EN SHUTDOWN THRESHOLD vs. TEMPERATURE
MAX5052 toc04
VIN SUPPLY CURRENT IN UNDERVOLTAGE LOCKOUT vs. TEMPERATURE
MAX5052 toc05
VIN SUPPLY CURRENT AFTER STARTUP vs. TEMPERATURE
1.5 VIN = 24V 1.4 IIN (mA)
1.30 UVLO/EN FALLING
52 51 50 49 VIN = 19V MAX5052 WHEN IN BOOTSTRAP UVLO MAX5053 WHEN UVLO/EN IS LOW
1.25 UVLO/EN (V) ISTART (A)
48 47 46 45 44 43
1.20
1.3
1.15
1.2
1.10 -40 -20 0 20 40 60 80 TEMPERATURE (C)
42 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.1 -40 -20 0 20 40 60 80 TEMPERATURE (C)
VCC REGULATOR SET POINT vs. TEMPERATURE
MAX5052 toc07
VCC REGULATOR SET POINT vs. TEMPERATURE
MAX5052 toc08
CURRENT-LIMIT TRIP THRESHOLD vs. TEMPERATURE
310 CURRENT-LIMIT TRIP THRESHOLD (mV) 305 300 295 290 285 280 275 270 -40 -20 0 20 40 60 80 -3 MEAN +3 TOTAL NUMBER OF DEVICES = 100
9.8 9.7 9.6 VCC (V) 9.5 9.4 9.3 9.2 -40 -20 0 20 40 60 80 TEMPERATURE (C) NDRV OUTPUT IS SWITCHING VIN = 19V NO LOAD NDRV OUTPUT IS NOT SWITCHING, VFB = 1.5V
8.9 VIN = 10.8V 8.8 8.7 10mA LOAD 8.6 VCC (V) 8.5 8.4 20mA LOAD 8.3 8.2 8.1 -40 -20 0 20 40 60 80 TEMPERATURE (C)
TEMPERATURE (C)
4
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Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
Typical Operating Characteristics (continued)
(UVLO = +1.4V, VFB = +1V, VCOMP = floating, VCS = 0V, TA = +25C, unless otherwise noted.)
SWITCHING FREQUENCY vs. TEMPERATURE
MAX5052 toc11 MAX5052 toc10
CURRENT-LIMIT TRIP THRESHOLD
30 25 PERCENTAGE OF UNITS (%) 20 15 10 5 0 260 270 280 290 300 310 320 CURRENT-LIMIT TRIP THRESHOLD (mV) 280 275 SWITCHING FREQUENCY (kHz) 270 265 260 255 250 245 240 -40 -3 MEAN +3 TOTAL NUMBER OF DEVICES = 200
SWITCHING FREQUENCY
TOTAL NUMBER OF DEVICES = 200
MAX5052 toc12 MAX5052 toc18 MAX5052 toc15
TOTAL NUMBER OF DEVICES = 100
30 25 PERCENTAGE OF UNITS (%) 20 15 10 5 0
-20
0
20
40
60
80
230
240
250
260
270
280
290
TEMPERATURE (C)
SWITCHING FREQUENCY (kHz)
PROPAGATION DELAY FROM CURRENT-LIMIT COMPARATOR INPUT TO NDRV vs. TEMPERATURE
MAX5052 toc13
UVLO/EN PROPAGATION DELAY vs. TEMPERATURE
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 -40 -20 0 20
MAX5052 toc14
REFERENCE VOLTAGE vs. TEMPERATURE
1.230 VIN = 12V
75
UNDERVOLTAGE LOCKOUT DELAY (s)
UVLO/EN RISING
REFERENCE VOLTAGE (V) UVLO/EN FALLING
70
1.229
tPWM (ns)
65
1.228
60
1.227
55
1.226
50 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.225 40 60 80 -40 -20 0 20 40 60 80 TEMPERATURE (C) TEMPERATURE (C)
INPUT CURRENT vs. INPUT CLAMP VOLTAGE
MAX5052 toc16
INPUT CLAMP VOLTAGE vs. TEMPERATURE
26.8 INPUT CLAMP VOLTAGE (V) 26.6 26.4 RON () 26.2 26.0 25.8 25.6 25.4 25.2 25.0 IIN = 2mA
MAX5052 toc17
NDRV OUTPUT IMPEDANCE vs. TEMPERATURE
2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 VIN = 24V SINKING 100mA
10 9 8 INPUT CURRENT (mA) 7 6 5 4 3 2 1 0
27.0
10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 INPUT VOLTAGE (V)
-40
-20
0
20
40
60
80
-40
-20
0
20
40
60
80
TEMPERATURE (C)
TEMPERATURE (C)
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5
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
Typical Operating Characteristics (continued)
(UVLO = +1.4V, VFB = +1V, VCOMP = floating, VCS = 0V, TA = +25C, unless otherwise noted.)
NDRV OUTPUT IMPEDANCE vs. TEMPERATURE
4.8 4.6 4.4 GAIN (dB) RON () 4.2 4.0 3.8 3.6 3.4 3.2 3.0 -40 -20 0 20 40 60 80 TEMPERATURE (C) VIN = 24V SOURCING 20mA
MAX5052 toc19
ERROR AMP OPEN-LOOP GAIN AND PHASE vs. FREQUENCY
120 100 80 60 40 20 0 -20 -40 -60 -80 -100 0.1 1 10 100 1k PHASE GAIN
MAX5052 toc20
5.0
50 30 10 -30 -50 -70 -90 -110 -130 -150 PHASE (DEGREES) -10
-170 10k 100k 1M 10M 100M
FREQUENCY (Hz)
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME UVLO/EN FB COMP CS GND NDRV VCC VIN FUNCTION Externally Programmable Undervoltage Lockout. UVLO programs the input start voltage. Connect UVLO to GND to disable the device. Error-Amplifier Inverting Input Error-Amplifier Output Current-Sense Connection for PWM Regulation and Overcurrent Protection. Connect to high side of sense resistor. An RC filter may be necessary to eliminate leading-edge spikes. 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. For bootstrapped operation (MAX5052) connect a startup resistor from the input supply line to VIN. Connect the bias winding supply to this point as well (see the Typical Operating Circuit). For the MAX5053, connect VIN directly to 10.8V to 24V supply.
Detailed Description
The MAX5052/MAX5053 are current-mode PWM controllers that have been specifically designed for use in isolated and nonisolated power-supply applications. A bootstrap UVLO with a large hysteresis (11.9V), very low startup current, and low operating current result in efficient universal-input power supplies. In addition to the internal bootstrap UVLO, these devices also offer programmable input startup voltage programmed through the UVLO/EN pin. This feature is useful in preventing the power supply from entering a brownout condition, in case the input voltage drops below its minimum value. This is important since switching power
6
supplies increases their input supply current as the input voltage drops in order to keep the output power constant. The MAX5052 is well suited for universal input (rectified 85VAC to 265VAC) or telecom (-36VDC to -72VDC) power supplies. The MAX5053 is well suited for low-input-voltage (10.8VDC to 24VDC) power supplies. Power supplies designed with the MAX5052 use a high-value startup resistor, R1, that charges a reservoir capacitor, C1 (see Figure 1). During this initial period, while the voltage is less than the internal bootstrap UVLO threshold, the device typically consumes only 45A of quiescent current. This low startup current and the large bootstrap UVLO hysteresis helps to minimize
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Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies
D1 VSUPPLY R2 Q1 VIN C1 C2 C3 COMP FB GND UVLO/EN R3 VCC MAX5052 NDRV CS R4 R5 T1 D2 VOUT
(max). V IN is the value of the input-supply voltage where the power supply must start. V - VULR2 R2 = IN x R3 VULR2 where IUVLO is the UVLO/EN pin input current (50nA), and VULR2 is the UVLO/EN wake-up threshold.
MAX5052/MAX5053
R1
C4
R6
MAX5052 Bootstrap Undervoltage Lockout
In addition to the externally programmable UVLO function offered in both the MAX5052 and MAX5053, the MAX5052 has an additional internal bootstrap UVLO that is very useful when designing high-voltage power supplies (see the Functional Diagram). This allows the device to bootstrap itself during initial power-up. The MAX5052 attempts to start when VIN exceeds the bootstrap UVLO threshold of 21.6V. During startup, the UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver shut down to reduce current consumption. Once VIN reaches 21.6V, 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 9.7V, the UVLO circuit will shut down the CPWM comparator, ILIM comparator, oscillator, and output driver returning the MAX5052/MAX5053 to the startup mode.
0V
Figure 1. Nonisolated Power Supply with Programmable InputSupply Start Voltage
the power dissipation across R1 even at the high end of the universal AC input voltage (265VAC). The MAX5052/MAX5053 include a cycle-by-cycle current limit that turns off the gate drive to the external MOSFET during an overcurrent condition. When using the MAX5052 in the bootstrapped mode (if the powersupply output is shorted), the tertiary winding voltage drops below the 10V threshold causing the UVLO to turn off the gate drive to the external power MOSFET. This reinitiates a startup sequence with soft-start.
MAX5052 Startup Operation
Normally VIN is derived from a tertiary winding of the transformer. However, at startup there is no energy delivered through the transformer, hence, a special bootstrap sequence is required. Figure 2 shows the voltages on VIN and VCC during startup. Initially, both VIN and VCC are 0V. After the line voltage is applied, C1 charges through the startup resistor, R1, to an intermediate voltage. At this point, the internal regulator begins charging C2 (see Figure 1). The MAX5052 uses only 45A of the current supplied by R1, and the remaining input current charges C1 and C2. The charging of C2 stops when the VCC voltage reaches approximately 9.5V, while the voltage across C1 continues rising until it reaches the wake-up level of 21.6V. Once V IN exceeds the bootstrap UVLO threshold, NDRV begins switching the MOSFET and transfers energy to the secondary and tertiary outputs. If the voltage on the tertiary output builds to higher than 9.9V (the bootstrap UVLO lower threshold), then startup has been accomplished and sustained operation commences.
MAX5052/MAX5053 Undervoltage Lockout
The MAX5052/MAX5053 have an input voltage UVLO/EN pin. The threshold for this UVLO is 1.28V. Before any operation can commence, the voltage on this pin has to exceed 1.28V. The UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver shut down to reduce current consumption (see the Functional Diagram). Use this UVLO function to program the input-supply start voltage. For example, a reasonable start voltage for a 36V to 72V telecom range might be set at 34V. Calculate the divider resistor values, R2 and R3 (see Figure 1) by using the following formulas: R3 VULR2 x VIN 500 x IUVLO ( VIN - VULR2 )
The value of R3 is calculated to minimize the voltagedrop error across R2 as a result of the input bias current of the UVLO/EN pin. VULR2 = 1.28V, IUVLO = 50nA
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7
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
VCC 2V/div
MAX5052 VIN PIN 5V/div
where IIN is the MAX5052's internal supply current after startup (1.4mA), Qgtot is the total gate charge for Q1, fSW is the MAX5052's switching frequency (262kHz), Vhyst is the bootstrap UVLO hysteresis (12V) and tss is the internal soft-start time (60ms). For example: Ig = (8nC) (262kHz) 2.1mA C1=
0V
(1.4m A+ 2.1m A) (6 0m s) =1 7.5F (1 2V)
100ms/div
Figure 2. VIN and VCC During Startup when Using the MAX5052 in Bootstrapped Mode (Figure 1)
If VIN drops below 9.9V before startup is complete, the device goes back to low-current UVLO. In this case, increase the value of C1 in order to store enough energy to allow for the voltage at tertiary winding to build up.
choose 15F standard value. Assuming C1 > C2, calculate the value of R1 as follows: V x C1 IC1 = SUVR (500ms) R1 = VIN(MIN) - VSUVR IC1 + ISTART
Startup Time Considerations For Power Supplies Using the MAX5052
The VIN bypass capacitor, C1, supplies current immediately after wake up (see Figure 1). The size of C1 and the connection configuration of the tertiary winding determine the number of cycles available for startup. Large values of C1 increase the startup time but also supply gate charge for more cycles during initial startup. If the value of C1 is too small, VIN drops below 9.9V because NDRV does not have enough time to switch and build up sufficient voltage across the tertiary output which powers the device. The device goes back into UVLO and does not start. Use a low-leakage capacitor for C1 and C2. As a rule of thumb, offline power supplies keep typical startup times to less than 500ms even in low-line conditions (85VAC input for universal offline or 36VDC for telecom applications). Size the startup resistor, R1, to supply both the maximum startup bias of the device (90A) and the charging current for C1 and C2. The bypass capacitor, C2, must charge to 9.5V and C1 to 24V, all within the desired time period of 500ms. Because of the internal 60ms soft-start time of the MAX5052, C1 must store enough charge to deliver current to the device for at least this much time. To calculate the approximate amount of capacitance required, use the following formula: Ig = Qgtot x fSW C1 =
where VIN(MIN) is the minimum input supply voltage for the application (36V for telecom), VSUVR is the bootstrap UVLO wake-up level (23.6V max.), ISTART is the VIN supply current at startup (90A, max). For example: IC1 =
(24V) (15F) = 0.72mA (500ms)
= 29.6k
R1 =
(36V) - (12V) (0.72mA) + (90A)
(IIN + Ig ) (tSS )
Vhyst
choose 32k standard value. Choose a higher value for R1 than the one calculated above if longer startup time can be tolerated in order to minimize power loss on this resistor. The above startup method is applicable to a circuit similar to the one shown in Figure 1. In this circuit, the tertiary winding has the same phase as the output windings. Thus, the voltage on the tertiary winding at any given time is proportional to the output voltage and goes through the same soft-start period as the output voltage. The minimum discharge voltage of C1 from 22V to 10V must be greater than the soft-start time of 60ms. Another method for bootstrapping the power supply is to have a separate bias winding than the one used for regulating the output voltage and to connect the bias winding so that it is in phase with the MOSFET ON time (see Figure 3). The amount of capacitance required is much
8
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Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
D1 +VIN R2 Q1 VIN C1 VCC MAX5052 U2 OPTO TRANS R5 FB R6 -VIN UVLO/EN R3 R7 COMP CS GND R4 U3 TL431 R10 U2 OPTO LED U1 NDRV R8 R9 C3 T1 D2 R1
VOUT
C4
C2
Figure 3. Secondary-Side Regulated, Isolated Power Supply
smaller. However, in this mode, the input voltage range has to be roughly 2:1. Another consideration is if the bias winding is in phase with the output, then the power supply hiccups and soft-start under output short-circuit conditions. Whereas, this property is lost if the bias winding is in phase with the MOSFET ON time.
1V/div
Soft-Start
The MAX5052/MAX5053 soft-start feature allows the load voltage to ramp up in a controlled manner, eliminating output voltage overshoot. Soft-start begins after UVLO is deasserted. The voltage applied to the noninverting node of the amplifier ramps from 0 to 1.23V in over a 60ms soft-start timeout period. Figure 4 shows the 5V output of the power-supply circuit in Figure 5 during startup. Note the staircase increase of the output voltage. This is a result of the digital soft-starting technique used. Unlike other devices, the MAX5052/MAX5053 reference voltage to the internal amplifier is soft-started; this method results in superior control of the output voltage under heavy- and light-load conditions.
0V 10ms/div
Figure 4. Output Voltage Soft-Start During Initial Startup for the Circuit of Figure 5
650mA/1000mA peak current, so select a MOSFET that yields acceptable conduction and switching losses.
Internal Oscillator
The internal oscillator switches at 1.048MHz and is divided down to 262kHz by two D flip-flops. The MAX5052A/MAX5053A invert the Q output of the last D flip-flop to provide a duty cycle of 50% (Figure 6). The MAX5052B/MAX5053B perform a logic NAND operation on the Q outputs of both D flip-flops to provide a duty cycle of 75%.
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 9.5V. 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 offline applications that use the discontinuous flyback topology, this requires a MOSFET rated at 600V. NDRV can source/sink in excess of the
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Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
IN D8 R6 33k +VIN +VIN C1 1F 100V C2 1F 100V R8* OPEN C10* OPEN D3* OPEN R1 22.6k 1% 2 R2 2.49k 1% R9 4.3k C9 2200pF 3 C14 0.022F 1 R3 1M 1% 7 R4 42.2k 1% C7 0.22F COMP 6 R10 0 CS 4 C8 OPEN GND 5 FB VIN 8 C11 0.22F 4 12 R11 100 R5 0.17 1% 56 78 N1 3 3 2 6 1 C6 0.0047F 250VAC 7 D1 C3 68F 6.3V D4 L1 C13 1F VOUT1 (+5V/1.5A) C4 22F 6.3V C16 15F 35V FB_P D6 L2 T1 5 4 8 9 10 D2 C5 47F 25V C15 1F VOUT2 (+15V/0.1A)
C12 0.22F
R7 1.2k
R12 1.2k
D7* OPEN
D5
SGND
SGND
FB_P
IN
U1
NDRV
+VIN
MAX5052A
UVLO/EN
VCC
-VIN
SHDN JU1
*COMPONENTS MARKED "OPEN" ARE OPTIONAL. (SEE MAX5052A EV KIT DATA SHEET.)
Figure 5. Primary Regulated, Dual-Output, Isolated Telecom Power Supply
D OSCILLATOR 1.048MHz
Q
D
Q
262kHz WITH 50% (MAX5052A/MAX5053A)
Q
Q
262kHz WITH 75% (MAX5052B/MAX5053B)
Figure 6. Internal Oscillator
Internal Error Amplifier
The MAX5052/MAX5053 include an internal error amplifier that can be used to regulate the output voltage in the case of a nonisolated power supply (see Figure 1) Calculate the output voltage using the following equation: R5 VOUT = 1 + VREF R6
where VREF = 1.23V. The amplifier's noninverting input is internally connected to a digital soft-start circuit that gradually increases the reference voltage during startup and is applied to this pin. This forces the output voltage to come up in an orderly and well-defined manner under all load conditions. The error amplifier may also be used to regulate the tertiary winding output which implements a primary-side regulated, isolated power supply (see Figure 5). Calculate the output voltage using the following equation: VOUT1 = NS NT R1 VREF + VD6 1 + R2
-
VD1
where NS is the number of secondary turns for VOUT1, NT is the number of tertiary winding turns, and both VD6 and VD1 are the diode drops at the respective outputs.
10
______________________________________________________________________________________
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies
Current Limit
The current-sense resistor (RCS), connected between the source of the MOSFET and ground, sets the current limit. The CS input has a voltage-trip level (V CS) of 291mV. 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) quickly terminates the current ON-cycle, typically within 60ns. In most cases, a small RC filter is required to filter out the leading-edge spike on the sense waveform. Set the corner frequency at a few megahertz.
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 MAX package have been placed to allow easy interfacing to the external MOSFET. For universal AC input design, all applicable safety regulations must be followed. Offline 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. 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.
MAX5052/MAX5053
Applications Information
Primary Regulated, Isolated Telecom Power Supply
Figure 5 shows a complete design of a dual-output power supply with a telecom voltage range of 36V to 72V. An important aspect of this power supply is its primary-side regulation. This regulation, through the tertiary winding, also acts as bias winding for the MAX5052. In the circuit of Figure 5, cross-regulation has been improved (tertiary and 5V outputs) by using chip inductors, L1 and L2, and R7||R2. R7||R2 presents enough loading on the tertiary winding output to allow 5% load regulation on the 5V output over a load current range from 150mA to 1.5A.
5V OUTPUT LOAD REGULATION
6.0 5.8 5.6 5.4 VOUT (V) 5.2 5.0 4.8 4.6 4.4 4.2 4.0 0.15 0.35 0.55 0.75 0.95 1.15 1.35
Chip Information
TRANSISTOR COUNT: 1449 PROCESS: BiCMOS
L1 12V R2 Q1 VIN C1 C2 C3 COMP FB GND UVLO/EN R3 VCC MAX5053 NDRV CS
D1
15V
C4 R5
R1
R6
0V
IOUT (A)
Figure 7. Output Voltage Regulation for the Figure 5 Circuit
Figure 8. 12V to 15V Out Boost Regulator
______________________________________________________________________________________
11
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies MAX5052/MAX5053
Functional Diagram
VIN
VIN CLAMP 26.1V BOOTSTRAP UVLO DIGITAL SOFT-START REFERENCE 1.23V 21.6V 9.74V **
IN REGULATOR REG_OK
VCC
VCC
VL
(INTERNAL 5.25V SUPPLY) UVLO 1.28V 1.23V DRIVER S FB ERROR AMP CPWM CS 1.4V VOPWM THERMAL SHUTDOWN VCS 0.3V OSCILLATOR 262kHz* GND R Q NDRV UVLO
COMP
MAX5052/MAX5053
ILIM
*MAX5052A/MAX5053A: 50% MAXIMUM DUTY CYCLE, MAX5052B/MAX5053B: 75% MAXIMUM DUTY CYCLE. **MAX5052 ONLY.
Typical Operating Circuit
D1 C5 VSUPPLY R2 Q1 VIN C1 C2 R5 C3 COMP FB R6 GND UVLO/EN R3 VCC MAX5052 NDRV CS R4 R7 T1 D2
Selector Guide
PART MAX5052A BOOTSTRAP UVLO Yes Yes No No STARTUP VOLTAGE 22V 22 V 10.8V* 10.8V* MAX DUTY CYCLE 50% 75% 50% 75%
C4 R1 D4
VOUT
MAX5052B MAX5053A MAX5053B
*The MAX5053 does not have an internal bootstrap UVLO. The MAX5053 starts operation as long as the VCC pin is higher than 7V (the guaranteed output with a VIN pin voltage of 10.8V) and the UVLO/EN pin is high.
0V
12
______________________________________________________________________________________
Current-Mode PWM Controllers with an Error Amplifier for Isolated/Nonisolated Power Supplies
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.)
8LUMAXD.EPS
MAX5052/MAX5053
8
4X S
8
INCHES DIM A A1 A2 b c D e E H MIN 0.002 0.030 MAX 0.043 0.006 0.037
MILLIMETERS MAX MIN 0.05 0.75 1.10 0.15 0.95
y 0.500.1 0.60.1
E
H
1
0.60.1
1
D
L
S
BOTTOM VIEW
0.014 0.010 0.007 0.005 0.120 0.116 0.0256 BSC 0.120 0.116 0.198 0.188 0.026 0.016 6 0 0.0207 BSC
0.25 0.36 0.13 0.18 2.95 3.05 0.65 BSC 2.95 3.05 4.78 5.03 0.41 0.66 0 6 0.5250 BSC
TOP VIEW
A2
A1
A
e
c b L
SIDE VIEW
FRONT VIEW
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 8L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0036
J
1 1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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