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MAX256ASA+;MAX256ASA+T;中文规格书,Datasheet资料

The MAX256 is an integrated primary-side controller and H-bridge driver for isolated power-supply circuits.The device contains an on-board oscillator, protection circuitry and internal FET drivers to provide up to 3W of power to the primary winding of a transformer. The MAX256 can be operated using the internal program-mable oscillator or can be driven by an external clock for improved EMI performance. Regardless of the clock source being used, an internal flip-flop stage guaran-tees a fixed 50% duty cycle to prevent DC current flow in the transformer.

The MAX256 operates from a single-supply voltage of +5V or +3.3V, and includes undervoltage lockout for controlled startup. The device prevents cross-conduc-tion of the H-bridge MOSFETs by implementing break-before-make switching. Thermal shutdown circuitry provides additional protection against damage due to overtemperature conditions.

The MAX256 is available in the 8-pin thermally-enhanced SO package. The device is specified for the automotive (-40°C to +125°C) temperature range.

Applications

Features

o Provides Up to 3W to the Transformer in Isolated Power Supplies o Single Supply +5V or +3.3V Operation

o Internal Resistor-Programmable Oscillator Mode o External Clock Mode with Watchdog o Disable Mode

o Undervoltage Lockout o Thermal Shutdown

MAX256

________________________________________________________________Maxim Integrated Products 1

For pricing, delivery, and ordering information,please contact Maxim/Dallas Direct!at

*EP = Exposed paddle.

*/V denotes an automotive qualified part.

+Denotes a lead(Pb)-free/RoHS-compliant package.T = Tape and reel.

Isolated Power Supplies

Industrial Process Control

Isolated Communications Links

Medical Equipment

Telecommunications

M A X 256

for Isolated Supplies

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ABSOLUTE MAXIMUM RATINGS

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.

(All voltages referenced to GND, unless otherwise noted.)

Supply Voltage V CC ..................................................-0.3V to +6V ST1, ST2, CK_RS, MODE (Note 1)................-0.3V to V CC + 0.3V ST1, ST2 Maximum Continuous Current (T A < +125°C)....±0.6A ST1, ST2 Maximum Continuous Current (T A < +100°C)....±0.9A ST1, ST2 Maximum Continuous Current (T A < +85°C)......±1.0A

Continuous Power Dissipation (T A = +70°C)

8-Pin SO (derate 18.9mW/°C above +70°C)..............1509mW Operating Temperature Range .........................-40°C to +125°C Storage Temperature Range.............................-65°C to +150°C Junction Temperature......................................................+150°C Lead Temperature (soldering, 10s).................................+300°C Soldering Temperature (reflow).......................................+260°C

Note 1:ST1 and ST2 are not protected against short circuits. Damage to the device may result from a short-circuit fault.

SO-EP

Junction-to-Ambient Thermal Resistance (θJA )...............53°C/W Junction-to-Case Thermal Resistance (θJC )......................5°C/W

PACKAGE THERMAL CHARACTERISTICS (Note 2)

MAX256

for Isolated Supplies

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TIMING CHARACTERISTICS

Note 4:Total driver resistance includes the on-resistance of the top and the bottom internal FETs. If R OH is the high-side resistance,

and R OL is the low-side resistance, R OHL = R OH + R OL .

M A X 256

for Isolated Supplies

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Typical Operating Characteristics

(V CC = +5.0V ±10%, T A = +25°C, unless otherwise noted.) (See Figure 8)

SUPPLY CURRENT vs. OSCILLATOR FREQUENCY

M A X 256t o c 01

OSCILLATOR FREQUENCY (kHz)

S U P P L Y C U R R E N T (m A )

90040023456711000

2003005006007008001000

OSCILLATOR FREQUENCY vs. R S (+1%)

R S (k ?)

O S C I L L A T O R F R E Q U E N C Y

(k H z )0

200400600800100012001400

1001000

R S vs. REQUIRED ET PRODUCT

REQUIRED ET PRODUCT (V μs)

R S (k ?)

100

1000

100

OUTPUT VOLTAGE vs. OUTPUT CURRENT (TYPICAL APPLICATION FIGURE 8)

OUTPUT CURRENT (mA)

O U T P U T V O L T A G E (V )

200400600800024

681012EFFICIENCY vs. OUTPUT CURRENT (TYPICAL APPLICATION FIGURE 8)

OUTPUT CURRENT (mA)

E F F I C I E N C Y

2004006008000

0.10.20.30.40.50.60.70.80.91.0OUTPUT VOLTAGE vs. OUTPUT CURRENT (TYPICAL APPLICATION FIGURE 9)

OUTPUT CURRENT (mA)

O U T P U T V O L T A G E (V )

0100

200300400500

1012EFFICIENCY vs. OUTPUT CURRENT

(CIRCUIT OF FIGURE 9)

OUTPUT CURRENT (mA)

E F F I C I E N C Y

100

200300400500

00.10.20.30.40.50.60.70.80.91.0OUTPUT VOLTAGE vs. OUTPUT CURRENT

(CIRCUIT OF FIGURE 10)

OUTPUT CURRENT (mA)

O U T P U T V O L T A G E (V )

406080100120

140

152025

303540EFFICIENCY vs. OUTPUT CURRENT

(CIRCUIT OF FIGURE 10)

OUTPUT CURRENT (mA)

E F F I C I E N C Y

406080100120

140

0.1

0.20.30.40.50.60.70.80.91.0

MAX256

for Isolated Supplies

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OPERATION AT 100kHz

M A X 256t o c 1

1μs/div CK_RS 5V/div ST15V/div ST25V/div

M A X 256t o c 11

100ns/div

CK_RS 5V/div ST15V/div ST25V/div

OPERATION WITH EXTERNAL 2MHz CLOCK

Typical Operating Characteristics (continued)

(V CC = +5.0V ±10%, T A = +25°C, unless otherwise noted.) (See Figure 8)

M A X 256

Detailed Description

The MAX256 is an integrated primary-side controller and H-bridge driver for isolated power-supply circuits.The device contains an on-board oscillator, protection circuitry, and internal FET drivers to provide up to 3W of power to the primary winding of a transformer. The MAX256 can be operated using the internal program-mable oscillator, or can be driven by an external clock for improved EMI performance. Regardless of the clock source being used, an internal flip-flop stage guaran-tees a fixed 50% duty cycle to prevent DC current flow in the transformer.

Oscillator Modes

The MAX256 is driven by the internal programmable oscillator or an external clock. The logic state of MODE determines the clock source (see Table 1). Drive MODE high to select the internal resistor programmable oscillator. Drive MODE low to operate the MAX256 with an external clock signal on CK_RS.

Internal Oscillator Mode

The MAX256 includes a 100kHz to 1MHz programma-ble oscillator. Set the oscillator frequency by connect-ing CK_RS to ground with a 10k ?or larger resistor.Leave CK_RS unconnected to set the oscillator to the minimum default frequency of 100kHz. CK_RS is inter-nally pulled to ground with a 165k ?resistor.

External Clock Mode

The MAX256 provides an external clock mode. When operating in external clock mode, an internal flip-flop divides the external clock by two in order to generate a switching signal with a guaranteed 50% duty cycle. As a result, the MAX256 outputs switch at one half the external clock frequency. The device switches on the rising edge of the external clock signal.

Watchdog

When the MAX256 is operating in external clock mode,a stalled clock could cause excessive DC current to

flow through the primary winding of the transformer.The MAX256 features an internal watchdog circuit to prevent damage from this condition. The MAX256 is disabled when the external clock signal on CK_RS remains at the same logic level for longer than 55μs (max). The device resumes normal operation upon the next rising edge on CK_RS.

Disable Mode

When using the internal oscillator, drive MODE low to disable the MAX256. The device is disabled within 55μs after MODE goes low. When operating in external clock mode, suspend the clock signal for longer than 55μs to disable the MAX256. The device resumes nor-mal operation when MODE is driven high or when the external clock signal resumes.

Power-Up and Undervoltage Lockout

The MAX256 provides an undervoltage lockout feature to ensure a controlled power-up state and prevent operation before the oscillator has stabilized. On power-up and during normal operation (if the supply voltage drops below 1.8V), the undervoltage lockout disables the device.

Thermal Shutdown

The MAX256 is protected from overtemperature dam-age by a thermal shutdown circuit. When the junction temperature (T J ) exceeds +165°C, the device is dis-abled. The device resumes normal operation when T J falls below +155°C.

ESD Protection

As with all Maxim devices, ESD-protection structures are incorporated on all pins to protect against electro-static discharges encountered during handling and assembly.

ESD Test Conditions

ESD performance depends on a variety of conditions.Please contact Maxim for a reliability report document-ing test setup, methodology, and results.

for Isolated Supplies

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former as close as possible to the MAX256 using short,wide traces.

When the device is operating with the internal oscillator,it is possible for high-frequency switching components on ST1 and ST2 to couple into the CK_RS circuitry

through PC board parasitic capacitance. This capacitive coupling can induce duty-cycle errors in the oscillator,resulting in a DC current through the transformer. To ensure proper operation, shield the CK_RS circuitry

MAX256

for Isolated Supplies

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Figure 1. Secondary-Side Rectification Topologies

M A X 256

from ST1 and ST2 by placing a grounded trace between these circuits. Place R S as close as possible to the CK_RS pin. An additional capacitance of 100nF from CK_RS to GND may be required in some applications.

Output Voltage Regulation

For many applications, the unregulated output of the MAX256 meets the supply voltage tolerances. This con-figuration represents the highest efficiency possible with the MAX256.

For applications requiring a regulated output voltage,Maxim provides several solutions. In the following examples, assume a tolerance of ±10% variation for the input voltage.

When a full-bridge power supply is operated under maximum input voltage and low output load current, the voltage at the output of the rectifier network can exceed the absolute maximum input voltage of the low dropout regulator (LDO). If the minimum output load current is less than approximately 5mA, connect a zener diode from the output voltage to ground (as shown in Figure 2) to limit the output to a safe value.

+3.3V to Isolated, Regulated +5.0V

In the circuit of Figure 2, the MAX1659 LDO regulates the output of the MAX256 to +5V. The Halo TG M-H281NF provides a center-tapped 1:2.6 turns ratio, and the secondary circuit implements a 4-diode bridge rec-tifier (Figure 1C).

For a minimum input voltage of +3.0V, the output volt-age of the bridge rectifier is approximately +5.5V at a current of 200mA. A 15V zener diode protects the LDO from high input voltages, but adds a few microamps to the no-load input current of the MAX256.

+5V to Isolated, Regulated +3.3V

In Figure 3, the MAX1658 LDO is used with the TG M-H281NF transformer and a 2-diode push-pull rectifier (Figure 1A). This topology produces approximately +4.5V at a current of 350mA. The MAX1658 produces a regulated +3.3V output voltage.

+5V to Isolated, Regulated +12V

In Figure 4, the 7812 LDO is used with the TG M-H281NF transformer and the voltage doubler network (Figure 1B). This circuit produces approximately +12.5V at a load current of 150mA. The 7812 produces a regulated +12V output.

+5V to Isolated, Regulated ±15V

In Figure 5, the MAX256 is used with two TGM-280NS transformers and voltage doubler networks (Figure 1B)to supply 20V to a pair of 7815 regulators. The circuit produces a regulated ±15V at 50mA.

Isolated DAC/ADC Interface for Industrial

Process Control

The MAX256 provides isolated power for data convert-ers in industrial process control applications (Figure 6).The 3W isolated power output capability allows for data converters operating across multiple isolation barriers.The power output capability also supports circuitry for signal conditioning and multiplexing.

Isolated RS-485/RS-232 Data Interfaces

The MAX256 provides power for multiple transceivers in isolated RS-485/RS-232 data interface applications. The 3W isolated power output capability of the MAX256allows more than ten RS-485 transceivers simultaneously.

Isolated Power Supply

The MAX256 allows a versatile range of secondary-side rectification circuits (see Figure 1). The secondary transformer winding can be wound to provide a wide range of isolated voltages. The MAX256 delivers 3W of power to the transformer with a +5V supply (-40°C to +85°C). The MAX256 produces up to 2.5W over the +85°C to +125°C temperature range. For a supply volt-age of +3.3V, the MAX256 delivers 2W of power to the transformer over the -40°C to +85°C temperature range, and 1.4W between +85°C and +125°C. Figure 8shows a +5V to isolated +5V application that delivers up to 500mA. In Figure 9, the MAX256 is configured to provide +5V from a +3.3V supply at 350mA, and in Figure 10, the MAX256 provides isolated +15V and -15V at a total current up to 75mA.

The MAX256 provides the advantages of the full-bridge converter topology, including multiple isolated outputs,step-up/step-down or inverted output, relaxed filtering requirements, and low output ripple.

Power-Supply Decoupling

Bypass V CC to ground with a 0.47μF ceramic capacitor as close to the device as possible. Additionally, place a 4.7μF capacitor from V CC to ground.

Exposed Paddle

Ensure that the exposed paddle is soldered to the bot-tom layer ground for best thermal performance. Failure to provide a low thermal impedance path to the ground plane will result in excessive junction temperatures when delivering maximum output power.

for Isolated Supplies

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MAX256

for Isolated Supplies

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Figure 3. +5V to Isolated Regulated +3.3V

Figure 4. +5V to Isolated Regulated +12V

M A X 256

Component Selection

Transformer Selection

Transformer selection for the MAX256 can be simplified by the use of a design metric, the ET product. The ET product relates the maximum allowable magnetic flux density in a transformer core to the voltage across a winding and switching period. Inductor current in the primary linearly increases with time in the operating region of the MAX256. Transformer manufacturers specify a minimum ET product for each transformer. For the MAX256, the requirement on ET product is calculat-ed as:

By choosing a transformer with sufficient ET product in

the primary winding, it is ensured that the transformer will not saturate during operation. Saturation of the magnetic core results in significantly reduced induc-tance of the primary, and therefore a large increase in current flow. Excessive transformer current results in a temperature rise and possible damage to the trans-former and/or the MAX256.

When CK_RS is unconnected, the internal oscillator is programmed for the minimum frequency. The default required ET product for the MAX256 is 42.3Vμs, (assum-ing +5.5V maximum VCC), or 27.7Vμs for +3.3V opera-tion (assuming +3.6V maximum VCC). Both of these ET products assume the minimum oscillator frequency of 65kHz. See the Typical Operating Characteristics plot,R S vs. Required ET Product to determine the required ET product for a given value of R S .

In addition to the constraint on ET product, choose a transformer with a low DC-winding resistance. Power dissipation of the transformer due to the copper loss is approximated as:

where R PRI is the DC-winding resistance of the primary,and R SEC is the DC-winding resistance of the sec-ondary. In most cases, an optimum is reached when:

For this condition, the power dissipation is equal for the primary and secondary windings.

R N R SEC PRI

=2P I N R R D TX LOAD PRI SEC _=×+????

2for Isolated Supplies

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MAXIM

MAX256ASA+MAX256ASA+T