The A2557xB, A2557xEB, and A2557xLB are specifically designed to provide a cost-effective solution for relay drive applications delivering up to 300 mA drive current per channel. They can also be used to drive incandescent lamps in applications where turn-on time is not a concern. Each of the four outputs will drop 300mA in the on state. The minimum breakdown voltage of the output is 60V and the sustain voltage is 40V. Low power sleep mode is initiated with "enable low" or "all inputs low". In this mode, the supply current drops below 100 microamps.

Overcurrent protection for each channel is designed into these devices and activates at a nominal 500mA . It protects each output from short circuits with supply voltages up to 32 volts. When the output experiences a short circuit, the output current is limited to 500 mA current clamp. Additionally, the folding circuit reduces the current limit and helps keep the device within its SOA (safe operating area) if excessive voltage is present at the output. An XOR circuit compares the input and output states of each driver. If a short or open load condition is detected, the single fault output turns on (active low). The UDx2547B/EB have similar devices for 1.3A operation.

Successive or multiple overload conditions that cause the channel temperature to reach approximately 165°C will cause an additional linear drop in the output current of the affected driver. If the fault condition is corrected, the output stage will return to normal saturation.

The first character of the part number suffix determines the operating temperature range of the device. Suffix 'S-' is standard -20°C to +85°C; suffix 'E-' is -40°C to +85°C; suffix 'K-' is industrial temperature range -40°C to +125°C . Package suffix '-B' devices are 16-pin power downtilts; suffix '-EB' devices are 28-lead power PLCCs; suffix '-LB' are 16-lead power wide-body SOICs for surface mount applications. All packs feature batwing construction for maximum pack power consumption.

feature

300 mA output current per channel; independent overcurrent protection and thermal limit for each driver; output voltage 60V; output SOA protection; fault detection circuitry for open or shorted loads; low quiescent current sleep mode; integrated output flyback/ Clamping diodes; TTL and 5V CMOS compatible inputs.

*The output current is limited to about 500mA per driver, and the junction temperature is limited if you try higher currents. The complete part number includes a suffix to identify the operating temperature range (E-, K-, or S-) and package type (-B, -EB, or -LB). Always order by the full part number, eg A2557KLB.

Functional block diagram

Typical operating characteristics

Circuit description and application

The A2557 low-current quad-supply driver provides the same protected output driver functionality (and is pin-compatible) as the UDx2543/49/59 devices, combined with a fault diagnostic scheme similar to the UDx2547, and an automatic low-current sleep mode capability. These devices monitor their outputs for faults (open or short). For each channel, input and output levels are compared. If these values differ from expected values, a fault condition is flagged by pulling the common fault output low.

The fault output only works when ENABLE is high. The output state is detected by monitoring the OUTn terminal with a comparator with a threshold typically 2.5v. To detect an open output, a 30µA current sink pulls the output below the comparator threshold. A minimum load of about 1 mA is required to ensure proper fault operation. When in "sleep" mode, the fault function is disabled, i.e. the fault goes high and the 30µA output receiver is turned off. The fault output is a typical 60µA switched current sink.

Each channel consists of a TTL/CMOS compatible logic input and a common enable input. A logic high on the input will provide the drive to open the output npn switch. Each output has a current limit circuit that limits the output current by sensing the voltage drop across a low value internal resistor in the output switch transmitter. If this dip reaches a threshold, the base driver of the output switch will decrease to maintain a constant current in the output.

To keep the device within its Safe Operating Area (SOA), this output current limit is further reduced:

8226 ; If power dissipation in the output device raises the local junction temperature above 165°C (nominal) to limit power dissipation (and thus limit local junction temperature). Since each channel has its own thermal limiting circuit, some independence is provided between the output channels, i.e. one channel can operate at thermally reduced current limit while the other channels can provide full drive capability.

• As a function of output voltage. The full current limit of 500 mA (nominal) reaches approximately VO = 8 volts; above this limit, it decreases linearly to approximately 350 mA at VO = 32 volts. This helps improve SOA by immediately reducing peak power pulses to short-circuit loads at high VO.

A logic low on the enable input causes all outputs to turn off, regardless of the state of the input terminals. Additionally, the device will enter a low quiescent current "sleep" mode, reducing the ICC to below 100 microamps. If ENABLE is set high, and any input goes high, the circuit will "auto-wake up". However, if the device is enabled but all inputs are held low, the circuit is still in "sleep" mode.

All outputs have internal flyback diodes with a common cathode connection at the K terminal.

Incandescent lamp driver

High incandescent lamp turn-on (in rapid flow) can lead to poor lamp reliability and destroy semiconductor lamp drivers. When an incandescent lamp is initially turned on, the cold filament has the least resistance and typically allows 10 to 12 times the inrush current.

Heating (parallel) or current limiting (series) resistors protect the driver and indicator, but use active power when the indicator is off or on, respectively. Lamps with steady state current ratings up to 300mA can be driven without the need for heating or current limiting resistors, regardless of the lamp's on time (10 s of ms).

With these drivers, during turn-on, an internal sense resistor senses a large inrush current, reducing the drive current to the output stage, and the output operates in linear mode with a load current limited to approximately 500mA. During lamp preheating, the filament resistance increases to a maximum value and the output driver goes into saturation, applying the maximum rated voltage to the lamp.

Inductive Load Driver

Dual (unipolar) stepper motors (and other inductive loads) can be driven directly. Internal diodes prevent damage to the output transistors by suppressing high voltage spikes that occur when turning off inductive loads. For fast current decay (fast turn-off speed), using a Zener diode will increase the flyback voltage and improve performance. However, the peak voltage must not exceed the specified minimum sustain voltage (VSUPPLY+VZ+VF

overcurrent condition

If the load is shorted or the motor stalls, the load current will try to increase. As mentioned above, the drive current to the affected output stage decreases linearly, resulting in linearization of the output (limiting the load current to about 500mA). As the junction temperature of the output stage rises, a thermal shutdown circuit will shut down the affected outputs. If the fault is corrected, the output driver will return to normal saturation.

Troubleshooting

The fault output requires a pull-up resistor or current source. This can be connected to any power supply level (within specification constraints) required by the following circuit. For a 5 V supply (ie Vcc), 150 kΩ or greater should be used. Since the fault diagnosis function is to indicate when the output state of any channel differs from the input state, the fault output waveform will obviously generate a pulse waveform after the combined duty cycle of all channels showing the fault state. Therefore, there are two basic ways to use functions in an application:

• As an interrupt in controller-based systems. If the system has a microcontroller, a fault low causes an interrupt, which then initiates a diagnostic sequence to find the culprit channel. This sequence typically consists of looping through each channel each time while monitoring the fault output. It is then easy to determine which channel's output is faulty, and how it failed (ie, short to power, open circuit, or short to ground). The system can then take whatever action is required, but can continue to operate the remaining "good" channels while disabling the signal of the faulty channel.

• As a simple "common" fault indication. If there is no controller in the system, the fault output can be set to indicate (via lights or LEDs, etc.) a fault condition anywhere on the four channels. Since the fault output depends on the state of the input and output (four possible), but is only indicated on two of those states, the duty cycle of the fault output will reflect the duty cycle of the fault channel input (or its inverse duty cycle, depending on the type of failure).

In a typical application (50% duty cycle) a simple solution is to make the pull-up current on the fault output much smaller than the pull-down current (60µA) and add a capacitor to give longer than the run period constant. For typical values, the device will produce a continuous DC output level. Component values need to be adjusted to suit different conditions.

In some cases, spurious faults may appear on the fault output when the load switches on and off:

• Light load shutdown. Under light load conditions, the turn-off delay of the output stage (see characteristics above) increases and can lead to false fault outputs of a few microseconds (duration proportional to turn-off delay). Since it is difficult to define this under all operating conditions, it is generally recommended to include a small (~0.01µF) smoothing/storage capacitor at the fault output if the particular application is sensitive to this type of fault.

• Incandescent lights are on. As mentioned above, driving an incandescent filament causes the driver to operate at the current limit for a period of time after it is turned on. During this time, the "Fault" status (overcurrent) will be displayed. As mentioned above, the period can be 10 s ms. To avoid this indication, the capacitor on the faulty output needs to be increased to provide the proper time constant. Alternatively, in microcontroller-based systems, code can be written to ignore fault conditions for an appropriate amount of time after the lights are turned on.

Unconnected outputs do not guarantee proper fault operation - unused outputs should not be turned on, or pulled high to >2.5 V, and/or the associated input pulled low.

Thermal factor

Device power consumption can be calculated as:

Note - ICC is also regulated by duty cycle, but for most purposes this is a reasonable approximation.

This can then be compared to the allowable package power dissipation using:

where Rθ is expressed as:

28-lead PLCC (part number suffix EB) = 36oC/W

16-pin PDIP (part number suffix B) = 43oC/W

16 lead SOIC (part number suffix LB) = 90-C/W

Rθ is measured with minimal copper ground area on a typical double-sided printed circuit board. The thermal resistance (Rθ) from the header to the power strip is about 6°C/W for the three package types, so by adding a piece of printed circuit board copper (typically 6 to 18 cm²) to the ground terminal of the power strip of the device ) area, which can increase power consumption by 20% to 30%. See Application Note 29501.5, Improving Batwing Power Consumption.

Dimensions of A2557EB, A2557KB and A2557SB

Dimensions: Inches (for reference only)

Notes: 1. Within the limits shown, the exact body and lead configuration is selected by the supplier.

2. Lead spacing tolerance is non-cumulative

3. The thickness of lead is measured at or below the seat surface.

4. Mesh lead frame. Conductors 4, 5, 12 and 13 are internally integral.

Dimensions of A2557ELB, A2557KLB and A2557SLB

Dimensions: Inches (for reference only)

Notes: 1. Within the limits shown, the exact body and lead configuration is selected by the supplier.

2. Lead spacing tolerance is non-cumulative

3. Lead thickness is measured at or below the seat surface.

4. Mesh lead frame. Conductors 4, 5, 12 and 13 are internally integral.

Dimensions of A2557EEB, A2557KEB and A2557SEB

Dimensions: Inches (for reference only)

Notes: 1. Within the limits shown, the exact body and lead configuration is selected by the supplier.

2. Lead spacing tolerance is non-cumulative

3. Mesh lead frame. Conductors 5 to 11 and 19 to 25 are integrated internally.