Datasheet UC1702, UC2702, UC3702 Datasheet (UNITRODE)

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Quad PWM Relay Driver

UC1702 UC2702 UC3702

FEATURES

Maintains Constant Average

•

Relay Voltage With Varying Supply Voltages

VBUS Voltages up to 42.5V

•

Up to 50mA per Relay

•

Integrated Schottky Flyback Diodes

•

Individual Relay Control Inputs

•

Short Circuit Protection

•

User Selectable Operating Center

•

Frequency and Relay Voltage

Global Reset

•

BLOCK DIAGRAM

DESCRIPTION

The UC3702 Quad Relay Driver is intended to drive up to four relays from logic inputs using an unregulated relay voltage supply. The relays are driven from the Bus supply in a power efficient PWM converter fashion. The relay coil is used as the inductive element. (See the application's sec tion concerning the relay selection.) VBUS must be higher then the rated relay voltage. Short circuit protection is provided on chip with periodic retry.

The UC3702 requires a +5V logic supply as a reference. Two external re sistors program the effective relay voltage and a capacitor sets the nominal operating frequency. Internal Schottky diodes eliminate the need for any external power components.

In typical applications, the UC3702 grants the user the flexibility to choose the most cost effective relay without the added burden of generating a sep arate relay supply voltage. The UC3702 will even function with a poorly regulated supply VBUS, containing significant 100Hz or 120Hz ripple.

SLUS368 - APRIL 1997

UDG-96009-1

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UC1702 UC2702 UC3702

ABSOLUTE MAXIMUM RATINGS

VBUS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +50V

VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +9V

Input Voltage . . . . . . . . . . . . . . . . . . . . . . –0.3V to VCC + 0.3V

Average Current per Relay. . . . . . . . . . . . . . . . . . . . . . . . 50mA

Total Output Capacitance. . . . . . . . . . . . . . . . . . . . . . . . . 25pF

Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C

Junction Temperature. . . . . . . . . . . . . . . . . . . –55°C to +150°C

Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C

Currents are positive into, negative out of the specified termi nal. Consult Packaging Section of Databook for thermal limita

CONNECTION DIAGRAM

DIL-16, SOIC-16 (Top View) J or N, D Package

tions and considerations of packages.

ELECTRICAL CHARACTERISTICS: Unless otherwise stated these specifications apply for T

UC1702; –25°C to +85°C for UC2702; 0°C to +70°C for UC3702; VBUS = 34V, VCC = 5V, RESET R1 = 203k

Supply Voltage Section

VCC 4.5 5 8 V

VCC Supply Current RESET

VBUS 42.5 V

Oscillator Section

NOMINAL Operating Frequency R1 = 203k

R1 100 202 500 k

R2 20 42.5 80 k

CT 100 pF

Output Driver VBUS = 20V

Rated Relay Current 50 mA

Short Circuit Current 100 175 mA

Diode Leakage Current Control Input Low, V

Diode Forward Voltage 500 mV

Averaged Output Voltage Error UC2702, UC3702 (Note 1) –10 10 %

Control Inputs (INA, INB, INC, IND)

Logic Low 0.5 V

Logic High 3 VCC V

TON / TOFF Delay 250 ns

Input Current –1 0 1

Input Current V

RESET

TON / TOFF Delay 2 s

Input Current V

Note 1: Programmed Average Voltage

; CT = 220pF, TA= TJ.

PARAMETER

106 1

TEST CONDITIONS MIN TYP MAX UNITS

, INA, INB, INC, IND = VCC 4 6 mA

, R2 = 43.2k , CT = 220pF 150 175 200 kHz

= VBUS 0.05 5 A

OUTX

UC1702 –20 20 %

= 5V 70 120 A

= 0.3V –8 –5 A

= 5V –10 0 10 A

••

RVCC

•

400

–

= –55°C to +125°C for

= VCC, R2 = 43.2k ;

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PIN DESCRIPTIONS

CT: A capacitor from the CT pin to ground sets the oscil

lator center frequency. Note that the oscillator period must be least an order of magnitude less than the relax ation time constant (L/R) of the relay coil. However, a needlessly high operating frequency only increases power dissipation. For best accuracy, CT should be 220pF or greater.

GND: Both ground pins must be connected to a low noise system ground.

INA, INB, INC, IND: Separate digital control inputs for each of the four relay drivers. An active high input (a logic high) turns on the respective relay. Active low dis ables the common-emitter drive transistor.

OUTA, OUTB, OUTC, OUTD: Each of these output pins can be connected to the low side of one relay coil. The rated relay coil voltage of all relays to be driven by a sin gle UC3702 must be the same. (It is permissible to use coils with different rated series resistances with a single UC3702, so long as the rated coil voltages are the same and care is taken with respect to the relaxation times of the different relay coils). Output transients are slew rate limited to decrease electro-magnetic radiation.

R1: User selected external resistor must be placed from the R1 pin to the VBUS voltage supply. Sensing the

VBUS amplitude, the oscillator varies its ON duty cycle

such that the average voltage across the driver coil is constant.

R2: User selected external resistor must placed from R2 pin to GND. This resistor, in conjunction with the external timing capacitor, CT, sets the OFF cycle time of the oscil lator. For best accuracy, use 42.3kΩ for R2.

RESET:

Digital reset pin is active low. When RESET is low, the oscillator stops running and all drivers are open collector.

VCC: Nominally 5V. In addition to supplying the voltage

for the driver logic and oscillator circuit, the VCC supply

is used as reference for the generating the average relay voltage. It is recommended that the VCC be regulated to ±5% of its nominal value or better to insure good regula

tion. Good decoupling with a minimum of 1µF is neces sary.

VBUS: This pin should be tied to a low impedance voltage source at some voltage higher than the rated voltage of the relay coil to be driven. VBUS may even be a poorly filtered rectified sin wave, as the UC3702 will regulate the correct duty cycle. Good decoupling with a minimum of 1µF is necessary.

UC1702 UC2702 UC3702

APPLICATIONS INFORMATION

The UC3702 must be programmed for a specific relay voltage. The table given below has some suggested val

Rated Relay

Voltage

9V 153kΩ 43.2kΩ 220pF

12V 203kΩ 43.2kΩ 220pF

24V 398kΩ 43.2kΩ 220pF

ues of R1, R2, and CT. The VCC supply voltage is as sumed to be 5V.

More generally, the following equation can be used to determine the equivalent average relay voltage:

RELAY

R1 R2 CT

VCC R

••

106 1

•

–

400

Industrial Relays

The UC3702 takes advantage of the inductance of the

relay coil to regulate a constant coil current in a manner very similar to a switch-mode power supply. To use the UC3702 correctly, the coil characteristics must be known. The rated relay voltage is what the relay manu facturers specify as voltage to drive the relay coil with if the coil were driven from a dc source. Most manufactur ers also specify the series resistance of the coil. This is the resistance of the copper wire and determines the

steady state coil ON current. For example, a 12V relay

with a 320

series coil resistance with a draw of 37.5mA of constant current. The relay inductance is not typically specified in the manufacturer’s short-form datasheet, so the engineer will need an LCR bridge to measure it or re quest the information from the relay manufacturer.

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APPLICATIONS INFORMATION (cont.)

Although the inductance may not be specified in the re lay datasheet, it is very repeatable since it is determined by the number of turns and geometry of the relay.

can be used to observe the coil current. (Do not use a

large series resistor, as this will impact observed relax ation time constant.)

UC1702 UC2702 UC3702

The relay’s coil inductance divided by its series resis tance is the relay coil’s relaxation time constant. It is im portant that the relaxation time constant be at least an order of magnitude longer than the nominal switching pe riod of the UC3702 to obtain a constant current through the relay coil. This means that some relays may be ill-suited for use with the UC3702.

For example, a common 12V industrial relay has a series resistance of 500

and a coil inductance of 500mH. Its relaxation time constant is 1ms, an order of magnitude less than the period of a 10kHz oscillator.

When performing an engineering evaluation of the UC3702 in a system, it is useful to have a current probe. Alternately, a 1

resistor can be placed in series with the

Layout Considerations

The layout of the printed circuit board and good decoup

ling of the power supplies is critical to proper operation of

the UC3702. For best results, high quality 4.7

lum capacitors should be placed as close as possible to the VCC and the VBUS pins.

In addition to decoupling considerations, the R1 and R2 pins are sensitive to capacitive coupling from any of the driver outputs (OUTA - D) which can slew at speeds of 300V/

s. The external resistors R1 and R2 should be placed as close as possible to their respective pins. Avoid routing the outputs directly past these pins without a low impedance trace (such as GND, VCC, or VBUS) in between to act as a capacitive shield.

relay coil in the prototype and differential voltage probes

Typical Microcontroller Application Driving 12V Industrial Relays

F tanta

UNITRODE CORPORATION 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 • FAX (603) 424-3460

UDG-96010-1