Power Electronics

ICs Drive Motors, Regulate Voltage, and Protect Batteries

As IC manufacturers develop processing technologies allowing higher integration levels, they can double up on the functions included in a single package or shrink the package size. Two examples are the dual motor driver IC from Allegro Microsystems and Microchip Technology's dual LDO. Shrinking feature size plays a role in Seiko's Li-ion battery protection IC, which is 1.8 mm × 2.0 mm and 0.75-mm high.

Dual Full-Bridge PWM Motor Driver IC

Intended for pulse-width modulated (PWM) current control of two dc motors, the Allegro Microsystems A3974SED with two full-bridge motor drivers makes it possible to drive two dc motors completely independent of one another. Using a third-generation BCD (Bipolar CMOS DMOS) process, it includes DMOS output drivers rated up to 50V and 1.5A. On-chip synchronous rectification control circuitry reduces power dissipation by up to 30%, which allows for faster switching speeds, higher operating currents, and lower silicon temperatures. Independent ENABLE input pins control the speed and torque of each dc motor. Fig. 1 is a simplified circuit of the device.

A serial interface programs the internal fixed off-time PWM current-control timing circuitry to operate in slow-, fast-, and mixed current-decay modes. The 3-wire serial port (clock, data, and strobe) programs the A3974SED using two 20-bit words: one bit selects the word, followed by 19 bits of data. This provides maximum flexibility in configuring the PWM to the motor drive requirements.

You can program the PWM timer to provide fixed off-time PWM signals to the control circuitry. In mixed current-decay mode, the first portion of the off time operates in fast decay, until it reaches the fast-decay time count, followed by slow decay for the rest of the off-time period. By setting the fast-decay time longer than the off time, the device effectively operates in fast-decay mode.

Synchronous rectification circuitry allows the load current to flow through the low RDS(on) of the DMOS output driver during the current decay. This eliminates the need for external clamp diodes in most applications — saving cost and component count, while minimizing power dissipation.

When triggering the PWM off cycle load current recirculates according to the decay mode selected by the control logic. After a short crossover delay, synchronous rectification turns on the appropriate MOSFETs during the current decay and shorts out the body diodes with the low RDS(on) driver. This lowers power dissipation significantly and can eliminate the need for external Schottky diodes.

The serial ports also allow you to configure synchronous rectification in active mode, passive mode, low side only, or disabled. The active mode prevents reversal of load current by turning off synchronous rectification when it detects a zero current level. Passive mode allows reversal of current, but turns off the synchronous rectifier circuit if the load current inversion ramps up to the current limit.

Internal circuit protection includes thermal shutdown with hysteresis, undervoltage monitoring of VDD and the charge pump, and crossover-current protection. The IC doesn't require special power-up sequencing.

The SLEEP pin puts the device into a minimum current draw mode. When asserted low, it resets the serial port to all zeros and disables all circuits.

Circuitry turns off all drivers if the junction temperature reaches 165°C (typical). It's intended only to protect the device from failures due to excessive junction temperatures, and should not imply that it permits output short circuits. Thermal shutdown has a hysteresis of approximately 15°C.

A 44-lead plastic PLCC supplies the A3974SED with four copper batwing tabs that provide a ϕJA of 40°C/W for maximum heat dissipation. The power tabs are at ground potential and need no electrical isolation.

Dual 150mA LDOs, Directed For Portable Applications

The TC1305 and TC1306 are new additions to the TC13XX series of power management combo ICs. They combine two fully independent CMOS low dropout regulators (LDOs) and a system supervisor. Select Mode operation allows the selection of one LDO output voltage from two values in the TC1306 and three values in the TC1305. This level of integration allows replacement of up to three discrete devices, resulting in a reduction in board space and cost. Fig. 2 shows a typical layout for the TC1305.

The two LDOs offer a dropout voltage of 150mV at 150mA, extending usable battery life. The total supply current is 200µA at full load, 20 to 60 times lower than bipolar regulators. Low supply current, user-selectable output voltage, and fast dynamic performance make these devices well-suited for a wide range of applications, including cellular and cordless phones, pagers, PDAs, notebooks, MP3 players, camcorders, cameras, and portable instruments.

Select Mode operation involves a tri-state input that allows the user to select the output voltage. In the TC1305, the Select Mode allows an output voltage selection of 2.5V, 2.8V, or 3.0V. For the TC1306, possible outputs are either 2.8V or 3.0V. For example, connecting the SELECT pin to GND in the TC1305 causes the two LDO outputs to supply 2.5V. Connecting the SELECT pin to VIN causes both outputs to supply a fixed 3.0V. Leaving the SELECT pin floating sets both voltages to 2.8V.

CMOS construction results in a very low supply current, which doesn't increase with load changes. In addition, the output voltage remains stable and within regulation at no load current. Both feature ultra low noise operation, fast response to step changes in load, exceptionally low crosstalk, and ±0.5% typical output voltage accuracy.

A 0.01µF capacitor connected from the bypass input to GND reduces noise present on the internal reference, which reduces output noise. If output noise isn't a concern, it's possible to leave the input unconnected. You can use larger capacitor values, but that results in a longer power-up.

The TC1305 and TC1306 are stable with a wide range of capacitor values and types; however, they require a minimum value of 1µF from OUT1 or OUT2 to ground. The output capacitor should have an equivalent series resistance (ESR) of 0.1Ω to 10Ω for a 1µF capacitor and 0.01Ω to 10Ω for a 10µF capacitor. Connect a 1µF capacitor from the VIN to GND if there's more than 10 in. of wire between the regulator and the ac filter capacitor, or if a battery is the power source. You can use an aluminum electrolytic, ceramic, or tantalum capacitor. Since many aluminum electrolytic capacitors freeze at approximately — 30°C, solid tantalums are recommended for applications operating below — 20°C. When operating from sources other than batteries, you can improve supply-noise rejection and transient response by increasing the value of the input and output capacitors and employing passive filters.

Applying a logic high to the ??? or ??? pin turns on the corresponding output. With a logic low, the corresponding regulator enters the shutdown mode. During the shutdown mode, the output voltage falls to zero, and regulator supply current reduces to 0.5µA (max). If the shutdown mode isn't necessary for either LDO, connect the corresponding ??? pin to VIN.

An integrated microprocessor supervisor monitors power-up, power-down, and brownout conditions. When the detected voltage (VDET) falls below the reset voltage threshold (2.63V) the device outputs an active low ??? signal, which remains low for 300 msec (typical) after VDET rises above the reset threshold.

The ??? output is valid to VDET = 1.0V (below this point it becomes an open circuit and doesn't sink current) and is able to reject negative-going transients (glitches) on the power supply line. You can improve transient immunity by adding a capacitor near the VDET pin.

The TC1305 and TC1306 have a fast turn-on time (40 µsec) when released from shutdown. This is the time the output voltage requires to be within 2% of its fully enabled value after releasing from shutdown. The output voltage turn-on time is dependent on load conditions and output capacitance on OUT1 or OUT2 (RC response).

The TC1305 is available in 10-pin MSOP packages, and the TC1306 is available in 8-pin MSOP packages. Both devices are fully specified from — 40°C to +125°C. In 1000 quantities, individual pricing for TC1305 starts at $1.07; the TC1306 starts at $1.02.

Note: The Microchip name and logo and Select Mode are registered trademarks of Microchip Technology Inc. in the USA and other countries. All other trademarks are the property of their respective owners.

IC Protects Rechargeable Li-Ion Batteries

The Seiko S-8261 series are Li-ion/Li-polymer rechargeable battery protection ICs incorporating a high-accuracy voltage detection and delay circuit. These ICs are suitable for protection of single-cell lithium ion/lithium polymer battery packs from overcharge, overdischarge, and overcurrent. Applications include hand-held scanners, notebook computers, and other portable electronic devices using lithium-ion rechargeable single-cell battery packs.

Fig. 3, on page 54, shows the simplified circuit of a typical S-8261 application.

Possibly the smallest li-ion battery protection IC, the S-8261 has accuracy for overcharge (±25mV), overdischarge (±50mV), and overcurrent (±15mV) detection voltages. For added protection, it incorporates a third level of overcurrent detection.

The S-8261 consumes only 3.5µA while operating and only 0.1µA in power-down mode. Housed in a 6-pin SNB(B) package, the S-8261 footprint measures 1.8 mm by 2.0 mm and 0.75 mm tall. It also comes in an SOT-23-6 package. Pricing begins at 35 cents and 30 cents in quantities of 10K for the SNB(B) packaging and SOT-23-6 packaging, respectively.

Allegro Microsystems, Worcester, Mass.
CIRCLE 346 on Reader Service Card
Microchip Technology Inc., Chandler, Ariz.
CIRCLE 347 on Reader Service Card
Seiko Instruments USA Inc., Torrance, Calif.
CIRCLE 348 on Reader Service Card

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