Switchmode Controller ICs Cover Range of Applications

Feb 1, 2001 12:00 PM
Sam Davis

Portable systems take advantage of small footprint battery charger and regulator ICs.

Small outline packages now house controller ICs that operate as battery chargers as well as switchmode converters powering systems with up to 2.2A peak loads. This edition of Analog Feedback describes a Lithium-ion (Li-ion) battery charger in an 8-pin MSOP, buck regulator in 8-pin SO8 that can provide 2.2A peak, and a step-down regulator in an 8-pin MSOP for Li-ion battery-powered applications.

Li-Ion Battery Charger IC The TC3827 controller IC, from TelCom Semiconductor, provides safe and fast charging of a single Li-ion cell. This device is specifically designed for a wide variety of applications ranging from personal digital assistants (PDAs), cradle chargers, desktop computers and cellular phones. Only four external components are required for a complete charger operating from a 4.5V to 5.5V input range.

A typical Li-ion cell should be charged at a controlled current until it reaches 4.1V or 4.2V (depending on the type of cell), then charged at this voltage. This is accomplished in the TC3827 with high accuracy (51%), low shutdown current (1A), and easy charge current programming. The device has an overall system accuracy of 1%, ensuring that the cell capacity is fully utilized without lifecycle degradation.

As shown in Fig. 1, the TC3827 controller only requires a P-channel power MOSFET, two small capacitors, and a sense resistor (R subscript SENSE) to provide an inexpensive Li-ion battery charger. Minimum values for the V subscript IN and V subscript OUT capacitors are 10F and 1F, respectively.

The external PMOS transistor regulates current from the source into the Li-ion cell. Specifications to consider when choosing an appropriate PMOS are minimum drain-source breakdown voltage, minimum turn-on threshold voltage (V subscript GSTH), drain current, and power-dissipation capabilities.

Initially, current limited charging occurs until a prespecified battery voltage is measured at the V subscript OUT pin. Then the controller IC switches to constant-voltage mode. During this mode the TC3827 works like a linear regulator, holding the output voltage within the specified accuracy. The charger output is sensed at the V subscript OUT pin.

The charging current follows the foldback characteristic. R subscript SENSE sets the maximum charging current, I subscript MAX. The voltage drop across the current sense resistor is sensed at the V subscript SNS input. An amplified copy of this sense voltage is provided as output on the current monitor pin (I subscript MON). When the battery is deeply discharged to a minimum voltage level, or if the battery is shorted, the current sense circuit folds back the charge current to limit the power dissipation of the PMOS.

The TC3827 also has a logic-level shutdown input, SHDN, which may be connected to the input voltage to enable the IC. Pulling it "low" or to ground will disable the PMOS drive (V subscript DRV pulled up to V subscript IN voltage). Also, a charger mode pin (MODE) is provided to drive an optional LED for a visual indication of current limited mode operation. The LED is turned off in constant voltage mode operation or if the battery is disconnected.

For optimum voltage regulation, place the load as close as possible to the device's V subscript OUT and GND pins. It is recommended to use dedicated p.c. board traces to connect the PMOS drain to the positive terminal and GND to the negative terminal of the load to avoid voltage drops along the high current carrying p.c. board traces.

Packaged in an 8-pin MSOP, this device is specified over the ambient operating range of 120C to `85C. Pricing for the TC3827 starts at $0.99 (5000).

SO8 SuperSwitcher Has 2.2A Current Limit Micrel's MIC4681 SuperSwitcher [TM] is a step-down (buck), voltage-mode, variable duty cycle switchmode regulator with an internal power switch. (Fig. 2). This 200 kHz regulator achieves over 1A of continuous output current over a 4V to 30V input range. Highest efficiency operation is from a supply voltage around `12V. A minimum number of external components are required and it can operate using a standard series of inductors and capacitors. Frequency compensation is provided internally for fast transient response and ease of use. In addition, it has excellent line, load, and transient response.

Featuring a 2.2A minimum current limit, the device is ideal for pulsed current applications such as GSM and TDMA cell phone battery chargers and power supplies. It can sustain an output of 4.2V/2A within a typical GSM charging environment.

In operation, a fixed-gain error amplifier compares the feedback signal with a 1.23V bandgap voltage reference. The resulting error amplifier output voltage is compared to a 200 kHz sawtooth output. A higher feedback voltage increases the error amplifier output voltage. A higher error amplifier voltage (comparator inverting input) causes the comparator to detect only the peaks of the sawtooth, reducing the duty cycle of the comparator output. A lower feedback voltage increases the duty cycle.

When the internal switch is ON, an increasing current flows from the supply V subscript IN, through external storage inductor L1, to output capacitor C subscript OUT and the load. Energy is stored in the inductor as the current increases with time. When the internal switch is turned OFF, the collapse of the magnetic field in L1 forces current to flow through fast recovery diode D1, charging C subscript OUT, which provides stabilization and reduces ripple.

During the ON portion of the cycle, the output capacitor and load currents return to the supply ground. During the OFF portion of the cycle, current is being supplied to the output capacitor and load by storage inductor L1, which means that D1 is part of the high-current return path.

It performs cycle-by-cycle current limiting and thermal shutdown for protection under fault conditions. A logic-low enables the TTL-compatible shutdown (SHDN) input. A logic-high shuts down the internal regulator, which reduces the current to typically 6A when V subscript SHDN4V subscript IN412V and 35A when V subscript SHDN45V.

The MIC4681 is available in 3.3V and 5V fixed output versions or adjustable output down to 1.25V. Fixed-voltage versions of the regulator have an internal resistive divider from the feedback (FB) pin. Adjustable versions require an external resistive voltage divider from the output voltage to ground, center tapped to the FB pin; they require a 1.23V feedback signal.

The MIC4681 employs the power-SO8 package that has a standard 8-lead small-outline package profile, but with much higher power dissipation than a standard SO8. The reason that the power SO8 has higher power dissipation (lower thermal resistance) is that pins five through eight and the die-attach paddle are a single piece of metal. The die is attached to the paddle with thermally conductive adhesive. This provides a low thermal resistance path from the junction of the die to the ground pins. It's good practice to connect pins five through eight to the largest ground plane that is practical for the specific design. This allows a 140C to `125C junction temperature range.

Pricing of the MIC4681BM in an SO8 package is $2.30 (1000).

A 1.4 MHz Buck Converter Delivers 95% Efficiency The LTC3404 from Linear Technology is a monolithic step-down dc-dc converter capable of handling up to a 600mA load. It's a high efficiency (up to 95%) synchronous buck regulator that employs constant frequency, current mode architecture (Fig. 3). The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode.

Its 1.4 MHz switching frequency allows use of small surface mount inductors and capacitors, which result in a very small converter size. For noise sensitive applications the LTC3404 can be externally synchronized from 1 MHz to 1.7 MHz.

Its input range is 2.65V to 6V, which allows operation with a single Li-ion or multiple NiCd/NiMH batteries. The device has 100% duty-cycle capability that provides low dropout operation, extending the battery run time in portable systems. Supply current during operation is only 10mA and drops to ,1mA in shutdown. Typical applications include wireless and communications equipment, such as cell phones, wireless modems, DSL modems, and PDAs.

On-chip are the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches. During normal operation, the top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch is controlled by the voltage on the I subscript TH pin, which is the output of error amplifier (EA). The V subscript FB pin allows EA to receive an output feedback voltage from an external resistive divider. When the load current increases, it causes a slight decrease in the feedback voltage relative to the 0.8V reference, which in turn, causes the I subscript TH voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse or the next clock cycle begins. Comparator OVDET guards against transient overshoots .6.25% by turning the main switch off and keeping it off until the fault is removed.

In burst mode operation, the internal power MOSFETs operate intermittently based on load demand. Burst mode can be disabled and the PWM pulse skipping mode enabled, which lowers efficiency at light loads, but becomes comparable to Burst Mode operation when the output load exceeds 50mA. The advantage of the pulse skipping mode is lower output ripple and less interference to audio circuitry. When the converter is in burst mode, the peak inductor current is set to about 250mA, even though the voltage at the I subscript TH pin indicates a lower value. The voltage at the I subscript TH pin drops when the inductor's average current is greater than the load requirement. As the I subscript TH voltage drops below about 0.55V, it turns off both power MOSFETs. The I subscript TH pin is then disconnected from the output of the EA amplifier and parked a diode voltage above ground.

In the sleep mode, both power MOSFETs are held off and a majority of the internal circuitry is partially turned off, reducing the quiescent current to 10A. The load current is now being supplied solely from the output capacitor. When the output voltage drops, the I subscript TH pin reconnects to the output of the EA amplifier and the top MOSFET is again turned on, and this process repeats.

If the output is shorted to ground, the frequency of the oscillator is reduced to about 200 kHz, 1/7 the nominal frequency. This frequency foldback ensures that the inductor current has ample time to decay, thereby preventing runaway. The oscillator's frequency will progressively increase to 1.4 MHz (or the synchronized frequency) when V subscript FB rises above 0.3V.

A phase-locked loop (PLL) in the LTC3404 allows the internal oscillator to be synchronized to an external source connected to the SYNC/MODE pin. When locked, the PLL aligns the turn-on of the top MOSFET to the rising edge of the synchronizing signal. When the LTC3404 is clocked by an external source, burst mode operation is disabled; the LTC3404 then operates in PWM pulse skipping mode. Increasing the output load slightly allows constant frequency PWM operation to resume. This mode exhibits low output ripple as well as low audio noise and reduced RF interference - while providing reasonable low current efficiency.

The LTC3404 is available in 8-lead MSOP packages, and is priced at $2.70 (1000).

TelCom Semiconductor, Mountain View, Calif.

Micrel, Inc., San Jose, Calif.

Linear Technology Corporation, Milpitas, Calif.