Supply ICs Boost Design Flexibility

Aug 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

The latest round of power supply ICs provide new features and better efficiency than their predecessors. Controller ICs provide multiple outputs, single-chip switchers handle higher loads, and charge pump devices deliver higher current.

Among the new multiple output devices, the LM2642 from National Semiconductor has two current-mode synchronous buck regulator controllers switching at 300kHz (Fig. 1). The two controllers operate 180° out of phase, which reduces the input ripple rms current, thereby significantly reducing the required input capacitance. You can independently adjust the output of each channel from 1.3V to 13.5V. Each output channel employs a pair of external power MOSFETs as synchronous rectifiers.

Current-mode feedback control assures line and load regulation and a wide loop bandwidth for excellent response to fast load transients. The IC senses current across the V_DS of the top MOSFET or across an external current sense resistor connected in series with the drain of the top MOSFET. Current limit is independently adjustable for each channel.

It features analog soft-start circuitry independent of the output load and output capacitance, making soft-start behavior more predictable and controllable. The ON/SS1 pin functions as both channel's enable and soft-start control.

A PGOOD1 pin monitors channel 1's dc output. Overvoltage protection (OVP) is available for both outputs. An undervoltage delay pin delays the IC's shutoff time during an output undervoltage event. During soft-start, overvoltage protection and current limit remain in effect.

You can implement sequential startup by connecting PGOOD1 to SS/ON2. Once channel 1 reaches 94% of nominal, PGOOD1 goes high, enabling SS/ON2. In this mode of operation, the state of channel 1 controls channel 2. If channel 1 falls out of the PGOOD1 window, channel 2 switches off immediately.

If the output voltage on either channel rises above 113% of nominal, overvoltage protection activates. Then, both channels latch off and the PGOOD1 pin goes low. Setting the OVP latch immediately turns off the high side MOSFET driver, HDRVx, and turns on the low side MOSFET driver, LDRVx, to discharge the output capacitor through the inductor. To reset the OVP latch, cycle the input voltage or switch off both channels. If the output voltage on either channel falls below 80% of nominal, undervoltage protection activates. An undervoltage (UV) event shuts off the UV_Delay MOSFET, allowing the UV_Delay capacitor to charge at 5µA (typical). At the UV_Delay threshold (2.3V typical), both channels latch off.

The LM2642, housed in a TSSOP-28 package, is shipping for $2.28 each in 1,000 units.

3A SPAK Buck Regulator

A member of Micrel's SuperSwitcher^™ family, the MIC4685 is a high-efficiency, internally compensated 200-kHz buck regulator (Fig. 2, on page 58). With an integral power switch, it provides 3A (continuous) and over 85% efficiency. Thermal performance of its 7-pin SPAK allows it to replace TO-220s and TO-263s (D²PAKs) in many applications. The SPAK saves board space with a 36% smaller footprint than a TO-263 package.

It maintains high efficiency over a wide output current range by using a boost capacitor to increase the voltage available to saturate the internal power switch. This technique employs the bootstrap (BS) pin and external capacitor, which provide a bias voltage higher than the input voltage to the internal power switch. This capacitor sees the dv/dt of the switching action at the SW pin as an ac voltage. The BS capacitor then couples the ac voltage back to the BS pin plus the dc offset of V_IN, where it is rectified and used to provide additional drive to the internal NPN power switch. As a result, it requires only the p.c. board's ground plane for a heatsink.

Its highest efficiency is from a supply of about +12V, yet it operates effectively with a 4V to 30V (34V transient) input range, allowing its use in applications where input voltage transients may be present. Safety features include cycle-by-cycle current limit, frequency-fold back short-circuit protection, and thermal shutdown.

Its enable (EN) input is TTL compatible and must be tied high if unused. A logic-high enables the regulator and a logic-low shuts down the internal regulator, reducing the current to typically 150µA when V_EN=0V.

As shown in Fig. 2, the IC requires an external resistive voltage divider (R1 and R2) from the output voltage to ground, center tapped to the FB (feedback) pin. The relationship of these two resistors is:

V_REF = 1.235V

A fixed-gain error amplifier compares the feedback signal with a 1.235V bandgap voltage reference. The resulting error amplifier output voltage is compared with a 200-kHz sawtooth waveform to produce a voltage-controlled variable duty cycle output.

A higher feedback voltage increases the error amplifier output voltage. A higher error amplifier voltage causes the comparator to detect only the sawtooth peaks, reducing the comparator output duty cycle. A lower feedback voltage increases duty cycle.

When the internal power switch is ON, an increasing current flows from V_IN through external storage inductor L1 to output capacitor C_OUT and the load. As the current increases with time, the inductor stores energy. When the internal power switch is OFF, the collapse of the magnetic field in Ll forces current to flow through fast recovery diode D1, charging external output capacitor C_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, storage inductor L1 supplies current to the output capacitor and load, which means D1 is part of the high-current return path.

The MIC4685 is available in a 7-lead SPAK package with a T_j range of -40°C to 125°C. Pricing is $2.85 in 1,000-piece quantities.

Step-Down DC-DC Converter

Linear Technology's LTC3250-1.5 is a switched capacitor step-down dc-dc converter delivering up to 250mA from a ThinSOT^™ package with efficiencies as high as 88% (81% with 3.6V input). It produces a 1.5V regulated output from an input range of 3.1V to 5.5V, making it ideal for driving the latest generation ICs used in single-cell Li-ion battery systems. A typical circuit requires only three small external capacitors (two 1 µF and one 4.7 µF). Simple to implement, it provides a tiny/low-profile footprint (Fig. 3).

The IC delivers good efficiency within the single-cell Li-ion voltage range; typical efficiencies are 40% higher than an equivalent LDO. A unique constant frequency architecture provides both a low noise regulated output as well as lower input noise than other conventional charge pump regulators. Output ripple is maintained at 4mVpk-pk in continuous mode, and only 10 mVpk-pk in Burst Mode^® operation, reducing output noise concerns. Integrated soft-start limits inrush current at turn on, while a low operating current of 35µA (<1 µA in shutdown) optimizes light load efficiency. High efficiency and a small footprint make it ideal for handheld applications.

The LTC3250-1.5 uses a switched capacitor charge pump to step down V_IN to a regulated 1.5V ±4% output. It regulates by sensing the output voltage through an internal resistor divider and modulating the charge pump output current based on the resulting error signal.

A 2-phase nonoverlapping clock activates the charge pump switches. On the first clock phase, current transfers from V_IN, through the 1 µF flying capacitor, to V_OUT. It delivers current to V_OUT on the first phase and also charges the flying capacitor. On the second clock phase, the IC connects the flying capacitor from V_OUT to ground, delivering the charge stored during the first clock phase to V_OUT. This method of switching delivers only half the output current from V_IN, providing 50% more efficiency than a conventional LDO. The sequence of charging and discharging the flying capacitor continues at a free running frequency of 1.5 MHz (typical). This constant frequency architecture provides a low noise regulated output as well as lower input noise.

In shutdown mode, all circuitry turns off, the IC disconnects V_OUT from V_IN, and then draws only leakage current from V_IN. It goes into shutdown by applying a logic low to the SHDN pin. The SHDN pin is a high-impedance CMOS input with a 0.8V threshold, so it should never float — a valid logic level must drive it.

The LTC3250-1.5 has built-in, short-circuit current limiting and overtemperature protection. During short-circuit conditions, it automatically limits the output to about 500mA. At higher temperatures, or if the input voltage is high enough to cause excessive self-heating on chip, thermal shutdown circuitry shuts down the charge pump once the junction temperature exceeds about 160°C. It re-enables the charge pump once the T_j drops back to about 150°C. The IC will cycle in and out of thermal shutdown without latch-up or damage until removal of the short-circuit on V_OUT. Avoid long-term overstress (I_OUT >350mA, and/or Tj >140°C) because it can degrade performance.

It has built-in soft-start circuitry, achieved by increasing the amount of current available to the output charge storage capacitor linearly over a period of approximately 500 µs. Soft-start is enabled when the device is brought out of shutdown and disabled after regulation is achieved.

To improve efficiency at low output currents, the IC includes a Burst Mode. In operation, an output current sense detects when the required output current drops below an internally set threshold (30mA, typical). When this occurs, the part shuts down the internal oscillator and goes into a low current operating state. It remains in this state until the output has dropped enough to require another burst of current. Unlike other charge pumps whose burst current is dependent on many factors, this IC's burst current is set by the burst threshold and hysteresis. Thus, the fixed V_OUT ripple voltage in Burst Mode is typically 10mV for a 4.7 µF output capacitor.

The LTC3250ES6-1.5 is available in a low profile (1 mm) ThinSOT package. Pricing starts at $1.60 each in 1,000-piece quantities.

National Semiconductor, Santa Clara, Calif. CIRCLE 346 on Reader Service Card

Micrel semiconductor, San Jose, Calif. CIRCLE 347 on Reader Service Card

Linear Technology, Milpitas, Calif. CIRCLE 348 on Reader Service Card

Acceptable Use Policy blog comments powered by Disqus