Battery-based applications require the power supply controller IC to need very few external components. Any that are used should be low-cost types, while the IC should be packaged in some form of small outline package to minimize size and weight. In addition, the application determines whether the controller should provide step-up, step-down, or some other topology.
One tradeoff in selecting a controller IC is whether it employs external or on-chip power MOSFET switches. On-chip devices minimize external components, but have the potential for increasing the junction temperature and degrading thermal performance. Depending on the package, this could also reduce the current carrying capacity of the IC. Some of the controller ICs described below have on-chip power MOSFETs; others require external MOSFETs.
Intersil's ISL6224 provides power control and protection for a single, adjustable output voltage required to power battery-based systems. Housed in a 16-pin SSOP package, the IC integrates control circuits and feedback compensation for a single synchronous buck converter. The switchmode controller regulates the output from batteries ranging from 4V to 24V. An external resistive divider sets the output in the range of 0.9V to 5.5V. This IC employs external MOSFETs configured as synchronous rectifiers, as shown in Fig. 1, for a two-step conversion application that operates at 600 kHz.
The converter's light-load efficiency is enhanced by a hysteretic operation, which automatically engages at light loads when the inductor current becomes discontinuous. As the filter inductor resumes continuous current, the IC automatically restores the PWM mode.
The ISL6224 uses an average current mode control scheme with input voltage feedforward ramp programming for better rejection of input voltage variations. With internal feedback compensation, the IC provides fast and firm handling of transients when powering advanced chip sets.
It uses the lower MOSFET's on-state resistance, RDS(ON), as the current-sensing element. Gate control logic translates the generated PWM control signals into the MOSFET gate drive signals, providing necessary amplification, level shifting, and shoot-through protection.
A capacitor connected from the SOFT pin to ground accomplishes soft start. By monitoring the output voltage, the ISL6224 issues a single power good signal, PGOOD, when soft-start completes and the output is within ±10% of its set point. After completing the soft-start sequence, undervoltage protection latches the chip off when any of the monitored outputs drop below 70% of its set point.
The PWM controller's overcurrent circuitry monitors the output current by sensing the voltage drop across the lower MOSFET. It implements a “soft-crowbar” function for an output overvoltage. If the output voltage goes above 120% of its nominal output level, the upper MOSFET turns off and the lower MOSFET turns on. It maintains this soft-crowbar condition until the output voltage returns to the regulation window and normal operation continues. This soft crowbar and monitoring of the output prevents the output voltage from ringing negative as the inductor current flows in the “reverse” direction through the lower MOSFET and output capacitors.
Internal MOSFET
The LTC3405A-1.5 and LTC3405A-1.8 from Linear Technology are high-efficiency monolithic synchronous buck regulators using a constant frequency, current mode architecture. Supply current during operation is only 20mA and drops to <1mA in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3405A-1.5/LTC3405A-1.8 ideal for single Li-ion battery-powered applications. Their 100% duty cycle provides low dropout operation, extending battery life in portable systems. Fig. 2, on page 60, shows the IC's functional block diagram.Switching frequency is internally set at 1.5 MHz, allowing the use of small surface-mount inductors and capacitors. The LTC3405A-1.5/LTC3405A-1.8 are specifically designed to work well with ceramic output capacitors, achieving very low output voltage ripple and a small p. c. board footprint. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode.
When the load current increases, the output voltage decreases, causing a slight decrease in VFB relative to the 1.2V reference. This, in turn, causes the EA amplifier's output 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 — as indicated by the current reversal comparator IRCMP — or the beginning of the next clock cycle.
Comparator OVDET guards against transient overshoots >7.8% by turning the main switch off and keeping it off until the fault is removed. The LTC3405A Series parts are capable of Burst Mode operation in which the internal power MOSFETs operate intermittently, based on load demand.
To enable Burst Mode operation, connect the MODE pin to GND. To disable Burst Mode operation and enable PWM pulse skipping mode, connect the MODE pin to VIN or drive it with a logic high (VMODE>1.5V). In this mode, the efficiency is lower at light loads but becomes comparable to Burst Mode operation when the output load exceeds 25mA. The advantages of pulse skipping mode are lower output ripple and less interference to audio circuitry.
Sync Rectifiers
Maxim's MAX1742/MAX1842 constant-off-time, PWM step-down dc-dc converters are ideal for use in 5V and 3.3V low-voltage conversion necessary in notebook and sub-notebook computers. These 16-Pin QSOP package devices feature internal synchronous rectification for high efficiency and reduced component count. They require no external Schottky diode. The internal 90mW PMOS power switch and 70mW NMOS synchronous-rectifier switch easily deliver continuous load currents up to 1A. The MAX1742/MAX1842 produce a preset 2.5V, 1.8V, or 1.5V output voltage or an adjustable output from 1.1V to VIN. They achieve efficiencies as high as 95%. Fig. 3 is a functional diagram and circuit for a step-down regulator.
Both devices are designed for continuous output currents up to 1A. The MAX1742 uses a peak current limit of 1.3A (min) and is suitable for applications requiring small external component size and high efficiency.
The MAX1842 has a higher current limit of 3.1A (min) and is intended for applications requiring an occasional burst of output current up to 2.7A. Both devices also feature an adjustable soft-start to limit surge currents during startup, a 100% duty cycle mode for low-dropout operation, and a low-power shutdown mode that disconnects the input from the output and reduces supply current below 1 µA. The MAX1742/MAX1842 are available in 16-pin QSOP packages.
Intersil, palm Bay, Fla. CIRCLE 348 on Reader Service Card
Linear Technology, Milpitas, Calif. CIRCLE 347 on Reader Service Card
Maxim Integrated Products, Sunnyvale, Calif. CIRCLE 346 on Reader Service Card
