| by John Seago As new products are developed, they always seem to require more power supply current than previous models. While microprocessor voltages are dropping into the 1.6V region, the required current is increasing. The current required from the traditional 5V and 3.3V supplies continues to increase in many new products. The LTC1438 is a good choice for powering the latest microprocessors and supplying logic-level outputs in the 8A range.The LTC1438 Optimizes Circuit PerformanceAs a fixed frequency, 2-output, current mode, synchronous buck controller, the LTC1438 is intended for applications where regulation, efficiency and transient response are important. The current mode architecture provides excellent load and line regulation, while internal slope compensation eliminates subharmonic oscillations. The synchronous buck configuration provides very good efficiency, permitting a surface mount design without external heat sinks. External loop compensation allows the control loop to be optimized for transient load conditions. The 1% tolerance voltage reference ensures good output voltage accuracy.The LTC1438 integrates two regulators into one package; this technique has several advantages. Both regulators share a common oscillator, common reference and a common bias supply, so fewer external parts are required. Since both regulators are synchronized to the same oscillator, troublesome beat-frequencies are eliminated. The input voltage to each regulator may be different, adding to the versatility of the IC. Circuit DescriptionFigure 1 shows a high current version of the standard 2-output circuit shown in the LTC1438 data sheet. The high current version uses the LTC1438-ADJ to regulate both 5V and 3.3V outputs with load currents ranging from no load to 8A. Although separate regulator circuits are used for each output voltage, both circuits are identical except for the lower feedback resistor, which determines the output voltage.The LTC1438 regulates the 5V output by controlling the duty cycle of the top MOSFET, Q1, so the average input voltage to the buck inductor, L1, is equal to the output voltage. The buck inductor and output capacitors, C21C23, integrate and filter the energy pulses from the top MOSFET for the DC output. After the top MOSFET turns off, the bottom MOSFET, Q2, turns on to conduct inductor current to the load. To avoid shoot-through current, there is a short dead time before each MOSFET turns on. During this dead time, inductor current flows through the commutating diode, D5, to the load.The internal error amplifier is connected to the output by feedback resistors R14R15. The frequency response of the error amplifier is controlled by loop-compensation components, R3 and C3C4. The internal current comparator senses inductor current by the voltage developed across the current-sense resistor, R10. The 3.3V regulator functions like the 5V regulator. Refer to the LTC1438 data sheet for a thorough discussion of the external components used with the LTC1438.Additional FeaturesThe circuit in Figure 1 shows some additional features that add to the versatility of the LTC1438. The low-battery comparator in the IC is used to flag a low input voltage condition. Normally, the LBO pin is high. It goes low when the input voltage is low.A complete power-on-reset (POR) circuit is integrated into the LTC1438. At startup, the POR pin is low. It goes high 65,536 oscillator cycles after the channel 2 output voltage reaches 95% of its programmed value. The POR pin will go low if the output voltage falls 7.5% from nominal.Each output has a RUN/SS pin that provides output voltage delay, output current soft start and on/off control. The value of the capacitor connected to the RUN/SS pin determines the output voltage delay and the inductor-current ramp time, both at a rate of 0.5s/µF. Pulling a RUN/SS pin low turns off that output voltage. Pulling both RUN/SS pins low shuts down the LTC1438, turning off all internal circuitry and limiting the input current to a mere 16µA. | |
