Analog and Mixed Signal ICs Fill Multiple Roles

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

We use analog ICs in many varied applications. The mixed signal devices described here include some digital circuits, which is characteristic of many of today's ICs that provide system functions.

Micrel's MIC280 is a digital thermal supervisor capable of measuring a remote PN junction and its own internal temperature. The remote junction may be an inexpensive commodity transistor, like the 2N3906, or an embedded thermal diode contained in the Intel Pentium^™ II/III/IV CPUs, AMD Athlon^™ CPUs, and Xilinx Virtex^™ FPGAs. The device can measure remote temperature with ±1°C accuracy and 9-bit to 12-bit resolution (programmable). The independent high-, low-, and over-temperature thresholds for each zone are also provided. A 2-wire SMBus 2.0 compatible serial interface transmits the temperature data via the host processor. Fig. 1 shows a typical application using the remote PN junction of a 2N3906 CPU diode.

Device operation depends on sensing the V_CB-E of a diode-connected PNP transistor at two different current levels. For remote temperature measurements, you can do this using an external diode connected between T1 and ground, as shown in Fig. 1. This technique relies on measuring the relatively small voltage difference resulting from the two levels of current through the external diode. Any resistance in series with the external diode will cause an error in the temperature reading. A good rule of thumb is each ohm in series with the external resistor will cause a 0.8°C error in the MIC280's temperature measurement. It's not difficult to keep the series resistance well below an ohm (typically <0.1Ω), so this will rarely be an issue.

For maximum accuracy and minimum guard banding, the MIC280's advanced integrating A/D converter and analog front-end reduce errors due to noise. The interrupt output signals temperature events to the host, including data-ready and diode faults. You can lock the critical device settings to prevent changes and insure failsafe operation. The clock, data, and interrupt pins are 5V-tolerant — regardless of the value of V_DD. They won't clamp the bus lines low — even if the device is powered down. Superior accuracy, failsafe operation, and small size make the MIC280 an excellent choice for the demanding thermal management applications.

The least-significant bit of each temperature register (high bytes) represents 1°C. The values are in a two's complement format, wherein the most significant bit (D7) represents the sign — zero for positive temperatures and one for negative temperatures.

Extended temperature resolution is provided for the external zone. The reported high and low temperature limits and the measured temperature for Zone 1 are 12-bit values stored in a pair of 8-bit registers. The measured temperature, for example, is reported in registers TEMPIh, the high-order byte, and TEMP1I, the low-order byte. The values in the low-order bytes are left-justified, four-bit binary values representing 1/16-degree increments. The A/D converter resolution for Zone 1 is selectable from nine to 12 bits via the configuration register. The MIC280 reports low-order bits beyond the selected resolution as zeroes.

Hysteretic PFET Buck Controller

The LM3485 from National Semiconductor allows the design of a small, low-cost, switching buck regulator using hysteretic control, which allows a simple selection of external components. P-channel power MOSFET (PFET) architecture allows lower component count, as well as ultra-low dropout operation. You get low switching losses and high efficiency because the PFET switches only when the output voltage reaches the threshold voltage of the feedback comparator. Fig. 2 shows LM3485's internal circuitry.

In operation, a hysteretic comparator with 10mV hysteresis (typical) compares the FB pin with an internal reference voltage, typically 1.24V. The positive input terminal of the hysteretic comparator is the FB pin, and the negative input terminal is connected to the internal reference voltage. As the FB pin voltage surpasses the internal reference voltage, the output of the hysteretic comparator is in its low state, and the PGATE responds by turning on the external PFET.

During PFET turn-on, the input power supply charges C_OUT and supplies current to the load via the series path provided by PFET and the inductor. Current through the inductor ramps up linearly. The output voltage increases as the FB voltage reaches the upper hysteretic threshold, which is the internal reference voltage plus 10mV. When the hysteretic comparator output changes from low to high, the PGATE turns off the external PFET. As the PFET turns off, the inductor generates a negative-going transient that the catch diode clamps at below ground. The catch diode turns on and the current through the inductor ramps down. When the inductor current reduces to zero, both the PFET and the catch diode are off and C_OUT alone supplies current to the load. As the output voltage reaches the internal reference voltage, the next hysteretic control cycle starts.

The LM3485 operates in discontinuous conduction mode at light-load current or continuous conduction mode at heavy-load current. In the discontinuous conduction mode, current through the inductor starts at zero and ramps up to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the internal reference voltage. Until then the inductor current remains zero, operating frequency is lower and switching losses are lower. In contrast, in the continuous conduction mode, current always flows through the inductor and never ramps down to zero.

An external resistor sets current limiting, accomplished by sensing V_DS of the external PFET. Usually, when too much load current flows, the PGATE driver will turn off the external PFET.

Use a low ESR bypass capacitor between the power supply input and ground and locate it near the external PFET's source pin. The input capacitor prevents large voltage transients at input and provides the instantaneous current when the PFET turns on. The voltage and RMS current rating are important parameters for the input capacitor. The voltage rating must be at least 1.25 times higher than the maximum input voltage.

The important parameters for the catch diode are the peak current, the peak reverse voltage, and average power dissipation. The off-state voltage across the catch diode is approximately equal to the input voltage. The peak reverse voltage rating must be greater than the input power supply voltage. To improve efficiency, we recommend a low reverse leakage diode. A Schottky diode is preferred in low-output voltage applications, however for high-temperature applications, a Schottky diode may exhibit high leakage current.

Important parameters for the PFET are the maximum drain-source voltage, V_DSS, the on-resistance, R_DS(on), current rating, and the input capacitance. The PFET's off-state voltage must be equal to the sum of the input voltage and the diode forward voltage. The V_DSS must be at least a few volts higher than the input voltage.

Li-Ion And Li-Pol Charge Management IC

The bq2420x series from Texas Instruments are simple li-ion linear charge management ICs targeted at low-cost and space-limited charger applications. The series offers an integrated power MOSFET, high-accuracy voltage regulation, temperature monitoring, charge status, and charge termination — in a single monolithic device. The bq2420x supports a precision li-ion or li-pol (lithium-ion polymer) charging system suitable for single-cells with either coke or graphite anodes. Fig. 3, on page 52, shows a typical charger circuit.

Because it's intended for a current-limited wall-mount transformer, the bq2420x doesn't provide current regulation. However, these devices offer a fixed internal current limit to prevent damage to its internal power MOSFET. A time-limited, pre-conditioning phase that conditions deeply discharged cells is also provided. Once the battery reaches the charge voltage, the high-accuracy voltage regulation loop takes over and completes the charge cycle. The charge terminates based on minimum current. An internal charge timer provides a backup safety for charge termination.

Other standard features include an automatic sleep mode activated when V_CC falls below the battery voltage, and a recharge feature activated when the battery voltage falls below the V_RCH threshold.

Besides its standard features, the core product provides two additional enhancements: temperature monitoring and status display. The temperature-sense circuit continuously measures battery temperature using an external thermistor and inhibits charge until the battery temperature is within the user-defined thresholds. The STAT pin indicates three conditions of charger operation: charge-in-progress, charge complete, and fault. You can use this output to drive an LED or an interface to a microcontroller.

The bq24200 and bq24201 continuously monitor battery temperature by measuring the voltage between the TS and VSS pins. A negative- or a positive-temperature coefficient thermistor (NTC, PTC) and an external voltage divider typically develop this voltage. The bq24200 and bq24201 compare this voltage against internal thresholds to determine whether to allow charging. The temperature-sensing circuit is immune to any fluctuation in V_CC since both the external voltage divider and the internal thresholds are referenced to V_CC.

Once a temperature outside the internal voltage thresholds is detected, the bq24200 and bq24201 immediately suspend the charge by turning off the power MOSFET and holding the timer value (i.e. timers aren't reset). Charge resumes when a normal temperature range returns.

Micrel, San Jose, Calif.
CIRCLE 346 on Reader Service Card

National Semiconductor, Santa Clara, Calif.
CIRCLE 347 on Reader Service Card

Texas Instruments, Dallas
CIRCLE 348 on Reader Service Card