Main Control Loop, High Current Output Drivers

AN42	APPLICATION NOTE

Main Control Loop

OSCILLATOR

			–
			+
			–
		–	+
		–
		+
VREF	4-BIT		1.24V
VREF	DAC	REFERENCE
	DAC	REFERENCE

RC5040	+5V
	+5V
	VIN
–
+
DIGITAL
CONTROL

POWER PWRGD

GOOD

VID0	65-5040-01

VID1

VID2

VID3

Figure 2. RC5040 Block Diagram

High Current Output Drivers

The RC5040 contains two identical high current output drivers that use high speed bipolar transistors in a push-pull conﬁguration. Each driver is capable of delivering 1A of cur- rent in less than 100ns. Each driver’s power and ground are separated from the chip power and ground for additional switching noise immunity. The HIDRV driver’s power sup- ply, VCCQP, is boot-strapped from a ﬂying capacitor as illustrated in Figure 3. Using this conﬁguration, C12 is charged from VCC via the Schottky diode DS2 and boosted when the FET is turned on. This scheme provides a VCCQP voltage equal to 2•VCC – VDS(DS2), or approximately 9.5V when VCC = 5V. This voltage is sufﬁcient to provide the 9V gate drive to the MOSFET that is required to achieve a low RDS(ON). Since the low side synchronous FET is referenced to ground (see Figure 4), boosting the gate drive voltage is not needed and the VCCP power pin can be tied to VCC.

Refer to Typical Operating Characteristics of the RC5040 data sheet for a full load VCCQP waveform.

Internal Voltage Reference

The reference used in the RC5040 is a precision band-gap voltage reference, with internal resistors precisely trimmed to provide a near zero temperature coefﬁcient, TC. Added to the reference voltage is the output from a 4-bit DAC. The DAC is provided meet Pentium Pro speciﬁcations, requiring a programmable converter output via a 4-bit voltage identiﬁ- cation (VID) code. This code scales the output voltage from 2.0V (no CPU) to 3.5V in 100mV increments. To guarantee stable operation under all loads, a 10KΩ pull-up resistor and 0.1∝F of decoupling capacitance should be connected to the VREF pin. No load should be imposed on this pin.

Power Good (PWRGD)

The RC5040 and RC5042 Power Good function has been designed according to Intel’s Pentium Pro DC-DC converter speciﬁcation. The Power Good function provides a constant voltage monitor on the VFB pin. The internal circuitry of the converter compares the VFB signal to the VREF voltage and outputs an active-low interrupt signal to the CPU when the power supply voltage exceeds ±7% of its nominal setpoint. The Power Good ﬂag provides no other control function to the RC5040.

Output Enable (OUTEN)

Intel speciﬁcations state that the DC-DC converter should accept an open collector signal for controlling the output voltage. A logic LOW for this signal disables the output volt- age. When disabled, the PWRGD output is in the low state. This feature is available for the RC5040 only.

Upgrade Present (UP#)

Intel speciﬁcations state that the DC-DC converter must accept an open collector signal that indicates the presence of an upgrade processor. The typical state is high (for a stan- dard P6 processor). When the signal is low or in theground state (for the OverDrive processor), the output voltage must be disabled unless the converter can supply the OverDrive processor’s power requirements. When disabled, the PWRGD output must be in the low state. Because the RC5040 and RC5042 can supply the OverDrive processor requirements, the UP# signal is not required.

RC5040, RC5042 specifications

The Fairchild RC5042 and RC5040 are versatile integrated circuits that stand out in the realm of high-performance analog applications. Designed to meet the demands of modern electronic systems, these devices integrate various features and technologies that contribute to their effectiveness in a multitude of applications.

The RC5040 is a precision voltage reference that offers a stable, low-noise output, making it ideal for applications such as instrumentation, data acquisition systems, and RF circuits. It boasts an operating temperature range of -40°C to +85°C, ensuring reliability in diverse environments. One of its most significant characteristics is its low-temperature drift, which minimizes variations in output voltage over temperature fluctuations, thereby enhancing the accuracy of devices that utilize it.

On the other hand, the RC5042 is designed as a high-speed comparator with an integrated voltage reference. This dual functionality allows for a more compact design in applications where space is a premium. The RC5042 features an ultra-fast response time and high input impedance, which contribute to its capability to handle rapidly changing signals without distortion. This makes it particularly useful in applications like analog signal processing and threshold detection.

Both devices utilize Fairchild's advanced BiCMOS technology, which combines the benefits of bipolar and CMOS processes. This technology allows the devices to operate with low power consumption while maintaining high speed and operational efficiency. The RC5042 and RC5040 also incorporate noise-reduction techniques, which help in minimizing unwanted disturbances that could impact circuit performance.

Another noteworthy characteristic of both the RC5040 and RC5042 is their ease of integration. They come in compact package sizes, making them easier to incorporate into various designs without compromising on performance. Furthermore, the availability of multiple output options allows engineers the flexibility to choose configurations that best suit their specific applications.

In conclusion, the Fairchild RC5042 and RC5040 are robust devices that offer essential functionality for various high-performance analog applications. With their precision, fast response time, and exceptional reliability, these integrated circuits are a valuable asset in the design of modern electronic systems, catering to the growing demands of the technology landscape.