AN42

APPLICATION NOTE

 

 

MOSFET Gate Bias

The MOSFET(s) can be biased using one of two methods: Charge Pump or 12V Gate Bias.

Charge Pump (or Bootstrap)

Figure 6 employs a charge pump to provide the MOSFET gate bias. The charge pump capacitor, CP, is used as a flying capacitor to boost the voltage of the RC5040 or RC5042 out- put driver. When the MOSFET switches off, the source of the MOSFET is at -0.6V. VCCQP is charged through the Schot- tky diode to 4.5V. Thus, the capacitor CP is charged to 5V. When the MOSFET turns on, the source of the MOSFET is at approximately 5V. The capacitor voltage follows, and hence provides a voltage at VCCQP equal to 10V. The Schot- tky is required to provide the charge path when the MOSFET is off, and then reverses bias when the VCCQP goes to 10V. The capacitor CP needs to be a high Q and high frequency capacitor. A 1F ceramic capacitor is recommended here.

+5V

 

 

DS2

 

 

VCCQP

M1

 

 

 

HIDRV

 

 

CP

L1

RS

PWM/PFM

 

VO

 

 

Control

 

 

 

DS1

CB

 

 

65-AP42-06

Figure 6. Charge Pump Configuration

12V Gate Bias

Figure 7 illustrates how an external 12V source can be used to bias VCCQP. A 47 Ω resistor is used to limit the transient current into the VCCQP pin, and a 1F capacitor filter is used to filter the VCCQP supply. This method provides a higher gate bias voltage (VGS) to the MOSFET, and there- fore reduces the RDS,ON and resulting power loss within the MOSFET. Figure 8 illustrates how RDS,ON decreases dra- matically as VGS increases. A 6.2V Zener (DS2) is used to clamp the voltage at VCCQP to a maximum of 12V and ensure that the absolute maximum voltage of the IC is not exceeded.

Warning: The 12V Gate Bias method applies only to the RC5042. The RC5040 has not been designed to accept an external 12V gate bias voltage, and may be damaged if this method is used.

 

+5V

 

+12V

47Ω

 

D1

 

 

 

 

6.2V

 

 

VCCQP

 

 

M1

 

 

HIDRV

 

 

L1

RS

PWM/PFM

 

VO

 

 

Control

 

 

 

DS1

CB

 

 

65-AP42-07

Figure 7. 12V Gate Bias Configuration

 

0.1

 

 

 

 

 

 

 

 

 

 

 

 

 

0.09

 

 

 

 

 

 

 

 

R(DS)Fuji

 

 

 

0.08

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R(DS)Fuji

 

 

)

0.07

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Ω

0.06

 

 

 

 

 

 

 

 

R(DS)706A

 

DS,ON

0.05

 

 

 

 

 

 

 

 

R(DS)-706AEL

 

0.04

 

 

 

 

 

 

 

 

 

 

 

 

R

0.03

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.02

 

 

 

 

 

 

 

 

 

 

 

 

 

0.01

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

1.5

2

2.5

3

3.5

4

5

6

7

8

9

10

11

Gate-Source Voltage, VGS (V)

Figure 8. RDS,ON vs. VGS for Selected MOSFETs

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Fairchild RC5042, RC5040 specifications Mosfet Gate Bias, Charge Pump or Bootstrap

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.