Positive to negative DC/DC conversion (negative output) is widely used in LCD devices, OLED displays, audio amplifiers, industrial equipment, measurement tools, test systems, LED drivers, and battery chargers. In all of these applications, inverting converters must be compact, support high power, and accommodate extended input voltage ranges. The LTC7149 meets all of these requirements. Its integrated 4A switch and wide input voltage range of 3.4V to 60V meet and exceed the requirements of most demanding applications, including those in automotive environments.

Circuit Description and Function

Figure 1 shows a positive-to-negative converter based on the LTC7149. This solution provides –10V/2A output from a 12V input voltage (eg, an automotive power rail). The power chain components are selected for a nominal 12V input, but with appropriate derating, the input voltage for this application can be as low as 4V or as high as 50V.

Figure 1: LTC7149 Positive-to-Negative Converter (VIN: 4V to 50V, VOUT: –10V/2A)

In automotive applications, the LTC7149’s ability to handle high voltage inputs eliminates the need for expensive voltage suppressors. Very low minimum input voltage keeps sensitive systems operating even in cold crank conditions. Guidelines for calculating voltage and current stress on components surrounding the LTC7149 can be found in the LTC7149 data sheet. Figure 2 shows an example of output current derating for input voltages below 12V.

Figure 2: Output Current Derating vs. Input Voltage (for the circuit of Figure 1)

The circuit of Figure 1 uses external loop compensation. As shown in Figure 3, connecting ITH to INTVCC allows the use of internal compensation. Connecting MODE/SYNC to GND enables Burst Mode operation. If desired, a sync pulse referenced to GND can be applied to this pin. The efficiency of this solution reached 94%.

Voltage Controlled Variable Negative Output Circuit

Dynamically changing negative bias is required for many applications, including LCDs, OLED monitors, and test equipment systems. The LTC7149 has features designed to simplify this task.

Figure 3 shows a negative voltage supply where the negative output is controlled by a positive signal voltage. A positive control voltage referenced to GND is applied to the VOUTSNS pin. In Figure 3, this is VCTRL in the 0V to 5V range. The resulting negative output voltage VOUT– is determined by:

VOUT– = –50μA • RSET + VCTRL

Figure 3: Positive-to-Negative Converter with Variable VOUT– (from –5V to –10V)

Low pass filter RF/CF provides noise rejection. The VOUTSNS pin cannot be floated under any circumstances, a voltage potential must always be present on this pin. If this requirement cannot be met (eg during system testing), resistor RP should be installed.

Figure 4 shows VOUT– as a function of VCTRL. Figure 5 illustrates the broad potential of this approach, as the VCTRL voltage is shaped into a sine wave with a 2.5V amplitude.

Figure 4: Variable negative output VOUT as a linear function of VCTRL.

Figure 5: Variable negative output VOUT– follows the sinusoidal waveform on VCTRL.

in conclusion

The LTC7149 is a high efficiency 50V, 4A synchronous monolithic regulator for negative output supplies that combines wide input and output voltage ranges with integrated switching transistors to simplify converter design. The solutions and circuits discussed in this Design Essentials assist in implementing this regulator in automotive and industrial applications, display and monitor systems.

Reviewing Editor: Tang Zihong

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