Inverting Regulator Takes Inputs Up to 50V and Supports Outputs to 4A Design Note 552 Victor Khasiev Introduction Positive-to-negative DC/DC conversion (inverting 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 cases, the inverting converter must be compact, support high power and accommodate an extended input voltage range. The LTC ®7149 satisfies all of these requirements. Its integrated 4A switches and wide 3.4V to 60V input voltage range exceed the requirements of the most demanding applications, including those in automotive environments. Circuit Description and Functionality Figure 1 shows a positive-to-negative converter based on the LTC7149. This solution delivers –10V at 2A from an input voltage of 12V—an automotive rail, for 511Ω ITH 10nF LTC7149EFE ISET MODE/SYNC RSET 200k VIN 332k 1M 20Ω 100Ω 137k RUN PGOOD RT PGDFB GND VOUTSNS VOUT– VOUT– SVOUT– INTVCC VINREG VIN VIN SW SW SW SW SW SW SW BOOST EXTVCC VOUT– CIN2 10µF + VIN CIN1 4V TO 50V 10µF 63V 2.6 2.4 2.2 L1 10µH 0.1µF 10Ω CIN1: SUN ELECT., 63CE10KX CIN2: MURATA, GRM32ER71J106KA2L COUT: TDK, C4532X7R1E226M250KC L1: COILCRAFT, SER1360-103KL COUT 22µF ×3 2.0 1.8 1.6 1.4 1.2 DN552 F01 Figure 1. LTC7149, Positive-to-Negative Converter (VIN: 4V – 50V, VOUT: –10V at 2A) 07/16/552 The circuit of Figure 1 uses external loop compensation. Connecting ITH to INTVCC allows internal compensation to be used, as shown in Figure 3. Tying the L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 2.2µF 100pF In automotive applications, the LTC7149’s ability to handle high voltage inputs eliminates the need for costly voltage suppressors. The very low minimum input voltage keeps sensitive systems operational even during cold crank conditions. Guidelines for calculating voltage and current stress on the components around the LTC7149 are detailed in the LTC7149 data sheet. As an example, derating of the output current at input voltages below 12V is shown in Figure 2. LOAD CURRENT (A) 4.7nF instance. The power train components were selected for a nominal 12V input, but with proper derating, the input voltage of this application can be as low as 4V or as high as 50V. VOUT– –10V 2A 1.0 4 6 8 10 12 INPUT VOLTAGE (V) 14 16 DN552 F02 Figure 2. Output Current Derating vs Input Voltage for Figure 1 MODE/SYNC to GND activates Burst Mode® operation. Synchronization pulses referenced to GND can be applied to this pin if needed. Efficiency of this solution reaches 94%. Voltage Controlled Variable Negative Output Circuit A significant number of applications require on-the-fly changes to the negative bias, including LCD, OLED monitors and test equipment systems. The LTC7149 includes features to simplify this task. Figure 3 shows a negative voltage source, where the negative output is controlled by a positive signal voltage. The positive control voltage, referenced to GND, is applied to the VOUTSNS pin. In Figure 3, this is VCTRL, in the range of 0V to 5V. The resulting negative output voltage VOUT– is determined by: VOUT– = –50µA • RSET + VCTRL 10nF Figure 4 shows VOUT– as a function of VCTRL. Figure 5 illustrates the broad application potential of this approach as the VCTRL voltage is shaped as a sine wave with a 2.5V amplitude. Conclusion The LTC7149 is a high efficiency 50V, 4A synchronous monolithic regulator for negative output power supplies. It combines wide input and output voltage ranges and integrated switching transistors, which simplify the converter design. The solutions and circuitry discussed in this design note can assist with the implementation of this regulator in automotive and industrial applications, display and monitor systems. 2.2µF LTC7149EFE INTVCC VINREG ISET VIN VIN MODE/SYNC ITH VIN 332k 20Ω VCTRL RF 0V TO 5V 100Ω CF 0.1µF RP 10k OPT RUN PGOOD RT PGDFB GND VOUTSNS VOUT– VOUT– SVOUT– SW SW SW SW SW SW SW BOOST EXTVCC VOUT– –12 CIN2 10µF + CIN1 10µF 63V VIN 4V TO 50V –8 L1 10µH 0.1µF 10Ω –6 –4 COUT 22µF ×3 –2 0 DN552 F03 VOUT– –5V TO –10V AT 2A Figure 3. Positive-to-Negative Converter with Variable VOUT– from –5V to –10V VCTRL 5V/DIV –10 VOUT– (V) RSET 200k + – The lowpass filter RF/CF provides noise suppression. The VOUTSNS pin cannot be left floating under any circumstances—some voltage potential must be present on this pin at all times. If this requirement cannot be met, for example during system testing, then resistor RP should be installed. 0 1 3 2 VCTRL (V) 4 5 DN552 F04 Figure 4. Variable Negative Output VOUT– as a Linear Function of VCTRL GND VOUT– 2V/DIV 500ms/DIV Figure 5. Variable Negative Output VOUT– Following the Sine Waveform on VCTRL Data Sheet Download www.linear.com/LTC7149 Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3161 dn552f LT/AP 0716 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2016