How to Produce Negative Output Voltages from Positive Inputs Using a µModule Step-Down Regulator Design Note 1021 Jaino Parasseril Introduction Linear Technology’s DC/DC step-down μModule ® regulators are complete switchmode power supplies in a surface-mount package. They include the DC/DC controller, inductor, power switches and supporting circuitry. These highly integrated regulators also provide an easy solution for applications that require negative output voltages. In other words, these products can operate as inverting buck-boost regulators. As a result, the lowest potential in the circuit is not the standard 0V, but –VOUT, which must be tied to the μModule regulator’s GND. All signals are now referred to –VOUT. For this discussion, the LTM®8025 (36V, 3A) is used to demonstrate how a buck μModule regulator can be altered to produce a negative output voltage with level-shifting circuitry for synchronization. This approach can be applied to other μModule regulators, such as the LTM8022 (36V, 1A), LTM8023 (36V, 2A) and LTM8027 (60V, 4A). Design Guide A conventional buck (step-down) μModule regulator can be easily conﬁgured to generate negative output voltages by conﬁguring it as an inverting buck-boost converter, as illustrated in Figure 1. The negative terminal of the input supply is connected to the VOUT pin of the μModule regulator and the GND pin is tied to the –VOUT rail. The actual input voltage (VIN’) seen by the μModule VIN + VIN VOUT μModule REGULATOR – regulator is the difference between the input supply (VIN) and the output voltage (–VOUT). This voltage must be within the allowable input range of the part. Additionally, the absolute value of the output voltage must not exceed the maximum output voltage rating of the μModule regulator. Since the part is now operating as an inverting buck-boost, the switch current is larger than in its buck counterpart. Hence, parameters such as output current, switching frequency, thermal performance, etc. must be considered to stay within the part’s limits. Refer to Appendix for detailed discussions and calculations. Refer to Table 1 for a selection guide of example buck μModule regulators conﬁgured as inverters. Table 1. Example of Buck (Step-Down) DC/DC μModule Regulators Conﬁgured as Inverters IOUT(MAX) 12VIN → –5VOUT μModule Regulator LTM8020 0.165A LTM8021 0.475A LTM8022 1A 1.6A LTM8025 2.95A 2.2A LTM8027 4A 3.65A L, LT, LTC, LTM, Linear Technology, the Linear logo and μModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN GND See LTM8025 and LTM8027 LTM8023 VOUT RLOAD 24VIN → –12VOUT VIN ’ = VIN – (–VOUT) + VIN ’: ACTUAL INPUT VOLTAGE SEEN BY μModule REGULATOR – VIN VOUT μModule REGULATOR RLOAD GND dn1021 F01 (a) Buck μModule Regulator Conﬁgured for Positive Output Voltages –VOUT (b) Buck μModule Regulator Conﬁgured for Negative Output Voltages Figure 1. How to Conﬁgure a Buck Module for Negative Output Voltages 12/11/1021 VIN 20V TO 24V VIN RUN/SS 4.7μF 750kHz 750kHz 5V 0V 0.01μF –7V –12V CMDSH2-3 100k VOUT AUX BIAS LTM8025 SHARE PGOOD SYNC RT ADJ GND 63.4k 22μF 34.8k –VOUT –12V AT 2A dn1021 F02 Figure 2. LTM8025 Schematic for –12V Output –12V Output Application The LTM8025 is a 36VIN, 3A step-down μModule converter that can support output voltages up to 24V. With minimal design effort, it can be easily conﬁgured to generate negative output voltages. Figure 2 shows an LTM8025 schematic generating –12V at 2A from an input range of 20V to 24V. The actual input voltage seen by the LTM8025 is VIN’ = VIN – (–VOUT). For instance, if VIN = 20V, VIN’ = 20V – (–12V) = 32V. Because the maximum input rating of the LTM8025 is 36V, the input supply in this speciﬁc application is limited to 24V. Additionally, the internal oscillator of the LTM8025 can be synchronized by applying an external 250kHz to 2MHz clock signal to the SYNC pin. For negative output voltages, the clock must be level-shifted to account for the lower potential. This example has a 0V to 5V, 750kHz input clock signal. By adding a few passive components, the input clock is level-shifted to produce a –12V to –7V signal, which is then applied to the SYNC pin of the LTM8025. Figure 3 shows the start-up waveforms for the –12V output application. Run/Shutdown The LTM8025 has a RUN/SS pin that provides shutdown along with soft-start functions. In order to shut down the part, the RUN/SS pin must be pulled below 0.2V. For negative output applications, the LTM8025 GND is tied to –VOUT. So, the RUN/SS voltage must be below 0.2V above –VOUT to turn off the part, whereas it must be tied to 2.5V above –VOUT for normal operation. Conclusion Step-down μModule regulators, such as the LTM8025, can be easily conﬁgured for negative output voltages. For negative outputs, the LTM8025 operates as an inverting buck-boost, so the maximum allowable output current is lower than typical buck topologies. If synchronization is desired, proper level-shifting circuitry is required. For a complete description of the LTM8025, including operation and applications information, refer to the data sheet. VIN 10V/DIV RUN/SS 2V/DIV VOUT 10V/DIV 200μs/DIV dn1021 F03 Figure 3. LTM8025 Start-Up Waveforms for –12V Output Data Sheet Download www.linear.com For applications help, call (408) 432-1900, Ext. 3747 Linear Technology Corporation dn1021 LT/TP 1211 REV A 305K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2011 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● LOGIC HIGH VIN VIN VOUT μModule REGULATOR CIN 0V R2 R1 R3 Figure 6. Step-Down μModule Regulator with Schottky Diode Protection for Negative Output dn1021 F04 Figure 4. Run Level-Shift Circuit for Negative Output Conﬁguration The shutdown threshold varies with each μModule regulator and is listed in their respective data sheet tables. Scale the resistors R2 and R3 according to the logic high input voltage and the μModule regulator’s shutdown threshold. Figure 5 shows an example of an LTM8027 –12V output application with the level-shifting circuitry. In this example, the LTM8027 has a 5V logic input and a Run pin resistor divider for about 2.5V, enough to exceed the part’s 1.4V shutdown threshold. VIN 20V TO 48V LOGIC INPUT –VOUT dn1021 F06 TO RUN PIN OF μMODULE REGULATOR –VOUT 5V COUT SCHOTTKY DIODE (OPTIONAL) GND Q1 LOGIC INPUT – Level-Shifting the Run Pin in a Negative Output Application Step-down μModule regulators are equipped with a Run pin to enable and shut down the part. For negative output applications, the Run voltage must be level-shifted to properly turn off the part. Using just a single PNP transistor and a few resistors, level-shifting can be achieved to utilize the shutdown feature, as seen in Figure 4. When the logic input is high, the Run voltage increases by an amount determined by the voltage divider resistors R2 and R3. Once the Run voltage exceeds the shutdown threshold, the μModule regulator will turn on; as a result, the output will drop to the programmed negative voltage. To shut down the part, apply a logic low input to force the Run voltage to the same potential as the negative output. External Schottky Diode for Start-Up Protection When conﬁguring a μModule regulator for negative output voltages, the combination of input and output capacitors creates an AC voltage divider at the output. During startup, the output (–VOUT ) will initially go positive for a short period of time before dropping down to the intended negative potential. The positive voltage peak is dependent on both the capacitance values and the input voltage step. To limit the amount of positive voltage, an external Schottky diode between –VOUT and the input supply ground may be required. Figure 6 shows a simpliﬁed μModule regulator schematic with the Schottky diode protection. + APPENDIX 4.7μF ×2 2N3906 0V 20k Design Considerations for Negative Output Applications For negative output applications, the input voltage seen by the μModule regulator (VIN′) is the difference between the input supply voltage (VIN) and the output voltage (–VOUT ): VIN′ = VIN – (–VOUT ) As a result, the maximum input voltage (VIN(MAX)′) must be below the μModule regulator’s abs max input voltage (VIN_MODULE(MAX)). VIN VOUT LTM8027 RUN BIAS1 SS BIAS2 RT 20k 48.7k 22μF ×4 AUX SYNC 20k (Equation 1) SCHOTTKY DIODE (OPTIONAL) ADJ GND 56.2k dn1021 F05 Figure 5. LTM8027 with Run Level-Shift Circuitry for –12V Output VOUT –12V 3A Additionally, the switch current is higher for inverting applications compared to the positive output conﬁguration. Hence, the maximum output current (IOUT(NEG)) must be derated from the μModule regulator’s typical rating (IOUT(POS)) according to the following equation: IOUT(NEG) ≤ (IOUT(POS)) • (1 – DCMAX) (Equation 2) where the max duty cycle, VOUT DCMAX = VIN(MIN) + VOUT VIN = 15V nominal (range: 12V to 18V) VOUT = –5V IOUT(NEG) = 2A Selected μModule regulator: LTM8025 VIN_MODULE(MAX) = 36V IOUT(POS) = 3A Calculations: Using Equations 1 to 3, the following values were determined: VIN(MAX)′ = VIN(MAX) – (VOUT ) = 18 – (–5) = 23V (Equation 3) Equation 2 is only an approximation. The following parameters need to be considered to get a more accurate value: switching frequency, inductor current ripple, efﬁciency, switch current limit derating at high duty cycle, etc. Design Example: Inverting power supply requirements: LTM8025 data sheet ratings: DCMAX = VOUT 5 = = 0.294 VIN(MIN) + VOUT 12+ 5 (IOUT(POS)) • (1 – DCMAX) = (3A) • (1 – 0.294) = 2.12A The above calculations determined that the LTM8025 is a good candidate for this inverting application. The maximum input voltage across the μModule regulator is 23V, well below the 36V maximum operating voltage. With a max duty cycle of 29.4%, the maximum output current is approximately 2.12A—sufﬁcient for the 2A requirement of this application.