# Intermediate Rail 2MHz Switching Power Supply Withstands Entire Automotive Input Voltage Range

Abstract: This application note illustrates an intermediate 8V switching power supply for an automotive radio and infotainment system. The design withstands the complete automotive input voltage range (including cold crank and load dump conditions), assuring a stable 8V supply for common subsystems such as a CD driver, LCDs, and a radio module in modern infotainment systems. To avoid disturbance in the AM and FM bands, the switching power supply runs at a fixed frequency of 2MHz, enabling an ideal solution for radio systems.

*EDN*, October 31, 2012.

## Introduction

**Figure 1**illustrates common automotive systems that require different architecture solutions.

*Figure 1. Automotive power-supply solutions.*

**Figure 2**shows the switching power-supply schematic. It incorporates the MAX15005 step-up controller and the MAX16952 step-down controller, with additional circuitry for proper operation. Both ICs are synchronized with an external 2MHz square-wave logic signal, provided by a microcontroller or dedicated IC. This allows great flexibility in choosing the optimal switching frequency for the power supply. During normal battery conditions, the MAX15005 is disabled and the MAX16952 regulates the 8V on the OUTB node. When the battery voltage decreases during cold crank, the MAX15005 is enabled and boosts the voltage on the OUTA node. This allows the MAX16952 to regulate the 8V on the OUTB node. The entire design can survive up to 40V automotive load dump, thanks to the robustness of the two ICs. The system has been set up and tested to provide 20W of power (8V at 2.5A) on its main output (OUTB), although the external components can be modified to reach higher output power.

*Figure 2. The switching power-supply schematic.*

## External Components for the MAX16952

### Output Voltage and Switching Frequency

_{22}and R

_{21}resistors). Choosing a 51kΩ low-side resistor-divider for R

_{22}(since the MAX16952 data sheet recommends low-side resistors of less than 100kΩ), the high-side resistor-divider must be selected using the following equation:

(Eq. 1) |

_{FB}= 1V (typ).

_{22}, the typical resulting output voltage value is:

(Eq. 2) |

(Eq. 3) |

(Eq. 4) |

_{FB}(MIN) is 0.985V and V

_{FB}(MAX) is 1.015V.

_{16}internal oscillator resistance that imposes an internal switching frequency that is less than 1.8MHz. For this reason, a 30kΩ resistor was selected for R

_{16}. To make the MAX16952 switch at a 2MHz fixed frequency, it is necessary to avoid the dropout condition. The MAX16952 avoids dropout until the turn-off time (t

_{OFF}) is higher than 100ns (typ). This implies that the system can never go over a maximum duty cycle of:

(Eq. 5) |

(Eq. 6) |

_{1}inductor and D

_{2}Schottky diode.

(Eq. 7) |

_{ON}) is 80ns (typ), which enables it to reach a minimum duty cycle of:

(Eq. 8) |

### Inductor and Current Sense

*Figure 3. The MAX16952 inductor current.*

**Figure 3**:

(Eq. 9) |

(Eq. 10) |

(Eq. 11) |

(Eq. 12) |

(Eq. 13) |

_{2}of 2.2µH results in a LIR factor of 0.24 with a peak inductor current of:

(Eq. 14) |

_{20}sense resistor reaches 68mV (min). Leaving margin for the tolerance on the inductor, the sense resistor has been sized to have a voltage drop of 60% of the current limit threshold when the inductor current reaches its peak value (I

_{PEAK}):

(Eq. 15) |

_{20}has been chosen.

## External Components for the MAX15005

### UVLO Threshold

_{5}, the following equation is applied to choose the R

_{4}resistor value:

(Eq. 16) |

_{4}.

### Overvoltage Input (OVI)

_{1}and the diode D

_{2}, the MAX15005 must turn on when the IN voltage goes below 11.5V. However, to optimize efficiency, the MAX15005 must not run when the battery voltage is at its normal level (IN = 12V).

_{2}resistor-divider equal to 20kΩ and considering that the MAX15005 should turn off when the input voltage rises above 11.6V, the high-side R

_{1}resistor-divider must be chosen according to the following formula:

(Eq. 17) |

_{1}resistor, the MAX15005 is disabled when the main voltage rises above 11.67V, leaving a margin of 330mV from the normal 12V IN battery voltage. Once considering the hysteresis on the OVI comparator, we can estimate the falling voltage value on the main supply that enables the MAX15005:

(Eq. 18) |

_{3}and D

_{1}) between the OVI pin and SS pin. When the MAX15005 is disabled, the SS pin is internally tied to ground, connecting R

_{3}in parallel with R

_{2}and resulting in an effective decrease of the hysteresis. Using a 180kΩ resistor for R

_{3}and neglecting the voltage drop across the diode, the new falling voltage threshold on the main supply becomes:

(Eq. 19) |

### Output Voltage

*Figure 4. The MAX15005 inductor current.*

_{ON}, as stated in the MAX15005 data sheet, under all application conditions. The minimum t

_{ON}results in a minimum duty cycle of 34% (with the 2MHz switching frequency), which limits the minimum output voltage that can be regulated with the MAX15005. To estimate this voltage threshold, it is necessary to consider the boost regulator duty cycle formula:

(Eq. 20) |

_{IN}) is at its maximum value (11.67V in this design) and the MAX15005 is operating. It is possible to estimate the minimum output voltage that is regulated by the MAX15005 in this limit condition by adapting the previous equation:

(Eq. 21) |

_{2}Schottky diode of 0.3V, and neglecting the voltage drop on the NMOS N1. Thus, the MAX15005 must regulate an output voltage greater than 17.38V to ensure the 2MHz switching frequency under all operating conditions.

_{13}equal to 10kΩ, it is possible to calculate the high-side feedback R

_{14}resistor-divider:

(Eq. 22) |

_{FB}(MIN) = 1.215V.

_{14}, the minimum output voltage regulated by the operating MAX15005 is:

(Eq. 23) |

_{2}Schottky diode. However, the C

_{7}output capacitor must be able to sustain the output voltage regulated by the MAX15005 itself.

### Synchronization and the Maximum Duty Cycle

_{6}and a 100pF capacitor for C

_{4}, the MAX15005 internal oscillator frequency is approximately 1MHz, allowing an external synchronization frequency of 2MHz.

_{4}capacitor is discharged through an internal 1.33mA current source (typ). When the voltage on this capacitor (RTCT pin) reaches 500mV, the C

_{4}capacitor is charged through R

_{6}, which is connected to the VREG5 pin until the next synchronization signal rising edge is detected. The discharge time (T

_{DISCHARGE}) determines the minimum t

_{OFF}for the regulator. If this time is less than 160ns (as in this case), the minimum t

_{OFF}is clamped to 160ns. In fact assuming a charge time (T

_{CHARGE}) of 340ns (T

_{P}= 500ns), the voltage on RTCT increases:

(Eq. 24) |

(Eq. 25) |

_{OFF}of 160ns implies a maximum duty cycle of 68%. Reusing the boost regulator duty cycle formula (Equation 20) applied in the case where the maximum duty cycle is required (lower input voltage, in this case 5V), the maximum voltage regulated by the MAX15005 on the OUTA pin is:

(Eq. 26) |

### Inductor Selection

(Eq. 27) |

_{IN}is at its maximum value (11.67V) with a corresponding duty cycle of 37%.

_{OUTA(MIN)}, sourced from the boost regulator. Merging these last considerations and solving the previous equation, the minimum inductance value is 1.32µH. For this design, a 2.2µH inductor was chosen for L

_{1}.

### Current Sense

(Eq. 28) |

(Eq. 29) |

_{10}.

## Bench Tests

### Cold-Crank Test

**Plot 1**shows, when the IN voltage decreases, the MAX15005 begins pumping OUTA voltage up to 17.5V. This allows the MAX16952 to keep 8V regulated on OUTB. On the other side, when the input voltage returns to its operating value, the MAX15005 stops and the OUTA voltage decreases to the IN voltage, less the drop on the D

_{2}diode and L

_{1}inductor. Every test has been performed with an 2.5A output load on the OUTB pin.

### Plot 1

**Plot 2**and

**Plot 3**illustrate a closer view of the cold-crank fall and rise phases, respectively.

### Plot 2

### Plot 3

### Analyzing the Frequency Domain

### Plot 4

### Plot 5

### Plot 6

### Plot 7

## Further Improvements

_{2}Schottky diode during the normal application condition when the MAX15005 is nonoperational. This is accomplished with an N-channel MOSFET connected in parallel with D

_{2}when the main supply is at its normal value. To reduce electromagnetic interference (EMI), slow the voltage edge on the MOSFET gate and add external resistors (R

_{8}, R

_{17}, R

_{18}, and R

_{19}), resulting in a trade-off of increased power dissipation. To filter out spikes on the current-sense waveform of the MAX15005, it can be useful to introduce a small RC filter (C

_{6}and R

_{9}). The MAX15005 current limit threshold can also be reduced by adding an offset to the R

_{7}resistor—this reduces power dissipation across the sense resistor R

_{10}.

#### References

- This estimate assumes an internal 1.33mA discharging current and an external charging current sourced from VREG5 through R
_{6}as VREG/R_{6}= 715µA. The latter estimation approximates the voltage on RTCT pin as 0V.