Objective:Design a power-supply solution to optimize the output of DAC81416 high-voltage output.

This application note describes how to design a power supply to power the high voltage output of the DAC81416 using a single 12-V input. The proposed power supply is capable of providing 500 mA of current for a high-voltage output from the DAC. The DAC81416, DAC71416, and DAC61416 (DACx1416) are a pin-compatible family of 16-channel, buffered, high-voltage output digital-to analog converters (DACs) with 16-bit, 14-bit, and 12-bit resolution. A user-selectable output configuration enables full-scale bipolar output voltages of ±20 V, ±10 V, ±5 V or ±2.5 V, and full-scale unipolar output voltages of 40 V, 20 V, 10 V or 5 V.

The DACx1416 requires five power-supply inputs: VIO, VDD, VAA, VCC, and VSS. VDD and VAA should be at the same level. For high-voltage output options like ±20 V, VCC and VSS should be ±24 V to account for the headroom and footroom requirements for the output buffer of the DAC81416. For unipolar output voltage (0 V to 40 V), the minimum VCC supply required is 42 V.

To generate the 24-V VCC supply, use LM51571 in boost configuration. The LM5157x-Q1 device is a wide input range, non-synchronous boost converter with integrated 50-V, 6.5-A (LM5157-Q1) or 50-V, 4.33-A (LM51571-Q1) power switch. The device can be used in boost, SEPIC, and flyback topologies.

The –24-V VSS supply is generated by using the LM25576 in an inverting buck boost configuration. Operating with an input voltage range of 6 V to 42 V, the LM25576 delivers 3 A of continuous output current with an integrated 170-mΩ N-Channel MOSFET

Custom Design With WEBENCH® Tools for LM51571

Click here to create a custom design using the device with the WEBENCH® Power Designer.

• Start by entering the input voltage (V_IN), output voltage (V_OUT), and output current (IOUT) requirements into WEBENCH® . For this design V_IN = 12 V (typical), V_OUT = 24 V, I_OUT = 500 mA
• Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial in WEBENCH Power Designer.
• Run electrical simulations to see important waveforms and circuit performance (If supported by the device). Otherwise download the spice model from website and simulate the circuit using PSpice® for TI

Design Notes for LM51571

• When selecting the inductor (L1), consider three key parameters: inductor current ripple ratio (RR), falling slope of the inductor current, and Right Half Plane (RHP) zero frequency (f_RHP). RR is selected to have a balance between core loss and copper loss. The falling slope of the inductor current must be low enough to prevent subharmonic oscillation at high duty cycle (additional RSL resistor is required, if not). Higher f_RHP (lower inductance) allows a higher crossover frequency and is always preferred when using a small value output capacitor. The inductance value can be selected to set the inductor current ripple between 30% and 70% of the average inductor current to achieve a good compromise between RR, f_RHP, and inductor falling slope
• To set the output regulation target, select the feedback resistor values as shown in the following equation:
  Equation 1. $V_{OUT} = V_{REF }\times \frac{R_{FB1}}{R_{FB2}}$

  where:

  • V_REF = 1 V (typical)
• There are a few ways to select the proper value of the output capacitor (C_OUT). Select the output capacitor value based on output voltage ripple, output overshoot, or undershoot due to load transient. The ripple current rating of the output capacitors must be enough to handle the output ripple current. By using multiple output capacitors, the ripple current can be split allowing for a capacitor with a lower ripple current to be chosen.
• A Schottky diode is the preferred type for a D1 diode due to its low forward voltage drop and small reverse recovery charge. Low reverse-leakage current is an important parameter when selecting the Schottky diode. The diode must be rated to handle the maximum output voltage plus any switching node ringing. Also, it must be able to handle the average output current.
• The switching frequency of the device can be set by a single resistor connected between RT and the GND pins using the following equation:
  Equation 2. $R_T = \frac{2.21 \times { 10}^{10}}{f_{RT\left(TYPICAL\right)}} - 955$

See the How to Design a Boost Converter Using LM5155 application report for calculating compensation components or using WEBENCH Power Designer.

Custom Design With WEBENCH® Tools for LM25576

• RT sets the oscillator switching frequency. Generally, higher-frequency applications are smaller but have higher losses. Operation at 500 kHz was selected for this example as a reasonable compromise for both small size and high efficiency. RT is calculated using the following equation:
  Equation 3. $R_{T }= \frac{\left(\frac{1}{f_S}\right) - 580\times {10}^{-9}}{135 \times {10}^{-2}}$
• The inductor value is determined based on the operating frequency, load current, ripple current, and the minimum and maximum input voltage (V_IN(min), V_IN(max)). To keep the circuit in continuous conduction mode (CCM), the maximum ripple current (I_RIPPLE) should be less than twice the minimum load current, or 0.1 A_p-p. Using this value of ripple current, the value of inductor (L1) is calculated using the following equation:
  Equation 4. $L1 = \frac{V_{OUT} \times \left(V_{IN\left(MAX\right)} - V_{OUT}\right)}{I_{RIPPLE} \times f_S \times V_{IN\left(MAX\right)}}$
• With the inductor value selected, the value of C_RAMP necessary for the emulation ramp circuit is calculated using the following equation:
  Equation 5. $C_{RAMP }= L1 \times {10}^{-5}$
• A Schottky type re-circulating diode is required for all LM25576 applications. Ultra-fast diodes are not recommended and may result in damage to the device due to reverse recovery current transients. The near-ideal reverse recovery characteristics and low forward voltage drop are particularly important diode characteristics for high input voltage and low output voltage applications common to the LM25576. The reverse recovery characteristic determines how long the current surge lasts each cycle when the buck switch is turned on. The reverse recovery characteristics of Schottky diodes minimize the peak instantaneous power in the buck switch occurring during turn-on each cycle. The resulting switching losses of the buck switch are significantly reduced when using a Schottky diode. The reverse breakdown rating should be selected for the maximum V_IN, plus some safety margin.
• The regulator supply voltage has a large source impedance at the switching frequency. Low ESR ceramic input capacitors are necessary to limit the ripple voltage at the V_IN pin while supplying most of the switch current during the on-time. When the buck switch turns on, the current into the V_IN pin steps to the lower peak of the inductor current waveform, ramps up to the peak value, then drops to zero at turn-off.

For more information, see the Working with Inverting Buck-Boost Converters application report.