CN0198

This circuit enhances the slew rate control and dynamic power control features of the AD5755 to create a more complete and robust DAC solution. By implementing a simple step-down dc-to-dc converter using the ADP2300, the circuit can provide higher than normal supply currents that are required when slewing the AD5755 outputs.

The AD5755 behaves like any standard DAC converting digital data to analog current (for example, 0 mA to 20 mA, 4 mA to 24 mA, or 0 mA to 24 mA) or voltage outputs (0 V to 5 V, 0 V to 10 V, ±5 V, or ±10 V). The AD5755 operates with an extended AV_SS power supply range to −26.4 V, and an AV_DD range to +33.0 V.

Power Dissipation Control

In standard, current controlled module or actuator designs, the load resistor value can range from typically 50 Ω to 750 Ω, but it can be as low as 10 Ω or as high as 1 kΩ. The 4 mA to 20 mA output driver stage must operate on a supply voltage that provides sufficient headroom for the full range of the load resistor values. For example, when driving 24 mA into a 1 kΩ load, a supply voltage of greater than 27 V is required, assuming a 3 V headroom is needed. In this case, the internal package power dissipation due to the output driver is 3 V × 24 mA = 72 mW. However, when driving a 10 Ω load with the same 27 V supply voltage, the internal power dissipation of the driver is approximately 27 V × 24 mA = 648 mW. For a quad DAC, this is greater than 2.5 W.

The AD5755 circuitry senses the output voltage and dynamically regulates the boost supply voltage to meet supply voltage requirements plus a sufficient amount of headroom. For 24 mA output into 10 Ω, the boost voltage of 7.4 V results in an internal power dissipation of only 7.4 V × 24 mA = 178 mW. This represents nearly a 4× reduction in power vs. the unregulated case.

A separate boost supply voltage is generated for each of the four DAC outputs by four independent dc-to-dc converters operating on a 5 V input.

DC-to-DC Converters

The AD5755 contains four independent, on-board dc-to-dc converters. They provide dynamic control of the V_{BOOST_X} supply voltage for each individual channel. Figure 2 shows the discrete components needed for the dc-to-dc circuitry, and the following sections describe the operation of this circuitry.

It is recommended to place a 10 Ω, 100 nF low-pass RC filter after C_DCDC. This consumes a small amount of power; however, it reduces the amount of ripple on the V_{BOOST_X} supply. The suggested component values for L_DCDC, C_DCDC, and D_DCDC are given in Table 1.

DC-to-DC Converter Operation

The on-board dc-to-dc converters use a constant frequency, peak current mode control scheme to step up an AV_CC input of 4.5 V to 5.5 V to drive the AD5755 output channel. These are designed to operate in discontinuous conduction mode (DCM) with a duty cycle of <90% typical.

Discontinuous conduction mode refers to a mode of operation where the inductor current goes to zero for an appreciable percentage of the switching cycle. The dc-to-dc converters are nonsynchronous; that is, they require an external Schottky diode.

DC-to-DC Converter Output Voltage

When a channel current output is enabled, the converter regulates the V_{BOOST_X} supply to 7.4 V (±5%) or (I_OUT × R_LOAD + Headroom), whichever is greater. The value of the headroom voltage is approximately 3 V. In the voltage output mode with the output disabled, the converter regulates the V_{BOOST_X} supply to +15 V (±5%). In current output mode with the output disabled, the converter regulates the V_{BOOST_X} supply to 7.4 V (±5%).

Within a channel, the V_{OUT_X} and I_{OUT_X} stages share a common V_{BOOST_X} supply so that the outputs of the I_{OUT_X} and V_{OUT_X} stages can be tied together.

DC-to-DC Converter Settling Time

When in current output mode, the settling time for a step greater than approximately 1 V (I_OUT × R_LOAD) is dominated by the settling time of the dc-to-dc converter. The exception to this is when the required voltage at the I_{OUT_X} pin plus the compliance voltage is below 7.4 V (±5%). The settling time for smaller loads is faster. The settling time for current steps less than 24 mA is also faster.

DC-to-DC Converter V_MAX Functionality

The maximum V_{BOOST_X} voltage is set in the dc-to-dc control register. On reaching this maximum voltage, the dc-to-dc converter is disabled, and the V_{BOOST_X} voltage is allowed to decay by approximately 0.4 V. After the V_{BOOST_X} voltage has decayed, the dc-to-dc converter is reenabled, and the voltage ramps up again to V_MAX, if still required.

As seen in Figure 3, the DC-DCx bit in the status register asserts when the AD5755 is ramping to the V_MAX value and deasserts when the voltage is decaying to V_MAX − 0.4 V.

AV_CC Supply Static Current Requirements

The dc-to-dc converter is designed to supply a V_{BOOST_X} voltage of V_BOOST = I_OUT × R_LOAD + Headroom

This means that, for a fixed load and output voltage, the dc-to-dc converter output current can be calculated by

where:

I_OUT is the output current from I_{OUT_X} in amps.
ηV_BOOST is the efficiency at V_{BOOST_X} as a fraction.

AV_CC Supply Slewing Current Requirements

The AI_CC current requirement while slewing is greater than in static operation because the output power increases to charge the output capacitance of the dc-to-dc converter. If not enough AI_CC current is provided, the AV_CC voltage drops. Due to this AV_CC drop, the AI_CC current required to slew increases further.

This means that the voltage at AV_CC drops further, and the V_{BOOST_X} voltage, and thus the output voltage, may never reach its intended value. Because this AV_CC voltage is common to all channels, this may also affect other channels.

ADP2300 AV_CC Supply

The ADP2300 and several discrete components are used to create a simple 5 V rail that meets the supply current demands of the AD5755 as previously described. The output voltage is set externally by a resistive voltage divider from the output voltage to the FB pin, as show in Figure 4.

Test Data and Results

All test data was gathered using the EVAL-AD5755SDZ, EVAL-SDP-CB1Z, and ADP2300-EVALZ boards. The integral nonlinearity (INL), differential nonlinearity (DNL), and total unadjusted error (TUE) of the system using the ADP2300 circuit is shown in Figure 5, Figure 6, and Figure 7, respectively. The AD5755 boost regulators were active for all the measurements.

Complete documentation for the system can be found in the CN0198 Design Support package.