Feedback Circuit Improves Hysteretic Control

Mar 1, 2008 12:00 PM
By Kevin Daugherty, National Semiconductor

Another benefit of the LM3485-type controller is that under light loads, very low regulator quiescent current is achievable since the p-channel MOSFET only switches when necessary without the need for a charge pump or boot-strap circuit.

With this design, no additional ESR other than that inherent in C2 (C_OUT) is used to set the switching frequency. Although frequency deviation with respect to V_IN is acceptable, it is important to emphasize that very wide variations will occur from board to board mainly because of variations in the C_OUT ESR and inductor tolerance. For example, if C_OUT is a Sanyo 6TPC100M 100 µF with a rated ESR of 45 mΩ at 100 kHz, then from Eq. 1 the calculated frequency is 376 kHz, where α equals 1, ESR equals 45 mΩ, V_HYS equals 10.5 mV, T_D equals 110 ns, L equals 22 µH, V_IN equals 13.7 V and V_OUT equals 3.3 V.

The measured frequency is actually 160 kHz because C_OUT ESR is much lower than rated and it varies with temperature, further widening the frequency spread. Table 1 shows the variation in switching frequency for input voltages from 8 V to 16 V. Here, the frequency deviation percentage is relative to 160 kHz.

Emulated Ripple Voltage Example

Next, we modified the same board to include emulated ripple voltage circuitry with C_FF, C_S and R_S as shown in Fig. 3. For example, calculated values of C_FF, CS and R_S are for the desired switching frequency of approximately 330 kHz, and measurements were taken to confirm the results.

For the design procedure:

• Set C_FF-based feedback resistor R1 (33 kΩ) to be << impedance at 330 kHz.

• Select C_FF equals 2.2 nF for an impedance of 1/(2 π FC) = 219 Ω.

• C_S equals approximately 10 to 20 times C_FF , so choose 68 nF, which is not critical.

• Duty cycle can be calculated based on a volts-seconds balance across the inductor for on and off cycles (measured at 26% for nominal V_IN of 13.7 V and V_OUT of 3.3 V).

• Calculate required R_S for desired frequency using V_HYS equals 10.5 mV and TD equals 110 ns, from the LM3485 data sheet using Eq. 3.

Note that a T_D of 110 ns is primarily the propagation delay of the LM3485 comparator. For a larger input capacitance of the selected MOSFET, between 10 ns to 20 ns may need to be added:

RS = 287 kΩ.

As shown in Table 2, test results of the same evaluation board, but with modified circuitry, came very close to what was expected, using R_S equals 287 kΩ, C_S equals 68 nF and C_FF equals 2.2 nF.

In this design, the C_OUT ESR does not appreciably impact the switching frequency. Actually, we don't require this type of capacitor when using the emulated ripple voltage method, and a smaller and less-expensive ceramic capacitor with very low ESR between 10 µF to 22 µF is preferable. Note that if an output capacitor with too large an ESR is selected, it will tend to increase the frequency from the summation of emulated and feedback ripple voltage.

Table 2 shows the variation in switching frequency for input voltages from 8 V to 16 V. The frequency deviation percentage is relative to 340 kHz.

PC-Board Design

Board layout is a separate topic, but it is critical to achieving desirable operation of the hysteretic controller, so a few points should be emphasized:

• Make the feedback trace thin and keep it well away from the inductor and high di/dt traces of the input and switch node.

• Place the feedback resistors very close to the controller.

• Connect the feedback network ground directly to the controller ground pin and run the controller signal ground path completely separate from the power ground return to the input source ground.

• Connect C1 (C_IN) and its ground as close as possible to the anode side of D1 to contain the high di/dt loop. For additional points, the LM3485 evaluation board (AN-1227) serves as an excellent example of proper layout.

The most important objectives are to quantify and narrow the switching frequency variations that can occur due to component characteristics as well as board layout. Compared with previous methods of creating feedback ripple voltage, the emulated approach provides a much-improved method to initially set frequency, and removes the two largest variables (C_OUT ESR and inductance) from the equation. Better control of the frequency improves overall system cost and reliability. This occurs because you do not have to size the design for a relatively low frequency; therefore, the inductor does not require as high of a peak current rating, and it allows for the optimization of input and output capacitors.

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