7.3.1 Overvoltage Protection

The LM3478 has over voltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (pin 3). If at anytime the voltage at the feedback pin rises to V_FB+ V_OVP, OVP is triggered. See Electrical Characteristics section for limits on V_FB and V_OVP.

OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage will drop. The LM3478 will begin switching again when the feedback voltage reaches V_FB + (V_OVP - V_OVP(HYS)). See Electrical Characteristics for limits on V_OVP(HYS).

OVP can be triggered if the unregulated input voltage crosses 7.2 V, the output voltage will react as shown in Figure 21. The internal bias of the LM3478 comes from either the internal LDO as shown in the block diagram or the voltage at the Vin pin is used directly. At Vin voltages lower than 7.2 V the internal IC bias is the Vin voltage and at voltages above 7.2V the internal LDO of the LM3478 provides the bias. At the switch over threshold at 7.2 V a sudden small change in bias voltage is seen by all the internal blocks of the LM3478. The control voltage shifts because of the bias change, the PWM comparator tries to keep regulation. To the PWM comparator, the scenario is identical to a step change in the load current, so the response at the output voltage is the same as would be observed in a step load change. Hence, the output voltage overshoot here can also trigger OVP. The LM3478 will regulate in hysteretic mode for several cycles, or may not recover and simply stay in hysteretic mode until the load current drops or Vin is not crossing the 7.2 V threshold anymore. Note that the output is still regulated in hysteretic mode.

Depending on the requirements of the application, there is some influence one has over this effect. The threshold of 7.2 V can be shifted to higher voltages by adding a resistor in series with V_IN. In case V_IN is right at the threshold of 7.2 V, the threshold could cross over and over due to some slight ripple on V_IN. To minimize the effect on the output voltage one can filter the V_IN pin with an RC filter.

Figure 21. The Feedback Voltage Experiences an Oscillation if the Input Voltage crosses the 7.2-V Internal Bias Threshold

7.3.2 Slope Compensation Ramp

The LM3478 uses a current mode control scheme. The main advantages of current mode control are inherent cycle-by-cycle current limit for the switch and simpler control loop characteristics. It is also easy to parallel power stages using current mode control since current sharing is automatic. However, current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 22.

A small increase in the load current causes the switch current to increase by ΔI₀. The effect of this load change is ΔI₁.

The two solid waveforms shown are the waveforms compared at the internal pulse width modulator, used to generate the MOSFET drive signal. The top waveform with the slope S_e is the internally generated control waveform V_C. The bottom waveform with slopes S_n and S_f is the sensed inductor current waveform V_SEN.

Figure 22. Sub-Harmonic Oscillation for D>0.5 and Compensation Ramp to Avoid Sub-Harmonic Oscillation

Sub-harmonic Oscillation can be easily understood as a geometric problem. If the control signal does not have slope, the slope representing the inductor current ramps up until the control signal is reached and then slopes down again. If the duty cycle is above 50%, any perturbation will not converge but diverge from cycle to cycle and causes sub-harmonic oscillation.

It is apparent that the difference in the inductor current from one cycle to the next is a function of S_n, S_f and S_e as shown in Equation 1.

Equation 1.

Hence, if the quantity (S_f - S_e)/(S_n + S_e) is greater than 1, the inductor current diverges and sub-harmonic oscillation results. This counts for all current mode topologies. The LM3478 has some internal slope compensation V_SL which is enough for many applications above 50% duty cycle to avoid sub-harmonic oscillation .

For boost applications, the slopes S_e, S_f and S_n can be calculated with Equation 2, Equation 3, and Equation 4.

When S_e increases, then the factor that determines if sub-harmonic oscillation will occur decreases. When the duty cycle is greater than 50%, and the inductance becomes less, the factor increases.

For more flexibility, slope compensation can be increased by adding one external resistor, R_SL, in the I_SEN's path. Figure 23 shows the setup. The externally generated slope compensation is then added to the internal slope compensation of the LM3478. When using external slope compensation, the formula for S_e becomes:

A typical value for factor K is 40 µA.

The factor changes with switching frequency. Figure 24 is used to determine the factor K for individual applications and Equation 6 gives the factor K.

It is a good design practice to only add as much slope compensation as needed to avoid sub-harmonic oscillation. Additional slope compensation minimizes the influence of the sensed current in the control loop. With very large slope compensation the control loop characteristics are similar to a voltage mode regulator which compares the error voltage to a saw tooth waveform rather than the inductor current.

Figure 23. Adding External Slope Compensation

Figure 24. External Slope Compensation
ΔV_SL vs R_SL

7.3.3 Frequency Adjust/Shutdown

The switching frequency of the LM3478 can be adjusted between 100 kHz and 1 MHz using a single external resistor. This resistor must be connected between FA/SD pin and ground, as shown in Figure 25. To determine the value of the resistor required for a desired switching frequency, refer to Typical Characteristics or use Equation 7:

Figure 25. Frequency Adjust

The FA/SD pin also functions as a shutdown pin. If a high signal (>1.35 V) appears on the FA/SD pin, the LM3478 stops switching and goes into a low current mode. The total supply current of the IC reduces to less than 10 µA under these conditions. Figure 26 shows implementation of the shutdown function when operating in frequency adjust mode. In this mode a high signal for more than 30 us shuts down the IC. However, the voltage on the FA/SD pin should be always less than the absolute maximum of 7 V to avoid any damage to the device.

Figure 26. Shutdown Operation in Frequency Adjust Mode

7.3.4 Short-Circuit Protection

When the voltage across the sense resistor measured on the I_SEN pin exceeds 343 mV, short circuit current limit protection gets activated. A comparator inside the LM3478 reduces the switching frequency by a factor of 5 and maintains this condition until the short is removed. In normal operation the sensed current will trigger the power MOSFET to turn off. During the blanking interval the PWM comparator will not react to an over current so that this additional 343 mV current limit threshold is implemented to protect the device in a short circuit or severe overload condition.