7

Rev.1.3

11 -

In this case, a third pole set by the compensation capacitor (C4) and the compensation resistor (R3) is used to

compensate the effect of the ESR zero on the loop gain. This pole is located at:

fp3 = 1 / (2π × C4 × R3)

The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The sys-

tem crossover frequency where the feedback loop has the unity gain is important. Lower crossover frequencies

result in slower line and load transient responses, while higher crossover frequencies could cause system insta-

bility. A good rule of thumb is to set the crossover frequency below one-tenth of the switching frequency.

To optimize the compensation components, the following procedure can be used.

1) Choose the compensation resistor (R3) to set the desired crossover frequency.

Determine the R3 value by the following equation:

R3 = [ (2π × Cout × fc) / (Gea × Gcs) ] × (Vout/Vfb) < [ (2π × Cout × 0.1 × fs) / (Gea × Gcs) ] × (Vout/Vfb)

Where fC is the desired crossover frequency which is typically below one tenth of the switching frequency.

2) Choose the compensation capacitor (C1) to achieve the desired phase margin. For applications with typical

inductor values, setting the compensation zero, fz1, below one-forth of the crossover frequency provides suffi-

cient phase margin. Determine the C1 value by the following equation:

C1 > 4 / (2π × R3 × fc)

Where R3 is the compensation resistor.

3) Determine if the second compensation capacitor (C4) is required. It is required if the ESR zero of the output

capacitor is located at less than half of the switching frequency, or the following relationship is valid:

1 / (2π × Cout × Resr) < fs/2

If this is the case, then add the second compensation capacitor (C4) to set the pole fP3 at the location of the

ESR zero. Determine the C4 value by the equation:

C4 = (Cout × Resr) / R3

8. External bootstrap diode

An external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external

BS diode are:

Vout = 5V or 3.3V, and duty cycle is high: D = Vout/Vin > 65%

In these cases, an external BS diode is recommended from the output of the voltage regulator to BS pin, as

shown in Figure 1.

SW

Cout

BS

Cbs

0.1 to 1µF

L

ELM613DA

External BS

diode IN4148

5V or 3.3V

◄

Figure 1. Add optional external bootstrap diode to enhance efficiency.

The recommended external BS diode is IN4148, and the BS capacitor is 0.1 to 1μF.