Data Sheet

ADPL42010A

analog.com

Rev. 0

17 of 28

For ease of use, low ESR polytantalum capacitors (POSCAP) are a good choice for both the transient response and

stability of the regulator. These capacitors have intrinsic ESR that improves the stability. Ceramic capacitors have

extremely low ESR, and while they are a good choice in many cases, placing a small series resistance element

sometimes achieves optimum stability and minimize ringing. In all cases, a minimum of 10µF is required while the

maximum 3Ω ESR is allowed.

The place where ESR is most helpful with ceramics is at low output voltages. At low output voltages, below 2.5V,

some ESR prevents oscillation when ceramic output capacitors are used. Also, some ESR allows a smaller capacitor

value to be used. When small signal ringing occurs with ceramics due to insufficient ESR, adding ESR or increasing

the capacitor value improves the stability and reduces the ringing. Table 4 gives the minimum values of ESR to

prevent oscillation during load transient testing. Additional ESR or capacitance are necessary for adequate margin.

Table 4. Capacitor Minimum ESR

VOUT

10µF

22µF

47µF

100µF

1.2V

20mΩ

15mΩ

10mΩ

5mΩ

1.5V

20mΩ

15mΩ

10mΩ

5mΩ

1.8V

15mΩ

10mΩ

10mΩ

5mΩ

2.5V

5mΩ

5mΩ

5mΩ

5mΩ

3.3V

0mΩ

0mΩ

0mΩ

5mΩ

≥5V

0mΩ

0mΩ

0mΩ

0mΩ

Figure 28 to Figure 30 show the effect of ESR on the transient response of the regulator. These scope captures show

the transient response for the ADPL42010A at a worst-case condition when the output voltage is 1.2V. As Table 4

shows, higher output voltages require proportionally less ESR and output capacitance for stability. The output load

conditions are the same for Figure 28 to Figure 30. In all cases, there is a DC load of 500mA. The load steps up to 1A at

the first transition and steps back to 500mA at the second transition. Load transient testing on actual hardware is a

good way to ensure circuit stability, especially if the output voltage is 5V or less or the output network is inductive

due to filtering or lead length.

In Figure 28, a minimum ESR of 50mΩ is required for adequate stability when COUT is 10µF. In Figure 29, a minimum

ESR of 10mΩ is required for adequate stability when COUT is 100µF. Capacitors with added ESR can be combined with

low ESR ceramic capacitors to achieve both good high frequency bypassing and fast settling time. Figure 30 shows

the improvement in transient response that can be seen when a parallel combination of a 10µF ceramic and a 100µF

capacitor in series with 40mΩ are used.