DocID025589 Rev 3

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L7987

Application information

37

Equation 9

negligible.

In addition to the above considerations, a ceramic capacitor with an appropriate voltage

rating and with a value 1

F or higher should always be placed across VIN and power

ground and across VCC and the IC GND pins, as close as possible to the L7987device. This

solution is necessary for spike filtering purposes.

5.2

Output capacitor selection

The output capacitor is very important in order to satisfy the output voltage ripple

requirement. Using a small inductor value is useful to reduce the size of the choke but

increases the current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is

required. Nevertheless, the ESR of the output capacitor introduces a zero in the open loop

gain, which helps to increase the phase margin of the system. If the zero goes to very high

frequency, typical drawback in case of ceramic output capacitor application, a type III

compensation network must be designed.

The current in the output capacitor has a triangular waveform which generates a voltage

ripple across it. This ripple is due to the capacitive component (charge and discharge of the

output capacitor) and the resistive component (due to the voltage drop across its ESR). So

the output capacitor must be selected in order to have a voltage ripple compliant with the

application requirements.

The amount of the voltage ripple can be estimated starting from the current ripple obtained

by the inductor selection. Assuming

is roughly overestimated by Equation 10.

Equation 10

Usually the resistive component of the ripple is much higher than the capacitive one, if the

output capacitor adopted is not a multi-layer ceramic capacitor (MLCC) with very low ESR

value.

The output capacitor is important also for loop stability: it fixes the double LC filter pole and

the zero due to its ESR.

The output capacitor is also the key component that provides the current to the load during

a load transient which exceeds the system bandwidth. So, if the high slew rate load

transient is required by the application, the output capacitor must be designed in order to

sustain the load transient or absorbs the energy stored in the inductor until the converter

reacts.

In fact, even if the controller detects immediately the load variation and sets the duty cycle at

100% or 0%, the output current slope is limited by the inductor value, the input and output

voltage.



D1 D

–

 I

OUT











+

=

V

OUT PP



I

L



I

L



OUT



-----------------------------------------

+



