The output capacitor is 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. The output

voltage has a drop or overshoot that depends on the ESR and capacitive charge/discharge, as roughly estimated

in Eq. (11)

ΔVOUT_LT≅ΔIOUT⋅RES,OUT+ΔIOUT⋅ L⋅ΔIOUT

2⋅COUT⋅ΔVL

(11)

where ∆VL is the voltage applied to the inductor during the load appliance or load release.

ΔVL= VIN−VOUT

VOUT

(12)

MLCC capacitors have a typically low ESR to minimize the ripple but also have a low capacitance that does not

minimize the voltage deviation during dynamic load variations.

Electrolytic capacitors, on the other hand, have a large capacitance which minimizes voltage deviation during load

transients whereas they do not show the same ESR values as the MLCCs, resulting then in higher ripple

voltages.

A mix between an electrolytic and MLCC capacitor can be used to minimize ripple as well as reducing voltage

deviation in dynamic mode.

The high bandwidth error amplifier of the A7987 and the external compensation feature let design a wide range of

output filter configurations (including all MLCC solutions) and perform a fast transient response.

6.3

Inductor selection

The inductance value fixes the current ripple flowing through the output capacitor. So the minimum inductance

value, in order to have the expected current ripple, must be selected.

The rule to fix the current ripple value is to have a ripple at 20%-40% of the output current.

In the continuous conduction mode (CCM), the required inductance value can be calculated by the following

equation:

L=VOUT⋅ 1−VOUTVIN

ΔIL⋅FSW

(13)

In order to guarantee a maximum current ripple in every condition, Eq. (13) must be evaluated in case of

maximum input voltage, assuming VOUT fixed.

Increasing the value of the inductance helps reduce the current ripple but, at the same time, strongly impacts the

converter response time to a dynamic load change. The response time is the time required by the inductor to

change its current from the initial to the final value. Until the inductor has finished its charging (or discharging)

time, the output current is supplied (or recovered) by the output capacitors.

Further, if the compensation network is properly designed, during a load variation the device is able to properly

change the duty cycle so improving the control loop transient response. When this condition is reached the

response time is only limited by the time required to change the inductor current, basically by VIN, VOUT and L.

Minimizing the response time, at the end, can help to decrease the output filter total cost and to reduce the

application area.

6.4

Compensation network

The compensation network must assure stability and good dynamic performance. The loop of the A7987 is based

on the voltage mode control. The error amplifier is an operational amplifier with a high bandwidth. So, by selecting

the compensation network the E/A is considered as ideal, that is, its bandwidth is much larger than the system

one.

A7987

Inductor selection

DS12928 - Rev 3

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