L7987L datasheet(22/41 Pages) STMICROELECTRONICS

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Application information

L7987L

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DocID026362 Rev 3

The output voltage has a drop or overshoot that depends on the ESR and capacitive

charge/discharge, as roughly estimated in Equation 11:

Equation 11

where

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

Equation 12

MLCC capacitors have typically low ESR to minimize the ripple but also have 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 L7987L and external compensation feature let

design a wide range of output filter configurations (including all MLCC solutions) and

perform fast transient response.

5.3

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 Equation 13:

Equation 13

In order to guarantee a maximum current ripple in every condition, Equation 13 must be

evaluated in case of maximum input voltage, assuming VOUT fixed.

Increasing the value of the inductance help to 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.

V

OUT LT

–

I

OUT

RES OUT



I

OUT

I

OUT



2COUT VL



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





V

DMAX VIN VOUT

–





VOUT













VOUT

VIN

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

–







L

FSW



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

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