A7986ATR datasheet(18/42 Pages) STMICROELECTRONICS

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Application information

A7986A

18/42

DocID022801 Rev 5

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. Section 6.4 illustrates how to consider its effect in the system

stability.

For example, with VOUT = 5 V, VIN = 24 V, ΔIL = 0.9 A (resulting by the inductor value), in

order to have a

ΔVOUT = 0.01·VOUT, if the multi-layer ceramic capacitor is adopted, 10 µF

are needed and the ESR effect on the output voltage ripple can be neglected. In the case of

not negligible ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into

account its ESR value. So, in the case of 330 µF with ESR = 730 m

Ω, the resistive

component of the drop dominates and the voltage ripple is 28 mV

The output capacitor is also important to sustain the output voltage when a load transient

with high slew rate is required by the load. When the load transient slew rate exceeds the

system bandwidth, the output capacitor provides the current to the load. So, if the high slew

rate load transient is required by the application, the output capacitor and system bandwidth

must be chosen in order to sustain the load transient.

In Table 8 below, some capacitor series are listed.

6.4

Compensation network

The compensation network must assure stability and good dynamic performance. The loop

of the A7986A is based on the voltage mode control. The error amplifier is a voltage

operational amplifier with high bandwidth. So by selecting the compensation network, the

EA is considered as ideal, that is, its bandwidth is much larger than the system one.

The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to

the OUT pin (see Figure 10) results:

Equation 16

where VS is the sawtooth amplitude. As seen in Section 5.1, the voltage feed-forward

generates a sawtooth amplitude directly proportional to the input voltage, that is:

Table 8. Output capacitors

Manufacturer

Series

Cap value (

μF)

Rated voltage (V)

ESR (m

Ω)

MURATA

GRM32

22 to 100

6.3 to 25

< 5

GRM31

10 to 47

6.3 to 25

< 5

PANASONIC

ECJ

10 to 22

6.3

< 5

EEFCD

10 to 68

6.3

15 to 55

SANYO

TPA/B/C

100 to 470

4 to 16

40 to 80

TDK

C3225

22 to 100

6.3

< 5

PW0

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