A7985ATR datasheet(20/44 Pages) STMICROELECTRONICS

20 / 44 page

Application information

A7985A

20/44

DocID023128 Rev 5

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

the zero due to its ESR. In Section 6.4, how to consider its effect in the system stability is

illustrated.

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

order to have a

VOUT = 0.01 · VOUT, if the multi-layer ceramic capacitors are adopted,

10 µF are needed and the ESR effect on the output voltage ripple can be neglected. In 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 = 70 m

the resistive

component of the drop dominates and the voltage ripple is 43 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 A7985A 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 E/A is considered as ideal, that is, its bandwidth is much larger than the system

one.

The transfer functions of the PWM modulator and the output LC filter are studied (see

Figure 10). The transfer function of the PWM modulator, from the error amplifier output

(COMP pin) to the OUT pin, results:

Equation 17

where VS is the sawtooth amplitude. As seen in Section 5.1 on page 11, the voltage feed-

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

Equation 18

Table 8. Output capacitors

Manufacturer

Series

Cap value (

F)

Rated voltage (V)

ESR (m

)

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

GPW0

VIN

---------

KVIN



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