Datasheet

Application information L7986
18/45 DocID022450 Rev 7
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 V
OUT
= 5 V, V
IN
= 24 V, ΔI
L
= 0.9 A (resulting from the inductor value), in
order to have a ΔV
OUT
= 0.01·V
OUT
, 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 the case of
non-negligible ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into
account its ESR value. So, in case of 330 µF with ESR = 30 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 some capacitor series are listed.
6.4 Compensation network
The compensation network must assure stability and good dynamic performance. The loop
of the L7986 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 16
where V
S
is the sawtooth amplitude. As seen in Section 5.1 on page 9, the voltage feed-
forward generates a sawtooth amplitude directly proportional to the input voltage, that is:
Equation 17
Table 8. Output capacitors
Manufacturer Series Cap value (µF) Rated voltage (V) ESR (mΩ)
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
G
PW0
V
IN
V
s
---------=
V
S
KV
IN
⋅=