19/30
L6997S
6.9 Soft start capacitor
Considering the soft start equations (Eq. 11) at page 10, it can be found:
C
SS
= 150pF
The equations are valid without load. When an active load is present the equations result more complex; further
some active loads have unexpected effect, as higher current than the expected one during the soft start, can
change the start up time.
In this case the capacitor value can be selected on the application; anyway the Eq11 gives an idea about the
C
SS
value.
6.10 Sink mode
Figure 16. Efficiency vs output current
7 15A DEMO BOARD DESCRIPTION
The evaluation board shows the device operation in these conditions: V
IN
= 3.3V V
OUT
= 1.8V I
OUT
= 15A, F
SW
= 200KHz without the integrator feature. The evaluation board has two different input voltages: V
CC
[from 3V to
5.5V] used to supply the device and the V
IN
[up to 35V] for the power conversion. In this way, changing the pow-
er components configuration (C
IN
, C
OUT
, MOSFETs, L) it is possible evaluate the device performance in differ-
ent conditions. It is also possible to mount a linear regulator on board used to generate the V
CC
. On the top side
are also present two switches and four jumpers. The two switches have different goals: the one nearest to the
V
CC
is used to turn on/off the device when the V
CC
and V
IN
are both present; the other one, near to R11 is used
to turn on/off the PFM feature. The device can be turned on also with the power supply, but a correct start up
sequence is mandatory. V
IN
has to be raised first and then the V
CC
can be applied too. If the correct sequence
is not respected the device will not start up. The jumpers are used to set the integrator feature and to use the
remote sensing; for more information refers to the Jumpers table. Sometimes when using the integrator config-
uration a low frequency filter is required in order to reduce the noise interaction. The pole value should be at
least five times higher than the switching frequency. The low pass filter should be inserted in this way: the re-
sistor, in the place of the INT jumper position and the capacitor between the resistor and ground (refers to the
schematic).
0,0
10,0
20,0
30,0
40,0
50,0
60,0
70,0
80,0
90,0
100,0
0,01,02,03,04,05,0
Current [A]
Eff [%]
