L6995
18/25
R10 = 1K
Ω
R9 = 390
Ω
The device output voltage is adjustable by connecting a voltage divider from output to VSENSE pin. Minimum
outpu t voltage is V
OUT
= V
REF
= 0 . 9V. Once output divi der and frequency div ider have been desi gned as to ob-
tain the r equired output v oltage and sw itching frequency, the following equation gives the smallest input voltage,
which allows L6995 to regulate (which corresponds to T
OFF
= T
OFF, MIN
):
Eq 25
where the K
OSC
/T
OFFMIN
ratio worst-case is given in electrical characteristic table (pag. 4).
3.6 Voltage Feed Forward
Choosing the swi tching frequenc y around 270K Hz from the Eq1. It c an be select ed the input divider . For exam-
ple:
R3=560K
Ω
R4=28K
Ω
In order to compensate the comparator delay R4 resistor should be increased around 20%.
R4=33K
Ω
3.7 Current limit resi stor
From the Eq13 can be set the valley current limit, knowing the low side R DS
ON
. To set the exact current limit it
must be considered the temperature effect. So two STS25NH3LL have 2.75m
Ω
@ 25°C, at 100°C can be con-
sidered 3.85m
Ω
.
R8 = 47K
Ω
3.8 Integrator capacitor
Let it be F
U
= 15kHz.
Since V REF = 0.9V, from Eq4, it fol lows
α
OUT
= 0.72 and, from Eq5 i t fol lows C
INT 1
= 330pF. Because the ri pple
is lower than 150mV the system doesn't need the second integrator capacitor.
3.9 Soft start capacitor
Considering the soft start equations can be found:
C
SS
= 200pF
These equations are valid whitout load. When an active load is pr esent the equantions result more complex;
further some active loads have unexpected effect, as higher current than the expected one during the start up,
that can change the start up time.
In this case the capacitor value can be selected on the application; anyway the E q11 gives an idea about the
C
SS
value.
δ 1
α
OSC
α
OUT
--------------
1
K
OS C
T
OFFMIN
------------------------
------------------------------
⋅–<
