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L6995
Eq 20
Considering a switching frequency around 270kHz from the equation above the ripple current is around 7A.
So the maximum ESR should be:
Eq 21
The dynamic specifications are somet imes mor e relax ed than the static requir ements so the ESR value ar ound
7m
Ω
should be enough.
The cur rent ripple fl ows through the output ca pacitor , so the output capa citors s hould be c alculated also to sus-
tain this ripple: the RMS current value is given from Eq22.
Eq 22
But this is usually a negligible constrain when choosing output capacitor.
To allow the device control loop to work properly output capacitor zero should be at the least ten times smaller
than swi tching freq uency. The output capac itor val ue (C
OUT
) and the output capac itor ESR ( ESR
OUT
) shoul d be
large enough and small enough, to keep the output voltage ripple within the specification and to give to the de-
vice a minimum signal to noise ratio.
3.4 Pow er MOS FETS and Schottky Diodes
Sinc e a 5V bus powers the gate drivers of the device, the use of lo gic-level MOS FETS is highly rec ommended,
especially for high cur rent applications. The breakdown voltage VBR
DSS
must be greater than VIN
MAX
with a
certain margin, so the selection will address 20V or 30V devices.
The RDS
ON
can be selected once the allowable power dissipation has been established. By selecting identical
Power MOSFET for the main switch and the synchronous rectifier, the total power they dissipate does not de-
pend on the duty cycle. Thus, if P
ON
is this power loss (few percent of the rated output power), the required
RDS
ON
(@ 25 °C) can be derived from:
Eq 23
α
is the temperature coefficient of RDS
ON
(typically,
α
= 510
-3
°C
-1
for these low-voltage classes) and
∆
T the
admitted temperature rise. It is worth noticing, however, that generally the lower RDS
ON
, the higher is the gate
charge Q
G
, which leads to a higher gate drive consumption. In fact, each switching cycle, a charge Q
G
moves
from the input source to ground, resulting in an equivalent driv e current:
Eq 24
The SCHOTTY diode placed i n parallel to the s ynchronous re ctifier mus t have a r everse v oltage VRRM greater
than VIN
MAX
. For application with low Duty Cycle, where the input vol tage is high (ar ound 20V) it is very impor-
tant to selec t the high si de MOSFET with low gate ch arge, to reduce the sw itchi ng loss es as STS 11NF3LL. For
the low side section should be selected a low RDS
ON
as STS25NH3LL.
3.5 Output voltage setting
To select the output div ider network there isn't a spec ific cri teri a, but a low divider network value ( around 100
Ω
)
reduces the efficiency at low current; instead a high value divider network (500K
Ω
) increase the noise effects.
A network divider values from 1K to 50K is right. From the Eq4:
I∆
Vin Vo–
L
---------------------- -
Vo
Vin
---------
T
sw
⋅⋅=
ESR
V
ripple
∆
I∆
2
-----
--------------------- 7mΩ==
Icout
rms
1
23
-----------
I
L
∆=
RDS
ON
P
ON
Iout
2
1 α T∆⋅+()⋅
----------------------------------------------------- -=
Iq Qg F
SW
⋅=
