l6995 STMicroelectronics, l6995 Datasheet - Page 17

• Manufacturer Part Number: l6995
• Description: Step Down Controller For High Differential Input-output Conversion
• Manufacturer: STMicroelectronics

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 sometimes more relaxed than the static requirements so the ESR value around
7m should be enough.
The current ripple flows through the output capacitor, so the output capacitors should be calculated 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 switching frequency. The output capacitor value (C
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 Power MOSFETS and Schottky Diodes
Since a 5V bus powers the gate drivers of the device, the use of logic-level MOSFETS is highly recommended,
especially for high current applications. The breakdown voltage VBR
certain margin, so the selection will address 20V or 30V devices.
The RDS
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
RDS
Eq 23
admitted temperature rise. It is worth noticing, however, that generally the lower RDS
charge Q
from the input source to ground, resulting in an equivalent drive current:
Eq 24
The SCHOTTY diode placed in parallel to the synchronous rectifier must have a reverse voltage VRRM greater
than VIN
tant to select the high side MOSFET with low gate charge, to reduce the switching losses as STS11NF3LL. For
the low side section should be selected a low RDS
3.5 Output voltage setting
To select the output divider network there isn't a specific criteria, but a low divider network value (around 100 )
reduces the efficiency at low current; instead a high value divider network (500K ) increase the noise effects.
A network divider values from 1K to 50K is right. From the Eq4:
is the temperature coefficient of RDS
ON
(@ 25 ° C) can be derived from:
MAX
ON
G
ESR
Icout
RD S
Iq
, which leads to a higher gate drive consumption. In fact, each switching cycle, a charge Q
I
can be selected once the allowable power dissipation has been established. By selecting identical
. For application with low Duty Cycle, where the input voltage is high (around 20V) it is very impor-
=
=
O N
Vin Vo
---------------------- -
Qg
rms
=
-------------------- -
=
L
–
V
=
F
rippl e
---- -
----------------------------------------------------- -
Iou t
2
---------- - I
2 3
I
SW
1
2
-------- -
Vin
Vo
=
L
P
7 m
1
O N
+
ON
T
s w
is this power loss (few percent of the rated output power), the required
ON
T
(typically,
ON
as STS25NH3LL.
= 510
OUT
) and the output capacitor ESR (ESR
-3
°C
-1
for these low-voltage classes) and T the
DSS
must be greater than VIN
ON
, the higher is the gate
OUT
) should be
MAX
G
L6995
moves
with a
17/25