l6995 STMicroelectronics, l6995 Datasheet - Page 10

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

L6995
be supposed linear function of the time.
Eq 8
so the output voltage is given by:
Eq 9
calling V
Eq 10
the minimum C
Eq 11
1.5 Current limit
The current limit comparator senses the inductor current through the low side MOSFET RDS
pares this value with the ILIM pin voltage value. While the current is above the current limit value, the control
inhibits the one-shot start.
To properly set the current limit threshold, it should be noted that this is a valley current limit. Average current
depends on the inductor value, V
The average output current in current limit is given by:
Eq 12
Thus, to set the current threshold, choose RILIM according to the following equation:
Eq 13
Where K
output voltage meets the undervolatge threshold. The system is capable to sink current, but it has not a negative
current limit.
The system accuracy is function of the exactness of the resistance connected to ILIM pin and the low side MOS-
FET RDS
tem linearity.
Figure 6. Current limit schematic
10/25
out
C
I
I
ON
T
I
V
I
OUT
m ax valley
o ut
is the current limit factor (Typ 1 A). In current limit the system keeps the current constant until the
L
out
as the V
o ut
accuracy. Moreover the voltage on ILIM pin must range between 10mV and 2V to ensure the sys-
t,C
=T
CL
C
SS
t,C
SS
SS
uvp
=
value is given imposing this condition:
SS
out
I
=
=
m ax valley
=
final value, the output charging time can be estimated as:
---------------- -
R ds
--------------------------------------------------------------------------- -
=
R
R
-------------------------------------------------------------------------------------- -
ILim
V
ilim
Q t,C
------------------------ -
out
o n
C
/R
R
o ut
+
i lim
ds on
IN
S S
---- -
K
V
C
2
I
C
V
SS
ou t
/R
OUT
Positive current limit
d so n
=
R
K
------------------------------------------------------------------------------ -
ILIM
C
and switching frequency.
R
C
SS
V
ilim
C
SS
ou t
K
I
SS
/R
C
PGN
D
ds on
C
I
SS
SS
t
V
5µA
SS
K
Comparator
2
C
LS
Current
C
0.5
SS
I
SS
To inductor
To
logic
PHASE
2
t
2
ON
drop and com-