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2.3.5
Softstart time, OLP blanking time and auto-restart time
According to the definition of the softstart of the IC in the datasheet, soft start is implemented by
sweeping the operating frequency from an initial high value until the control loop takes over. The softstart
time depends on a few components, such as the
20ms target rising time of the output voltage, the customer can start with
The Timer pin is used to set the blanking time
RC parallel circuit,
C
T
OLP blanking time with
=
−
20
*
T
ms
R
C
OLP
T
The restart time can be calculated as:
=
−
*
*
ln(
T
R
C
restart
T
T
2.3.6
Load pin setting
One of the functions of the LOAD pin is to detect the over-load or open-loop faults. Once the voltage at this pin
is higher than 1.8V, IC will start internal and external timer and determine the entering of protection mode. The
resistor divider
and
R
1
FT
bottom resistor
R
connected to GND pin should be far bigger than the
2
FT
regulation. As an example, assuming
overload happens. The reference voltage at frequency pin is 2V. Then the voltage at LOAD pin
R
=
2
*
FT
V
+
LOAD
R
R
1
2
FT
FT
=
0
2 .
We can find
R
1
FT
pin.
2.3.7
Current sense
Application Note
and
, are connected to this pin. Based on the definition in the datasheet, the
R
T
= M
Ω
=
1
and
R
C
T
T
V
−
=
*
ln(
1
)
TH
T
*
R
I
T
BL
V
−
=
−
6
6
)
10
*
10
*
TL
V
TH
should be designed properly to ensure OLP is functional as required. The
R
2
FT
= M
2
R
2
FT
=
2
. 1
82
V
Ω
. A small capacitor of 1nF is usually connected to decouple noise at LOAD
M
, the value of
R
min
F
and restart time
T
OLP
1
can be calculated as:
uF
−
−
6
6
20
1000
*
10
*
10
. 0
525
=
ln(
* )
1000
2030
4
Ω
, the target voltage at Load pin is 1.82V when
13
and the value of
R
ocp
=
2
2 .
.
C
uF
SS
for over load protection. The
T
restart
4
−
*
ln(
1
−
6
6
10
*
20
*
10
ms
, in order not to affect normal
R
FMIN
. For a
C
SS
=
)
240
ms
[20]
2011-07-06
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