Application Note
13
2011-07-06
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
min
F
R
, the value of
ocp
R
and the value of
SS
C
. For a
20ms target rising time of the output voltage, the customer can start with
uF
C
SS
2
.
2
=
.
The Timer pin is used to set the blanking time
OLP
T
and restart time
restart
T
for over load protection. The
RC parallel circuit,
T
C
and
T
R
, are connected to this pin. Based on the definition in the datasheet, the
OLP blanking time with
Ω
=
M
R
T
1
and
uF
C
T
1
=
can be calculated as:
ms
I
R
V
C
R
ms
T
BL
T
TH
T
T
OLP
240
)
10
*
20
*
10
4
1
ln(
*
10
*
10
*
1000
20
)
*
1
ln(
*
*
20
6
6
6
6
=
−
−
=
−
−
=
−
−
The restart time can be calculated as:
ms
V
V
C
R
T
TH
TL
T
T
restart
2030
1000
*
)
4
525
.
0
ln(
*
10
*
10
)
ln(
*
*
6
6
=
−
=
−
=
−
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
1
FT
R
and
2
FT
R
should be designed properly to ensure OLP is functional as required. The
bottom resistor
2
FT
R
connected to GND pin should be far bigger than the
FMIN
R
, in order not to affect normal
regulation.
As an example, assuming
Ω
=
M
R
FT
2
2
, the
target voltage at Load pin is 1.82V when
overload happens. The reference voltage at frequency pin is 2V. Then the voltage at LOAD pin
V
R
R
R
V
FT
FT
FT
LOAD
82
.
1
2
*
2
1
2
=
+
=
[20]
We can find
Ω
=
M
R
FT
2
.
0
1
.
A small capacitor of 1nF is usually connected to decouple noise at LOAD
pin.
2.3.7 Current
sense