HV302 / HV312
Design Information-
continued
Programming Current Limit and Circuit Breaker
Feedback Capacitor Mode Operation
Calculate C2 (feedback capacitor) discharge current
I
C2
=
10
µ
A
−
I
SINK
=
10
µ
A
−
2.5
µ
A
=
7.5
µ
A
If Auto-Retry is disabled an adjustment must be made to I
C2
I
AUTO
=
V
t
R
DISABLE
=
4V
=
1.6
µ
A
2.5M
Ω
In this operating mode the circuit breaker trip current and the
inrush current limit can be independently programmed. In fact the
circuit breaker can be completely disabled by setting R
SENSE
= 0Ω.
The circuit breaker will trip in less than 5µs when the voltage on
the SENSE pin is raised 100mV above the V
EE
pin and the value of
the sense resistor may be calculated from the following equation:
R
SENSE
=
V
SENSE-CB
100mV
=
I
CB
I
CB
Where Vt is the maximum threshold voltage of the MOSFET.
Therefore, the adjusted value of I
C2
is:
I
C2
=
10
µ
A
−
I
SINK
−
I
AUTO
I
C2
=
10
µ
A
−
2.5
µ
A
−
1.6
µ
A
=
5.9
µ
A
In this example we assume that Auto-Retry is enabled and
therefore use I
C2
= 7.5µA.
dv
dv
and
I
CL
=
C
LOAD
×
dt
dt
Since V
IN
is fixed and V
RAMP
is constant during limiting, then
dv
dv
across C
LOAD
=
across C2 as they share a common node
dt
dt
and their other terminals are at fixed voltages during inrush current
I
I
I
×
C
LOAD
limiting. Therefore,
C2
=
CL
or C2
=
C2
.
C2 C
LOAD
I
CL
As previously calculated and by conservation of charge on RAMP
node I
C2
=7.5µA based on the chosen inrush current limit of I
CL
=1A.
Given that C
LOAD
=100µF the required value for C2 can be
calculated.
Note that
I
C2
=
C2
×
Therefore
C2
=
I
C2
×
C
LOAD
7.5
µ
A
×
100
µ
F
=
=
0.75nF
I
CL
1A
For an 8A circuit breaker:
R
SENSE
=
100mV
=
12.5mΩ
8A
The power rating of the sense resistor must be greater than or
equal to I
CB
x V
SENSE-CB
.
The following diagrams depict the equivalent circuitry to clarify the
feedback capacitor operation for programming the inrush current
limit.
Note that during initial power application the RAMP pin is voltage
protected by the capacitive AC voltage divider consisting of C
LOAD
,
C2 and C
RAMP
and the GATE pin is internally clamped.
Servo Control Mode Operation
The circuit breaker will trip in less than 5µs when the voltage on
the SENSE pin is raised 100mV above the V
EE
pin and the value of
the sense resistor may be calculated from the following equation:
R
SENSE
=
V
SENSE-CB
100mV
=
I
CB
I
CB
The inrush current limit may be programmed as follows:
Choose inrush current limit, for example I
CL
= 1A
Calculate
I
SINK
I
×
R
SENSE
1A
×
12.5m
Ω
=
2.5
µ
A
=
=
CL
5k
Ω
5k
Ω
For a 2A circuit breaker:
R
SENSE
=
100mV
=
50mΩ
2A
If the Circuit Breaker function is disabled by setting R
SENSE
= 0Ω,
then I
SINK
= 0A. However, in this example we assume that the
Circuit Breaker function is enabled and therefore use I
SINK
= 2.5µA.
The power rating of the sense resistor must be greater than or
equal to I
CB
x V
SENSE-CB
.
7
Rev. D
04/17/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com
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