HV302 / HV312
Design Information- continued
Calculate C2 (feedback capacitor) discharge current
Programming Current Limit and Circuit Breaker
IC2 = 10µA −ISINK = 10µA − 2.5µA = 7.5µA
Feedback Capacitor Mode Operation
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 RSENSE = 0Ω.
If Auto-Retry is disabled an adjustment must be made to IC2
Vt
RDISABLE 2.5MΩ
4V
IAUTO
=
=
= 1.6µA
The circuit breaker will trip in less than 5µs when the voltage on
the SENSE pin is raised 100mV above the VEE pin and the value of
the sense resistor may be calculated from the following equation:
Where Vt is the maximum threshold voltage of the MOSFET.
Therefore, the adjusted value of IC2 is:
VSENSE-CB
=
100mV
ICB
RSENSE
=
ICB
IC2 = 10µA −ISINK −IAUTO
For an 8A circuit breaker:
100mV
IC2 = 10µA − 2.5µA −1.6µA = 5.9µA
In this example we assume that Auto-Retry is enabled and
RSENSE
=
= 12.5mΩ
8A
therefore use IC2 = 7.5µA.
The power rating of the sense resistor must be greater than or
dv
dt
dv
dt
equal to ICB x VSENSE-CB
.
Note that IC2 = C2×
and
ICL = CLOAD
×
Since VIN is fixed and VRAMP is constant during limiting, then
The following diagrams depict the equivalent circuitry to clarify the
feedback capacitor operation for programming the inrush current
limit.
dv
dt
dv
dt
across CLOAD
=
across C2 as they share a common node
and their other terminals are at fixed voltages during inrush current
IC2
ICL
I
C2 × CLOAD
limiting. Therefore,
=
or C2 =
.
C2 CLOAD
ICL
As previously calculated and by conservation of charge on RAMP
node IC2=7.5µA based on the chosen inrush current limit of ICL=1A.
Given that CLOAD=100µF the required value for C2 can be
calculated.
I
C2 × CLOAD 7.5µA ×100µF
Therefore C2 =
=
= 0.75nF
ICL
1A
Note that during initial power application the RAMP pin is voltage
protected by the capacitive AC voltage divider consisting of CLOAD
,
C2 and CRAMP 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 VEE pin and the value of
the sense resistor may be calculated from the following equation:
VSENSE-CB
=
100mV
ICB
RSENSE
=
ICB
The inrush current limit may be programmed as follows:
Choose inrush current limit, for example ICL = 1A
For a 2A circuit breaker:
100mV
2A
RSENSE
=
= 50mΩ
I
CL ×RSENSE 1A ×12.5mΩ
Calculate ISINK
=
=
= 2.5µA
5kΩ
5kΩ
The power rating of the sense resistor must be greater than or
equal to ICB x VSENSE-CB
.
If the Circuit Breaker function is disabled by setting RSENSE = 0Ω,
then ISINK = 0A. However, in this example we assume that the
Circuit Breaker function is enabled and therefore use ISINK = 2.5µA.
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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