HV302 / HV312
Design Information- continued
The inrush current limit can be calculated as follows:
The timing functions are defined by the following equations:
CRAMP
VSENSE-CL
RSENSE
50mV
RSENSE
ICL
=
=
tSTART = 2.4
IRAMP
Thus the inrush current limit for a 2A circuit breaker:
50mV
CRAMP
tTH = VGS(th)
IRAMP
tPOR = tSTART + tTH
ICL
=
= 1A
50mΩ
Compensation components from gate to source of the external
MOSFET may be required to reduce peaking of the inrush current.
CRAMP
IRAMP
0.9ILIMIT
tRISE
≈
fs
RSENSE
RFB
g
−
CLOAD
ILIMIT
tLIMIT ≈ V
IN
CRAMP
IRAMP
tPWRGD−A
=
(
V
− VGS(LIMIT) −1.2
)
INT
These equations assume that the load is purely capacitive and the
following definitions apply.
CRAMP is the external capacitor connected to the RAMP pin.
IRAMP is the output current from the RAMP pin, nominally
Compensation can be accomplished as follows:
1. Start with a 2nF capacitor from gate to source.
2. Increase capacitor value up to 10nF if needed.
3. If needed, add a 1kΩ resistor in series with the above
capacitor.
10µA, when the voltage drop on RSENSE resistor is zero.
VINT is the internally regulated supply voltage and can range
from 9V to 11V.
VGS(th) is the gate threshold voltage of the external pass
transistor and may be obtained from its datasheet.
Servo Mode Timing
VGS(limit) is the external pass transistor gate-source voltage
required to obtain the limit current. It is dependent on the
pass transistor’s characteristics and may be obtained from
the transfer characteristics on the transistor datasheet.
g
fs is the transconductance of the external pass transistor and
may be obtained from its datasheet.
RFB is the internal feedback resistor and is nominally 5KΩ.
ILIMIT is the load current when the voltage drop on RSENSE
resistor is 50mV.
These equations may be used to calculate the minimum value of
CRAMP for the most critical system performance characteristics.
For maximum contact bounce duration protection choose a value
for tPOR and use the following equation:
tPOR ×IRAMP
2.4 + VGS(limit)
CRAMP
=
If control of PWRGD active delay is the critical system parameter,
then choose a value for tPWRGD-A and use the following equation:
tPWRGD−A ×IRAMP
CRAMP
=
V
− VGS(limit) −1.2
INT
8
Rev. D
04/17/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com
SUPERTEX [ Supertex, Inc ]
