HV100/HV101
Application Information, cont’d.
Auto-Retry
Not only does the HV100/HV101 provide short circuit protection
in a 3-pin package, it also includes a 2.5s built in auto-restart
timer. The HV100/HV101 will continuously try to turn on the
system every 2.5s, providing sufficient time for the pass element
to cool down after each attempt.
Programming the HV100/HV101
The HV100/HV101 require no external components other than a
pass element to provide the functionality described thus far. In
some applications it may be useful to use external components
to adjust the maximum allowable inrush current, adjust UVLO, or
to provide additional gate clamping if the supply rails have rise
times below 1ms.
All of the above are possible with a minimum number of external
components.
i)
To adjust inrush current with an external component simply
connect a capacitor (C
FB
) from drain to gate of the MOSFET.
The inrush calculation then becomes:
I
INRUSH
= (C
ISS
/ (C
RSS
+ C
FB
)) * 2.5e3 * C
LOAD
Note that a resistor (approximately 10KΩ) needs to be
added in series with C
FB
to create a zero in the feedback loop
and limit the spurious turn on which is now enhanced by the
larger divider element.
ii)
To increase undervoltage lockout simply connect a Zener
diode in series with the V
PP
pin.
2A/div
Calculating Inrush Current
As can be seen in the diagram below, for a standard pass
element, the HV100/HV101 will normalize the hotswap time
period against load capacitance. For this reason the current limit
will increase with increasing value of the load capacitance.
iii) If the V
PP
rises particularly fast (>48e6V/s) then it may be
desirable to connect a capacitor from gate to source of the
MOSFET to provide a path for the power application tran-
sient spike, which is now too fast for the internal clamping
mechanism.
iv) To limit the peak current during a short circuit, a resistor in
series with the source of the MOSFET may help.
Implementing PWRGD Control
Due to the HV100/HV101’s small footprint, it is possible to create
an open drain PWRGD signal using external components and
still maintain a size comparable with the smallest hotswap
controllers available elsewhere. To accomplish this an external
MOSFET may be used in conjunction with the gate output.
Simply use a high impedance divider (10MΩ) sized so that the
open drain PWRGD MOSFET threshold will only be reached
once the HV100/HV101’s gate voltage rises well above the
current limit value required by the external MOSFET pass
device. Alternatively a Zener diode between the gate output and
the PWRGD MOSFET gate set at a voltage higher than the
maximum pass element Vt will also work.
Inrush can be calculated from the following formula:
I
INRUSH(PEAK)
= (C
ISS
/ C
RSS
) * 2.5e3 * C
LOAD
This is a surprisingly consistent result because for most MOSFETs
of a particular type the ratio of C
ISS
/ C
RSS
is relatively constant
(though notice from the plot that there is some variation) even
while the absolute value of these and other quantities vary.
Based on this, the inrush current will vary primarily with C
LOAD
.
This makes designing with the HV100/HV101 particularly easy
because once the pass element is chosen, the period is fixed and
the inrush varies with C
LOAD
only.
HV100
PWGRD
08/26/02 rev.3b
©2002 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited.
5
1235 Bordeaux Drive, Sunnyvale, CA 94089
TEL: (408) 744-0100 • FAX: (408) 222-4895
www.supertex.com
SUPERTEX [ Supertex, Inc ]
