HV100/HV101
Auto-Retry
Not only do the HV100 and HV101 provide short circuit pro-
tection in a 3-pin package, they also includes a 2.5s built in
auto-restart timer. The HV100 and 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.
All of the above are possible with a minimum number of ex-
ternal 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(PEAK)
= (C
FB
+ 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.
To increase undervoltage lockout simply connect a Zener
diode in series with the V
PP
pin.
ii)
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 applica-
tion transient spike, which is now too fast for the internal
clamping mechanism.
Calculating Inrush Current
As can be seen in the diagram below, for a standard pass
element, the HV100 and 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.
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 MOS-
FETs of a particular type the ratio of C
ISS
/ C
RSS
is relatively
constant (though notice from the plot that there is some varia-
tion) even while the absolute value of these and other quanti-
ties vary. Based on this, the inrush current will vary primarily
with C
LOAD
. This makes designing with the HV100 and HV101
particularly easy because once the pass element is chosen,
the period is fixed and the inrush varies with C
LOAD
only.
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 and HV101’s small footprint, it is possible
to create an open drain PWRGD signal using external com-
ponents 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 V
t
will also work.
Programming the HV100 and HV101
The HV100 and 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.
HV100
PWGRD
6
SUPERTEX [ Supertex, Inc ]
