6

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.

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

This is a surprisingly consistent result because for most MOS-

FETs of a particular type the ratio of C

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

particularly easy because once the pass element is chosen,

the period is fixed and the inrush varies with C

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.

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

MOSFET. The inrush calculation then becomes:

I

Note that a resistor (approximately 10KΩ) needs to be

added in series with C

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

iii) If the V

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.

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

PWGRD

HV100