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HV101X

3-Pin Hotswap, Inrush Current Limiter Controllers (Negative Supply Rail)

Supertex Inc 

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.

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.

HV100/HV101

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 (CFB) from drain to gate of the MOSFET.

The inrush calculation then becomes:

I = (C / (C + C )) * 2.5e3 * C

INRUSH

ISS

RSS

FB

LOAD

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

added in series with CFB 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 VPP pin.

iii) If the VPP 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.

Inrush can be calculated from the following formula:

IINRUSH(PEAK) = (CISS / CRSS) * 2.5e3 * CLOAD

This is a surprisingly consistent result because for most MOSFETs

of a particular type the ratio of CISS / CRSS 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 CLOAD.

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 CLOAD only.

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.

HV100

PWGRD

©2002 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited.

5

08/26/02 rev.3b

1235 Bordeaux Drive, Sunnyvale, CA 94089

TEL: (408) 744-0100 • FAX: (408) 222-4895

www.supertex.com

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