PI2061 Datasheet, PDF (11/16 Pages) Vicor Corporation – High Side High Voltage Load Disconnect Switch Controller IC

Not Recommended for New Designs

N-Channel MOSFET Selection:
Several factors affect MOSFET selection including cost and
following ratings; on-state resistance (R_DS(on)), DC current,
short pulse current, avalanche, power dissipation, thermal
conductivity, drain-to-source breakdown voltage (BV_DSS),
gate-to-source voltage (V_GS), and gate threshold voltage
(V_{GS (TH)}).
The first step is to select a suitable MOSFET based on the
BV_DSS requirement for the application. The BV_DSS voltage
rating should be higher than the applied Vin voltage plus
expected transient voltages. Stray parasitic inductance in
the circuit can also contribute to significant transient
voltage condition, particularly during MOSFET turn-off
after an over current fault has been detected.
In a disconnect switch application when the output is
shorted, a large current is sourced from the power source
through the MOSFET. Depending on the input impedance
of the system, the current may get very high before the
MOSFET is turned off. Make sure that the MOSFET pulse
current capability can withstand the peak current. Also,
such high current conditions will store energy even in a
small parasitic inductance. The PI2061 has a very fast
response time to terminate a fault condition achieving
120ns typical and 200ns maximum. This fast response time
will minimize the peak current to keep stored energy and
MOSFET avalanche energy very low to avoid damage
(electrical stress) to the MOSFET.
Peak current during output short is calculated as follows,
assuming that the input power source has very low
impedance and it is not a limiting factor:
Where:
:
Peak current in the MOSFET right before
it is turned off.
:
Input voltage at MOSFET drain before
output short condition occurred.
: Over current turn-off time. This will
include PI2061 delay and the MOSFET
turn off time.
: Circuit parasitic inductance
The MOSFET avalanche energy during an input short is
calculated as follows:
Where:
:
:
Avalanche energy
MOSFET breakdown voltage
MOSFET R_DS(on) and maximum steady state power
dissipation are closely related. Generally the lower the
MOSFET R_DS(on), the higher the current capability and the
lower the resultant power dissipation for a given current.
This leads to reduced thermal management overhead, but
will ultimately be higher cost compared to higher R_DS(on)
parts. It is important to understand the primary design goal
objectives for the application in order to effectively trade
off the performance of one MOSFET versus another.
Power dissipation in load switch circuits is derived from the
total drain current and the on-state resistance of the
selected MOSFET.
MOSFET power dissipation:
Where :
:
:
MOSFET Drain Current
MOSFET on-state resistance
Note:
In the calculation use R_DS(on) at maximum MOSFET
temperature because R_DS(on) is temperature dependent.
Refer to the normalized R_DS(on) curves in the MOSFET
manufacturer’s datasheet. Some MOSFET R_DS(on) values
may increase by 50% at 125°C compared to values at 25°C.
The Junction Temperature rise is a function of power
dissipation and thermal resistance.
Where:
:
MOSFET Junction-to-Ambient thermal
resistance
Picor Corporation • picorpower.com
PI2061
Rev 1.4
Page 11 of 16