HV320
Pd * (Rθjc + Rθcs + Rθsa) + TA
≤
Tj_derated
Start Up Overload Protection
If there is an output overload or short circuit during start
up, the circuit breaker will trip when the voltage at the
sense pin reaches 100mV. The gate is clamped low
indefinitely until input power is cycled, the UV pin is pulsed
low (<1.16V), or the OV pin is pulsed high (>1.26V). See
Figure 2 on Page 5.
Circuit Breaker
The circuit breaker will trip in less than 5µs when the
voltage on the SENSE pin reaches a nominal 100mV. A
resistor in series with the SENSE pin and a capacitor
connected between the SENSE and V
EE
pins may be
added to delay the rate of voltage rise on the SENSE pin,
thus permitting a current overshoot and delaying Circuit
Breaker activation. See Figure 1 on Page 5.
Selection of External R_sense
As a design example, consider a 500W load of a –48V
rectifier: at a minimum regulation voltage of -42V, the
input current is 11.9A, assuming that the trip point is set
for 16A, the value of the sense resistor.
R_sense = 80mV / I
TRIP
= 0.08/16 = 0.005 Ohm
Where 80mV is the minimum circuit breaker trip level,
the maximum circuit breaker threshold is 120mV. This will
make the current trip level at 24A.
The power dissipation of the sense resistor is:
P
RSENSE
= (V
SENSE-
CB max)
/ R-sense = (0.12) / 0.005
= 2.88W
Two 0.01 Ohm, 2W, 2512 size SMT resistor may be used
in parallel. See Kelvin Connection to Sense Resistor.
Selection of External Pass Devices
The N-Channel may be selected based on maximum input
operating voltage, RDS, maximum operating load current
and peak short circuit current.
Continuing with the example, the lowest Rds(on)
International Rectifier N-Channel MOSFET at 100V Vdss
is 14 m-Ohm. The IRFB4710* (TO-220) may be used for
this application.
FET Power Dissipation = (I
CBmax
^2) x Rds(on) x K = 24^2 x
0.014 x 1.4 = 11.3W
The K=1.4 factor is increased Rdson with respect to
temperature rise. Assuming Rθjc = 0.74°C/W and Rθcs =
0.5°C/W and the maximum operating temperature being
55°C, then the needed heat sink thermal resistance can
be calculated per:
To Negative
Terminal of
Power Source
To Source
of MOSFET
2
2
11.3 * (0.74 + 0.5 +Rθsa) + 55°C
≤
150°C
Rθsa
≤
7°C
Kelvin Connection to Sense Resistor
Physical layout of the printed circuit board is critical for
correct current sensing. Ideally trace routing between the
current sense resistor and the V
EE
and SENSE pins should
be direct and as short as possible with zero current in the
sense traces. The use of Kelvin Connection from SENSE
pin and V
EE
pin to the respective ends of the current sense
resistor is recommended.
To
Vee
Pin
To
Sense
Pin
Sense Resistors
7
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