LTC4251 LINER [Linear Technology], LTC4251 Datasheet - Page 13

• Manufacturer Part Number: LTC4251
• Description: Negative Voltage Hot Swap Controllers in SOT-23
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
capacitors. If LTC4251, LTC4251-1 or LTC4251-2s are
used throughout, they respond by limiting the inrush cur-
rent to a value of 100mV/R
capacitors will recharge long before C
MOSFET SELECTION
The external MOSFET switch must have adequate safe
operating area (SOA) to charge the load capacitance on
start-up and handle short-circuit conditions until TIMER
latchoff. These considerations take precedence over DC
current ratings. A MOSFET with adequate SOA for a given
application can always handle the required current, but the
opposite cannot be said. Consult the manufacturer’s MOSFET
data sheet for safe operating area and effective transient
thermal impedance curves.
MOSFET selection is a three-step process. First, R
calculated, and then the time required to charge the load
capacitance is determined. This timing, along with the
maximum short-circuit current and maximum input voltage
defines an operating point that is checked against the
MOSFET’s SOA curve.
To begin a design, first specify the required load current and
load capacitance, I
point (50mV/R
mum load current. Note that maximum input current to a
DC/DC converter is expected at V
by:
R
S
200mV/DIV
=
–48RTN
50V/DIV
10V/DIV
I
5V/DIV
SENSE
40
L MAX
TIMER
GATE
(
Figure 4. Output Short-Circuit Behavior
(All Waveforms are Referenced to V
mV
S
)
) should be set to accommodate the maxi-
L
U
and C
ANALOG
CURRENT LIMIT
FAST CURRENT
LIMIT
SUPPLY RING
OWING
TO CURRENT
OVERSHOOT
ONSET OF OUTPUT
SHORT-CIRCUIT
C
TIMER
L
U
. The circuit breaker current trip
2ms/DIV
RAMP
S
. If C
SUPPLY (MIN)
T
SUPPLY RING
OWING TO
MOSFET
TURN-OFF
LATCH OFF
W
is sized correctly, the
T
times out.
425112 F04
EE)
TRACE 1
TRACE 2
TRACE 3
TRACE 4
. R
U
S
is given
(3)
S
is
where 40mV represents the guaranteed minimum circuit
breaker threshold.
During the initial charging process, the LTC4251/
LTC4251-1/LTC4251-2 may operate the MOSFET in cur-
rent limit, forcing 80mV to 120mV across R
mum inrush current is given by:
Maximum short-circuit current limit is calculated using
maximum V
The TIMER capacitor C
slowest expected charging rate; otherwise TIMER might
time out before the load capacitor is fully charged. A value
for C
load capacitor to charge. That time is given by:
Substituting Equation (4) for I
(6) with (2) gives:
Returning to Equation (2), the TIMER period is calculated
and used in conjunction with V
I
tive MOSFET.
As a numerical design example, consider a 30W load,
which requires 1A input current at 36V. If V
72V and C
Equation (7) gives C
R
(4V), the calculated value should be multiplied by 1.5,
giving a nearest standard value of C
If a short-circuit occurs, a current of up to 120mV/40mΩ
= 3A will flow in the MOSFET for 5.7ms as dictated by
SHORT-CIRCUIT(MAX)
SENSE
I
I
t
C
SHORT CIRCUIT MAX
INRUSH MIN
CL CHARGE
T
T
is calculated based on the maximum time it takes the
=
, C
C
T
−
L
(
, TIMER current (230µA) and TIMER threshold
L
= 100µF, Equation (3) gives R
SENSE
•
V
LTC4251/LTC4251-1/
)
=
SUPPLY MAX
=
C V
80
, or:
(
R
(
•
I
4
to check the SOA curves of a prospec-
mV
T
S
V
(
= 207nF. To account for errors in
=
)
•
T
80
=
must be selected based on the
C
120
L
)
mV
I
•
•
INRUSH MIN
R
R
V
INRUSH(MIN)
)
mV
S
S
SUPPLY MAX
•
230
LTC4251-2
(
T
= 330nF.
SUPPLY(MAX)
µ
(
A
)
SENSE
and equating
SUPPLY(MAX)
)
S
. The mini-
= 40mΩ;
13
(4)
(5)
(6)
(7)
425112fa
and
=