LTC4253AIGN#TR Linear Technology, LTC4253AIGN#TR Datasheet - Page 19

• Manufacturer Part Number: LTC4253AIGN#TR
• Description: MS-Hot Swap/High Voltage, Neg. 48V Hot Swap With 1% UV, Sequencer
• Manufacturer: Linear Technology

Specifications of LTC4253AIGN#TR

• Family Name: LTC4253A
• Package Type: SSOP N
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Product Depth (mm): 3.99mm
• Product Height (mm): 1.5mm
• Mounting: Surface Mount
• Pin Count: 16
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
MOSFET SELECTION
The external MOSFET switch must have adequate safe
operating area (SOA) to handle short-circuit conditions
until TIMER times out. These considerations take prece-
dence over DC current ratings. A MOSFET with adequate
SOA for a given application can always handle the required
current but the opposite may not be true. Consult the
manufacturer’s MOSFET data sheet for safe operating area
and effective transient thermal impedance curves.
MOSFET selection is a 3-step process by assuming the
absence of soft-start capacitor. First, R
then the time required to charge the load capacitance is de-
termined. This timing, along with the maximum short-cir-
cuit 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 Ioad capacitance, I
current trip point (V
the maximum load current. Note that maximum input
current to a DC/DC converter is expected at V
R
where V
sents the guaranteed minimum circuit breaker threshold.
During the initial charging process, the LTC4253/LTC4253A
may operate the MOSFET in current limit, forcing (V
between 80mV to 120mV (V
LTC4253A) across R
given by:
Maximum short-circuit current limit is calculated using
the maximum V
S
R
I
I
is given by:
SHORTCIRCUIT MAX
INRUSH MIN
S
=
CB(MIN)
V
I
L MAX
CB MIN
(
(
(
)
= 40mV (45mV for the LTC4253A) repre-
=
)
SENSE
)
(
V
U
CB
ACL MIN
R
S
. This gives
/R
)
. The minimum inrush current is
(
S
=
S
L
) should be set to accommodate
V
U
ACL MAX
)
and C
ACL
R
(
S
is 54mV to 66mV for the
L
. The circuit breaker
)
W
S
is calculated and
SUPPLY(MIN)
U
(10)
ACL
(8)
(9)
)
.
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:
The maximum current flowing in the DRAIN pin is given by:
Approximating a linear charging rate, I
I
be approximated with 0.5 • I
equation, TIMER capacitor C
Returning to Equation (3), the TIMER period is calculated
and used in conjunction with V
I
tive MOSFET.
As a numerical design example for the LTC4253, consider
a 30W load, which requires 1A input current at 36V. If
V
(8) gives R
account for errors in R
TIMER threshold (4V), R
DRAIN voltage clamp (V
be multiplied by 1.5, giving the nearest standard value of
C
If a short-circuit occurs, a current of up to
120mV/40mΩ = 3A will flow in the MOSFET for 6.3ms as
dictated by C
be selected based on this criterion. The IRF530S can
handle 100V and 3A for 10ms and is safe to use in this
application.
DRN(MAX)
SHORTCIRCUIT(MAX)
SUPPLY(MAX)
T
I
t
C
= 680nF.
DRN MAX
CL CHARGE
T
T
is calculated based on the maximum time it takes the
(
=
(
t
CL CHARGE
to zero, the I
S
)
(
= 40mΩ; Equation (13) gives C
T
=
= 72V and C
)
= 680nF in Equation (3). The MOSFET must
=
V
LTC4253/LTC4253A
SUPPLY MAX
C V
to check the SOA curves of a prospec-
•
I
)
• (
DRN
DRNCL
T
200
=
(
S
D
must be selected based on the
L
, C
C
, DRAIN current multiplier and
component in Equation (3) can
R
4
= 100µF, R
µ +
L
V
D
T
T
)
), the calculated value should
A
I
•
INRUSH MIN
DRN(MAX)
, TIMER current (200µA),
−
is given by:
V
SUPPLY MAX
V
4
DRNCL
•
I
(
DRN MAX
D
SUPPLY(MAX)
. Rearranging the
(
= 1MΩ, Equation
DRN
(
)
T
)
= 414nF. To
drops from
)
)
19
425353afc
(11)
(12)
(13)
and