LTC4253CGN Linear Technology, LTC4253CGN Datasheet - Page 20

• Manufacturer Part Number: LTC4253CGN
• Description: IC HOT SWAP CONTRLR -48V 16-SSOP
• Manufacturer: Linear Technology
• Type: Hot-Swap Controllerr

Specifications of LTC4253CGN

• Applications: General Purpose
• Internal Switch(s): No
• Voltage - Supply: 11.2 V ~ 14.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP (0.150", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LTC4253/LTC4253A
applicaTions inForMaTion
MOSFET SELECTION
The external MOSFET switch must have adequate safe op-
erating area (SOA) to handle short-circuit conditions until
TIMER times out. 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 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
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 Ioad capacitance, I
rent 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
20
S
R
I
I
is given by:
INRUSH(MIN)
SHORTCIRCUIT(MAX)
S
=
CB(MIN)
V
I
L(MAX)
CB(MIN)
=
= 40mV (45mV for the LTC4253A) repre-
SENSE
CB
V
ACL(MIN)
/R
R
S
. This gives
S
. The minimum inrush current is
=
S
) should be set to accommodate
L
V
and C
ACL(MAX)
ACL
R
S
L
. The circuit breaker cur-
is 54mV to 66mV for the
S
is calculated and
SUPPLY(MIN)
(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
the 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
can be approximated with 0.5 • I
the equation, TIMER capacitor C
Returning to Equation (3), the TIMER period is calcu-
lated 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
To 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 = 3 A
will flow in the MOSFET for 6.3ms as dictated by C
in Equation (3). The MOSFET must 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
= 680nF .
I
C
t
DRN(MAX)
CL(CHARGE)
T
T
=
is calculated based on the maximum time it takes
t
CL(CHARGE)
to zero, the I
S
= 40mΩ; Equation (13) gives C
=
= 72V and C
=
V
SUPPLY(MAX)
C • V
to check the SOA curves of a prospec-
I
• (200µA + 4 •I
=
DRNCL
T
DRN
D
C
must be selected based on the
R
4V
L
S
, DRAIN current multiplier and
L
D
, C
= 100µF , R
• V
I
INRUSH(MIN)
component in Equation (3)
), the calculated value should
− V
T
SUPPLY(MAX)
, TIMER current (200µA),
T
DRNCL
DRN(MAX)
is given by:
DRN(MAX)
D
= 1MΩ, Equation
SUPPLY(MAX)
DRN
. Rearranging
drops from
)
T
T
= 414nF .
= 680nF
425353afd
(11)
(12)
(13)
and