LTC4253ACUF-ADJ#TR Linear Technology, LTC4253ACUF-ADJ#TR Datasheet - Page 20

• Manufacturer Part Number: LTC4253ACUF-ADJ#TR
• Description: IC,Power Control/Management,CMOS,LLCC,20PIN,PLASTIC
• Manufacturer: Linear Technology

Specifications of LTC4253ACUF-ADJ#TR

• Family Name: LTC4253A-ADJ
• Package Type: QFN EP
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Product Depth (mm): 4mm
• Product Length (mm): 4mm
• Mounting: Surface Mount
• Pin Count: 20
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
LTC4253A-ADJ
MOSFET selection is a 3-step process by assuming the
absense 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
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
mum circuit breaker threshold.
During the initial charging process, the LTC4253A-ADJ
may operate the MOSFET in current limit, forcing (V
between 54mV to 66mV across R
current is given by:
Maximum short-circuit current limit is calculated using
the 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:
The maximum current flowing in the DRAIN pin is given by:
20
S
R
I
I
I
t
is given by:
SHORTCIRCUIT MAX
DRN MAX
INRUSH MIN
CL CHARGE
S
T
is calculated based on the maximum time it takes the
(
=
(
CB(MIN)
V
I
L MAX
CB MIN
(
(
)
(
=
)
)
=
= 45mV represents the guaranteed mini-
=
)
SENSE
V
)
(
SUPPLY MAX
C V
V
U
CB
ACL MIN
•
I
R
. This gives
/R
)
(
=
S
T
=
S
(
, must be selected based on the
L
) should be set to accommodate
V
U
C
R
ACL MAX
)
and C
L
D
R
)
I
•
INRUSH MIN
(
−
S
V
SUPPLY MAX
V
S
DRNCL
L
. The minimum inrush
. The circuit breaker
)
W
(
S
(
is calculated and
)
)
SUPPLY(MIN)
U
(10)
(11)
(12)
(13)
ACL
(9)
)
.
Approximating a linear charging rate, I
I
be approximated with 0.5 • I
equation, TIMER capacitor C
Returning to Equation 4, 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
R
gives C
current (200µA), TIMER threshold (4V), R
rent multiplier and DRAIN voltage clamp (V
calculated value should be multiplied by 1.5, giving the
nearest standard value of C
If a short-circuit occurs, a current of up to
66mV/45mΩ = 1.65A will flow in the MOSFET for 9.1ms
as dictated by C
be selected based on this criterion. The IRF530S can
handle 100V and 2A for 22.5ms and is safe to use in this
application.
Computing the maximum soft-start capacitor value during
soft-start to a load short is complicated by the nonlinear
MOSFET’s SOA characteristics and the R
An overconservative but simple approach begins with the
maximum circuit breaker current, given by:
From the SOA curves of a prospective MOSFET, determine
the time allowed, t
In the above example, 55mV/40mΩ gives 1.375A. t
the IRF530S is 47.6ms. From Equation 16, C
DRN(MAX)
SHORTCIRCUIT(MAX)
S
I
C
C
= 45mΩ; use R
CB MAX
SS
T
(
=
T
=
= 619nF. To account for errors in R
t
2 48
CL CHARGE
t
to zero, the I
)
SOA MAX
.
L
=
(
= 100µF, R
V
(
•
CB MAX
T
R
= 1µF in Equation 4. The MOSFET must
R
(
SS
S
SOA(MAX)
S
to check the SOA curves of a prospec-
)
= 40mΩ for more margin. Equation 14
)
• (
)
DRN
200
D
4
component in Equation 4 can
. C
µ +
T
V
T
= 1MΩ, Equation 9 gives
A
= 1µF.
DRN(MAX)
SS
is given by:
is given by:
4
•
I
DRN MAX
SUPPLY(MAX)
. Rearranging the
DRN
SS
(
D
C
SUPPLY(MAX)
, DRAIN cur-
S
SS
, C
SS
drops from
)
DRNCL
)
response.
T
= 384nF.
, TIMER
SOA
4253a-adjf
), the
(14)
(15)
(16)
and
for
=