LTC4252A-2CMS#TR Linear Technology, LTC4252A-2CMS#TR Datasheet - Page 20
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LTC4252A-2CMS#TR
Manufacturer Part Number
LTC4252A-2CMS#TR
Description
IC CTRLR HOTSWAP NEG VOLT 10MSOP
Manufacturer
Linear Technology
Type
Hot-Swap Controllerr
Specifications of LTC4252A-2CMS#TR
Applications
General Purpose
Internal Switch(s)
No
Operating Temperature
0°C ~ 70°C
Mounting Type
Surface Mount
Package / Case
10-TFSOP, 10-MSOP (0.118", 3.00mm Width)
Family Name
LTC4252A-2
Package Type
MSOP
Operating Temperature (min)
0C
Operating Temperature (max)
70C
Operating Temperature Classification
Commercial
Product Depth (mm)
3mm
Product Height (mm)
0.86mm
Product Length (mm)
3mm
Mounting
Surface Mount
Pin Count
10
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
APPLICATIO S I FOR ATIO
LTC4252-1/LTC4252-2
LTC4252A-1/LTC4252A-2
Approximating a linear charging rate as I
I
be approximated with 0.5 • I
tion, 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, consider a 30W load,
which requires 1A input current at 36V. If V
72V and C
40mΩ; Equation (13) gives C
errors in R
(4V), R
clamp (V
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 5.6ms as dictated
by C
selected based on this criterion. The IRF530S can handle
100V and 3A for 10ms 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 overly conservative but simple approach begins with
the maximum circuit breaker current, given by:
where V
From the SOA curves of a prospective MOSFET, determine
the time allowed, t
In the above example, 60mV/40mΩ gives 1.5A. t
for the IRF530S is 40ms. From Equation (15), C
437nF. Actual board evaluation showed that C
20
DRN(MAX)
SHORTCIRCUIT(MAX)
I
C
C
CB MAX
SS
T
T
(
= 680nF in Equation (3). The MOSFET must be
=
D
=
CB(MAX)
, DRAIN current multiplier and DRAIN voltage
DRNCL
t
to zero, the I
CL CHARGE
S
L
0 916
)
t
, C
=
.
= 100µF, R
SOA MAX
(
T
V
, TIMER current (230µA), TIMER threshold
), the calculated value should be multiplied
CB MAX
(
= 60mV (55mV for the LTC4252A).
•
R
(
R
SOA(MAX)
S
to check the SOA curves of a prospec-
U
SS
)
)
•
DRN
)
(
D
230
T
= 1MΩ, Equation (8) gives R
is given by:
U
component in Equation (3) can
4
. C
µ +
DRN(MAX)
V
A
SS
T
= 441nF. To account for
is given by:
4
•
W
I
DRN MAX
. Rearranging equa-
SUPPLY(MAX)
SS
(
DRN
C
SUPPLY(MAX)
SS
drops from
)
T
SS
)
response.
U
= 680nF.
SOA(MAX)
= 100nF
SS
(13)
(14)
(15)
and
S
=
=
=
was appropriate. The ratio (R
good gauge as a large ratio may result in the time-out
period expiring. This gauge is determined empirically with
board level evaluation.
SUMMARY OF DESIGN FLOW
To summarize the design flow, consider the application
shown in Figure 2 with the LTC4252A. It was designed for
80W.
Calculate the maximum load current: 80W/43V = 1.86A;
allowing for 83% converter efficiency, I
Calculate R
Calculate I
Select a MOSFET that can handle 3.3A at 71V: IRF530S.
Calculate C
C
riod t = 5.6ms.
Consult MOSFET SOA curves: the IRF530S can handle
3.3A at 100V for 8.2ms, so it is safe to use in this
application.
Calculate C
C
FREQUENCY COMPENSATION
The LTC4252A typical frequency compensation network
for the analog current limit loop is a series R
C
between the compensation capacitor C
C
for C
mized values for C
MOSFETs. Differences in the optimized value of C
the starting value are small. Nevertheless, compensation
values should be verified by board level short-circuit
testing.
T
SS
C
ISS
I
= 680nF, which gives the circuit breaker time-out pe-
SHORTCIRCUIT MAX
connected to V
= 68nF.
. The line in Figure 7 is used to select a starting value
C
based upon the MOSFET’s C
SHORTCIRCUIT(MAX)
S
T
SS
: from Equation (8) R
: from Equation (13) C
: using Equations (14) and (15) select
(
EE
C
. Figure 7 depicts the relationship
)
=
are shown for several popular
20
66
m
mV
: from Equation (10)
SS
Ω
• C
=
S
ISS
SS
3 3
= 20mΩ.
.
) to t
C
T
specification. Opti-
A
and the MOSFET’s
IN(MAX)
= 322nF. Select
CL(CHARGE)
C
(10Ω) and
= 2.2A.
C
versus
425212fb
is a