LTC4252-1CMS8 Linear Technology, LTC4252-1CMS8 Datasheet - Page 20

• Manufacturer Part Number: LTC4252-1CMS8
• Description: IC CNTRLR HOTSWAP NEGVOLT 8-MSOP
• Manufacturer: Linear Technology
• Type: Hot-Swap Controllerr

Specifications of LTC4252-1CMS8

• Applications: General Purpose
• Internal Switch(s): No
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-TSSOP, 8-MSOP (0.118", 3.00mm Width)
• Family Name: LTC4252-1
• 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: 8
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LTC42521CMS8

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