LTC4253CGN Linear Technology, LTC4253CGN Datasheet - Page 21

• 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

applicaTions inForMaTion
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:
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
the IRF530S is 40ms. From Equation (15), C
Actual board evaluation showed that C
propriate. The ratio ( R
gauge as large ratios may result in the time-out period
expiring prematurely. This gauge is determined empirically
with board level evaluation.
SUMMARY OF DESIGN FLOW
To summarize the design flow, consider the application
shown in Figure 3 for the LTC4253A. It was designed for
80W and C
Calculate maximum load current: 80W/43V = 1.86A;
allowing for 83% converter efficiency, I
Calculate R
Calculate I
I
Select a MOSFET that can handle 3.3A at 71V: IRF530S.
Calculate C
C
period t
SHORTCIRCUIT(MAX)
T
= 680nF , which gives the circuit breaker time-out
I
C
C
CB(MAX)
SS
SS
=
=
MAX
CB(MAX)
0.916 • R
2.48 • R
t
t
SOA(MAX)
L
S
SOA(MAX)
=
T
= 100µF .
SHORT-CIRCUIT(MAX)
= 5.9ms.
: from Equation (8) R
: from Equation (13) C
V
CB(MAX)
is 60mV (55mV for the LTC4253A).
R
SS
SOA(MAX)
S
SS
= 3.3A.
for the LTC4253A
for the LTC4253
SS
• C
. C
SS
SS
) to t
: from Equation (10)
is given by:
S
= 20mΩ.
CL(CHARGE)
SS
T
IN(MAX)
SS
= 302nF . Select
= 100nF was ap-
C
SS
SS
response.
= 2.2A.
is a good
= 437nF .
SOA
(14)
(15)
for
Consult MOSFET SOA curves: the IRF530S can handle 3.3A
at 100V for 8.3ms, so it is safe to use in this application.
Calculate C
C
FREQUENCY COMPENSATION
The LTC4253 typical frequency compensation network
for the analog current limit loop is a series R
and C
relationship between the compensation capacitor C
the MOSFET’s C
a starting value for C
specification. Optimized values for C
eral popular MOSFETs. Differences in the optimized value
of C
compensation values should be verified by board level
short-circuit testing.
As seen in Figure 5, at the onset of a short-circuit event,
the input supply voltage can ring dramatically due to series
inductance. If this voltage avalanches the MOSFET, current
continues to flow through the MOSFET to the output. The
analog current limit loop cannot control this current flow
and therefore the loop undershoots. This effect cannot be
eliminated by frequency compensation. A Zener diode is
required to clamp the input supply voltage and prevent
MOSFET avalanche.
SS
= 33nF .
C
C
versus the starting value are small. Nevertheless,
connected from GATE to V
Figure 6. Recommended Compensation
Capacitor C
60
50
40
30
20
10
0
SS
0
IRF530
: using Equations (14) and (15) select
ISS
IRF740
LTC4253/LTC4253A
IRF540
C
. The line in Figure 6 is used to select
2000
vs MOSFET C
IRF3710
C
MOSFET C
based upon the MOSFET’s C
4000
ISS
(pF)
ISS
EE
6000
NTY100N10
for the LTC4253
. Figure 6 depicts the
C
are shown for sev-
4253 F06
8000
C
21
(10Ω)
425353afd
C
and
ISS