Datasheet

14
4251b12f
LTC4251B/LTC4251B-1/
LTC4251B-2
MOSFET selection is a three-step process. First, R
S
is
calculated, and then the time required to charge the load
capacitance is determined. This timing, along with the
maximum short-circuit current and maximum input volt-
age defines an operating point that is checked against the
MOSFET’s SOA curve.
To begin a design, first specify the required load current
and load capacitance, I
L
and C
L
. The circuit breaker current
trip point (50mV/R
S
) should be set to accommodate the
maximum load current. Note that maximum input current
to a DC/DC converter is expected at V
SUPPLY (MIN)
. R
S
is
given by:
R
S
=
40mV
I
L(MAX)
(3)
where 40mV represents the guaranteed minimum circuit
breaker threshold.
During the initial charging process, the LTC4251B/
LTC4251B-1/LTC4251B-2 may operate the MOSFET in
current limit, forcing 80mV to 120mV across R
S
. The
minimum inrush current is given by:
I
INRUSH(MIN)
=
80mV
R
S
(4)
Maximum short-circuit current limit is calculated using
maximum V
SENSE
, or:
I
SHORT-CIRCUIT(MAX)
=
120mV
R
S
(5)
The TIMER capacitor C
T
must be selected based on the
slowest expected charging rate; otherwise TIMER might
time out before the load capacitor is fully charged. A value
for C
T
is calculated based on the maximum time it takes
the load capacitor to charge. That time is given by:
t
CL CHARGE
=
C•V
I
=
C
L
•V
SUPPLY(MAX)
I
INRUSH(MIN)
(6)
Substituting Equation (4) for I
INRUSH(MIN)
and equating
(6) with (2) gives:
C
T
=
C
L
•V
SUPPLY (MAX)
•R
S
• 230μA
(4V • 80mV)
(7)
Returning to Equation (2), the TIMER period is calcu-
lated and used in conjunction with V
SUPPLY(MAX)
and
I
SHORT-CIRCUIT(MAX)
to check the SOA curves of a prospec-
tive MOSFET.
As a numerical design example, consider a 30W load,
which requires 1A input current at 36V. If V
SUPPLY(MAX)
=
72V and C
L
= 100μF, Equation (3) gives R
SENSE
= 40mΩ;
Equation (7) gives C
T
= 207nF. To account for errors in
R
SENSE
, C
T
, TIMER current (230μA) and TIMER threshold
(4V), the calculated value should be multiplied by 1.5,
giving a nearest standard value of C
T
= 330nF.
If a short-circuit occurs, a current of up to 120mV/40mΩ
= 3A will flow in the MOSFET for 5.7ms as dictated by
C
T
= 330nF in Equation (2). 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.
SUMMARY OF DESIGN FLOW
To summarize the design flow, consider the application
shown in Figure 2, which was designed for 50W:
Calculate maximum load current: 50W/36V = 1.4A; allow-
ing 83% converter efficiency, I
IN (MAX)
= 1.7A.
Calculate R
S
: from Equation (3) R
S
= 20mΩ.
Calculate C
T
: from Equation (7) C
T
= 150nF (including
1.5X correction factor).
Calculate TIMER period: from Equation (2) the short-circuit
time-out period is t = 2.6ms.
Calculate maximum short-circuit current: from Equation
(5) maximum short-circuit current could be as high as
120mV/20mΩ = 6A.
Consult MOSFET SOA curves: the IRF530S can handle
6A at 72V for 5ms, so it is safe to use in this application.
APPLICATIONS INFORMATION