LTC4253ACGN#TRPBF【PDF 20/34 页】IC HOT SWAP CONTRLR -48V 16-SSOP

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Sheet目录42

16页 17页 18页 19页 [20页]21页 22页 23页 24页

LTC4253/LTC4253A

425353aff

For more information www.linear.com/4253

MOSFET SELECTION

The external MOSFET switch must have adequate safe op-

erating area (SOA) to handle short-circuit conditions until

TIMER times out. These considerations take precedence

over DC current ratings. A MOSFET with adequate SOA for

a given application can always handle the required current

but the opposite may not be true. Consult the manufacturers

MOSFET data sheet for safe operating area and effective

transient thermal impedance curves.

MOSFET selection is a 3-step process by assuming the

absence of soft-start capacitor. First, R

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 voltage, defines an

operating point that is checked against the MOSFETs

SOA curve.

To begin a design, first specify the required load current

and Ioad capacitance, I

and C

. The circuit breaker cur-

rent trip point (V

) 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)

is given by:

CB(MIN)

L(MAX)

(8)

where V

CB(MIN)

= 40mV (45mV for the LTC4253A) repre-

sents the guaranteed minimum circuit breaker threshold.

During the initial charging process, the LTC4253/LTC4253A

may operate the MOSFET in current limit, forcing (V

ACL

)

between 80mV to 120mV (V

ACL

is 54mV to 66mV for the

LTC4253A) across R

. The minimum inrush current is

given by:

INRUSH(MIN)

ACL(MIN)

(9)

Maximum short-circuit current limit is calculated using

the maximum V

SENSE

. This gives

SHORTCIRCUIT(MAX)

ACL(MAX)

(10)

The TIMER capacitor C

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

is calculated based on the maximum time it takes

the load capacitor to charge. That time is given by:

CL(CHARGE)

C V

SUPPLY(MAX)

INRUSH(MIN)

(11)

The maximum current flowing in the DRAIN pin is given by:

DRN(MAX)

SUPPLY(MAX)

DRNCL

(12)

Approximating a linear charging rate, I

DRN

drops from

DRN(MAX)

to zero, the I

DRN

component in Equation (3)

can be approximated with 0.5 " I

DRN(MAX)

. Rearranging

the equation, TIMER capacitor C

is given by:

CL(CHARGE)

(200礎 + 4 I

DRN(MAX)

)

(13)

Returning to Equation (3), the TIMER period is calcu-

lated and used in conjunction with V

SUPPLY(MAX)

and

SHORTCIRCUIT(MAX)

to check the SOA curves of a prospec-

tive MOSFET .

As a numerical design example for the LTC4253, consider

a 30W load, which requires 1A input current at 36V . If

SUPPLY(MAX)

= 72V and C

= 100礔 , R

= 1M? Equation

(8) gives R

??0m? Equation (13) gives C

= 414nF .

To account for errors in R

, C

, TIMER current (200礎),

TIMER threshold (4V), R

, DRAIN current multiplier and

DRAIN voltage clamp (V

DRNCL

), the calculated value should

be multiplied by 1.5, giving the nearest standard value of

??80nF .

If a short-circuit occurs, a current of up to 120mV/40m=?A

will flow in the MOSFET for 6.3ms as dictated by C

= 680nF

in Equation (3). 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.

APPLICATIONS INFORMATION

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