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4251b12f

LTC4251B/LTC4251B-1/

LTC4251B-2

MOSFET selection is a three-step process. First, RS 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, IL and CL. The circuit breaker current

trip point (50mV/RS) should be set to accommodate the

maximum load current. Note that maximum input current

to a DC/DC converter is expected at VSUPPLY (MIN). RS is

given by:

RS =

40mV

IL(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 RS. The

minimum inrush current is given by:

IINRUSH(MIN)=

80mV

(4)

Maximum short-circuit current limit is calculated using

maximum VSENSE, or:

ISHORT-CIRCUIT(MAX) =

120mV

(5)

The TIMER capacitor CT 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 CT is calculated based on the maximum time it takes

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

tCL CHARGE =

C• V

CL •VSUPPLY(MAX)

IINRUSH(MIN)

(6)

Substituting Equation (4) for IINRUSH(MIN) and equating

(6) with (2) gives:

CT =

CL •VSUPPLY (MAX) •RS • 230μA

(4V • 80mV)

(7)

Returning to Equation (2), the TIMER period is calcu-

lated and used in conjunction with VSUPPLY(MAX) and

ISHORT-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 VSUPPLY(MAX) =

72V and CL = 100μF, Equation (3) gives RSENSE = 40mΩ;

Equation (7) gives CT = 207nF. To account for errors in

RSENSE, CT, TIMER current (230μA) and TIMER threshold

(4V), the calculated value should be multiplied by 1.5,

giving a nearest standard value of CT = 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

CT = 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, IIN (MAX) = 1.7A.

Calculate RS: from Equation (3) RS = 20mΩ.

Calculate CT: from Equation (7) CT = 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.

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