LT4239 datasheet(16/24 Pages) AD

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LT4239

Rev. 0

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Design Example

As a design example, consider the following specifica-

tions: VIN = 12V, ILOAD(MAX) = 100A, CLOAD = 10mF and

VUV = 9.7V, using a trickle MOSFET with current limit set

at 50mV for start-up and a group of parallel MOSFETs

with current limit set at 10mV for load current conduction

(see Figure 1).

First, select the appropriate value of the current sense

resistors RS1 and RS2. Calculate RS2 value based on the

maximum load current ILOAD(MAX) and the lower limit for

the circuit breaker trip threshold ∆VSENSE2(CB)(MIN) (see

Equation 2).

RS2 =

∆VSENSE2(CB)(MIN)

ILOAD(MAX)

9.5mV

100A

= 0.095mΩ

(2)

As the sense resistor value is very low, several larger value

sense resistors are connected in parallel to improve the

sensing accuracy and keep the power dissipation within

limits. In this design, 12 parallel sense resistors of 1mΩ

value with 1% tolerance are used for RS2.

To achieve the specific resistance with the parallel sense

resistors, the averaging resistors, RA, should be selected

with the same ratio, k, as the sense resistors they con-

nect to. See Figure 5. This allows the current limit circuit

to measure the correct sense voltage for this effective

sense resistor. The smallest averaging resistor should not

exceed 1Ω.

SENSE2

OUTK

OUT

RSENSE2A

k • RS

RSENSE2B

k • RA

4239 F05

Figure 5. Weighted Averaging Sense Voltages

Next, calculate the RS1 value based on the inrush current

IINRUSH and the lower limit for the current limit threshold

∆VSENSE1(ACL)(MIN). The inrush current is set to 4A to fully

charge 10mF of CLOAD to 12V in 30ms. A capacitor, CG1,

is added to the gate of M1 to achieve the inrush current

(Equation 3).

CG1 =

CLOAD •IGATE1(UP)

IINRUSH

10mF • 10µA

= 25nF

(3)

Choose a practical value of 33nF with 10% tolerance for

CG1 (Equation 4).

RS1 =

∆VSENSE1(ACL)(MIN)

IINRUSH

45mV

= 11mΩ

(4)

Choose a 10mΩ sense resistor with 1% tolerance for RS1.

The MOSFET M1 is selected to handle the power dissi-

pation during inrush when load capacitor CLOAD is being

charged at power up. The method to determine the power

dissipation is based on the principle that the energy dis-

sipated in the MOSFET is the same as the energy stored

in the load capacitor, and is given by Equation 5.

ECL =

CLOADVIN2 =

(10mF)(12V)2

= 0.72J

(5)

Calculate the time it takes to charge CLOAD (Equation 6).

tCHARGE =

CLOAD • VIN

IINRUSH

10mF • 12V

= 30ms

(6)

Use Equation 7 to calculate the average power dissipated

in the MOSFET M1.

PDISS =

ECL

tCHARGE

0.72J

30ms

= 24W

(7)

The SOA (Safe Operating Area) curves of the selected

MOSFET must be evaluated to ensure that the thermal

capacity of the package tolerates 24W for 30ms. The SOA

curve of the PSMN6R1-30YLD shows 30W (3A at 10V)

for 100ms, satisfying this requirement.

If the circuit starts up into an output-short, the short-circuit

current will be limited for 100µs before the fault is latched-

off. Use Equation 8 to calculate the maximum short-circuit

current using the maximum active current limit threshold,

∆VSENSE1(ACL)(MAX) and minimum RS1 value.

ISHORT1(MAX) =

∆VSENSE1(ACL)(MAX)

RS1(MIN)

55mV

9.9mΩ

= 5.6A (8)

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