MIC28304 datasheet(25/39 Pages) MICREL

25 / 39 page

Micrel, Inc.

MIC28304

The short-circuit current limit can be programmed by

using Equation 3.

( )

)

(

CLIM

)

(

R15

∆

−

Eq. 3

Where:

ICLIM = Desired current limit

RDS(ON) = On-resistance of low-side power MOSFET,

57mΩ typically

VCL = Current-limit threshold (typical absolute value is

14mV per the Electrical Characteristics

(4))

ICL = Current-limit source current (typical value is 80µA,

per the Electrical Characteristics table).

ΔI

L(PP) = Inductor current peak-to-peak, since the inductor

is integrated use Equation 4 to calculate the inductor

ripple current.

The peak-to-peak inductor current ripple is:

)

IN(max)

OUT

IN(max)

OUT

L(PP)

−

∆

Eq. 4

The MIC28304 has 4.7µH inductor integrated into the

module. The typical value of RWINDING(DCR) of this particular

inductor is in the range of 45mΩ.

In case of hard short, the short limit is folded down to

allow an indefinite hard short on the output without any

destructive effect. It is mandatory to make sure that the

inductor current used to charge the output capacitance

during soft start is under the folded short limit; otherwise

the supply will go in hiccup mode and may not be

finishing the soft start successfully.

The

MOSFET

RDS(ON) varies

30%

40%

with

temperature; therefore, it is recommended to add a 50%

margin to ICLIM in Equation 3 to avoid false current limiting

due to increased MOSFET junction temperature rise.

Table 2 shows typical output current limit value for a

given R15 with C6 = 10pF.

Table 2. Typical Output Current-Limit Value

R15

Typical Output Current Limit

1.81k

2.7k

6.3A

March 25, 2014

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