MIC28304 Datasheet, PDF (25/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
The short-circuit current limit can be programmed by
using Equation 3.
R15 = ^{(ICLIM − ∆IL(PP) × 0.5) × RDS(ON) + VCL}
I_CL
Eq. 3
Where:
I_CLIM = Desired current limit
R_DS(ON) = On-resistance of low-side power MOSFET,
57mΩ typically
V_CL = Current-limit threshold (typical absolute value is
14mV per the Electrical Characteristics⁽⁴⁾)
I_CL = 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:
MIC28304
The MOSFET R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a 50%
margin to I_CLIM 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
1.81kΩ
3A
2.7kΩ
6.3A
∆I_L(PP)
=
V_OUT × (V_IN(max) − V_OUT )
V_IN(max) × f_sw × L
Eq. 4
The MIC28304 has 4.7µH inductor integrated into the
module. The typical value of R_WINDING(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.
March 25, 2014
25
Revision 1.1