MIC28303 Datasheet, PDF (23/38 Pages) Microchip Technology – 50V, 3A Power Module

During discontinuous mode, the bias current of most
circuits is substantially reduced. As a result, the total
power supply current during discontinuous mode is
only about 400 μA, allowing the MIC28303-1 to achieve
high efficiency in light load applications.
4.3 Soft-Start
Soft-start reduces the input power supply surge current
at startup by controlling the output voltage rise time.
The input surge appears while the output capacitor is
charged up. A slower output rise time will draw a lower
input surge current.
The MIC28303 implements an internal digital soft-start
by making the 0.8V reference voltage V_REF ramp from
0 to 100% in about 5 ms with 9.7 mV steps. Therefore,
the output voltage is controlled to increase slowly by a
stair-case V_FB ramp. Once the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption. P_VDD must be powered up at the same
time or after V_IN to make the soft-start function
correctly.
4.4 Current Limit
The MIC28303 uses the R_DS(ON) of the low side
MOSFET and external resistor connected from I_LIM pin
to SW node to decide the current limit.
VIN
2.2μF
x3
R15
C6
MIC28303
VIN
BST
SW
SW
CS
ILIM
FB
PGND
FIGURE 4-4:
Current-Limiting Circuit
In each switching cycle of the MIC28303, the inductor
current is sensed by monitoring the low-side MOSFET
in the OFF period. The sensed voltage V_(ILIM) is
compared with the power ground (P_GND) after a
blanking time of 150 ns. In this way the drop voltage
over the resistor R15 (V_CL) is compared with the drop
over the bottom FET generating the short current limit.
The small capacitor (C6) connected from the I_LIM pin to
P_GND filters the switching node ringing during the
off-time allowing a better short limit measurement. The
time constant created by R15 and C6 should be much
less than the minimum off time.
 2016 Microchip Technology Inc.
MIC28303
The V_CL drop allows programming of short limit through
the value of the resistor (R15), If the absolute value of
the voltage drop on the bottom FET is greater than V_CL.
In that case the V_(ILIM) is lower than P_GND and a short
circuit event is triggered. A hiccup cycle to treat the
short event is generated. The hiccup sequence
including the soft start reduces the stress on the
switching FETs and protects the load and supply for
severe short conditions.
The short-circuit current limit can be programmed by
using Equation 4-3.
EQUATION 4-3:
R15 = -^--^I--^C----^L---^I--^M-----–----^----^I--^L---^--^P---^P----^---^----0---.--5---^----^-----^R----^D---^S---^--^O----^N---^---+-----^V----^C---^L-
I_CL
Where:
I_CLIM
R_DS(ON)
V_CL
I_CL
∆I_L(PP)
Desired Current Limit
On-Resistance of Low-Side Power
MOSFET, 57 mΩ Typically
Current-Limit Threshold (Typical
Absolute Value is 14 mV per Table 1-1)
Current-Limit Source Current (Typical
Value is 80 µA, per Table 1-1)
Inductor Current Peak-to-Peak.
Because the inductor is integrated, use Equation 4-4 to
calculate the peak-to-peak inductor ripple current.
EQUATION 4-4:
I _{L  P P }
=
V_{----O----U----T----}_-----_--V_{----I--N-----M-----A---X------–-----}V_{---O----U----T---}
V_{INMAX}  f_SW  L
The MIC28303 has 4.7 µH inductor integrated into the
module. The typical value of R_WINDING(DCR) of this
particular inductor is in the range of 45 mΩ.
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 R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a
50% margin to I_CLIM in Equation 4-3 to avoid false
current limiting due to increased MOSFET junction
temperature rise. Table 4-1 shows typical output
current limit value for a given R15 with C6 = 10 pF.
TABLE 4-1: TYPICAL OUTPUT
CURRENT-LIMIT VALUES
R15
1.81 kΩ
2.7 kΩ
Typical Output Current-Limit
3A
6.3A
DS20005464A-page 23