MIC28303 datasheet(23/38 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005464A-page 23

MIC28303

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 VREF 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 VFB ramp. Once the soft-start cycle ends, the

related circuitry is disabled to reduce current

consumption. PVDD must be powered up at the same

time or after VIN to make the soft-start function

correctly.

4.4

Current Limit

The MIC28303 uses the RDS(ON) of the low side

MOSFET and external resistor connected from ILIM pin

to SW node to decide the current limit.

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 (PGND) after a

blanking time of 150 ns. In this way the drop voltage

over the resistor R15 (VCL) is compared with the drop

over the bottom FET generating the short current limit.

The small capacitor (C6) connected from the ILIM pin to

PGND 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.

The VCL 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 VCL.

In that case the V(ILIM) is lower than PGND 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:

Because the inductor is integrated, use Equation 4-4 to

calculate the peak-to-peak inductor ripple current.

EQUATION 4-4:

The MIC28303 has 4.7 µH inductor integrated into the

module. The typical value of RWINDING(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 RDS(ON) varies 30% to 40% with

temperature; therefore, it is recommended to add a

50% margin to ICLIM 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.

2.2μF

MIC28303

BST

PGND

VIN

ILIM

R15

TABLE 4-1:

TYPICAL OUTPUT

CURRENT-LIMIT VALUES

R15

Typical Output Current-Limit

1.81 kΩ

2.7 kΩ

6.3A

R15

CLIM

LPP



0.5





–

 R

DS ON





-----------------------------------------------------------------------------------------------------

Where:

ICLIM

Desired Current Limit

RDS(ON)

On-Resistance of Low-Side Power

MOSFET, 57 mΩ Typically

VCL

Current-Limit Threshold (Typical

Absolute Value is 14 mV per Table 1-1)

ICL

Current-Limit Source Current (Typical

Value is 80 µA, per Table 1-1)

∆IL(PP)

Inductor Current Peak-to-Peak.

LPP





OUT

IN MAX



OUT

–





IN MAX





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