MIC28512

DS20005524A-page 20

 2016 Microchip Technology Inc.

FIGURE 4-4:

MIC28512-1 Control Loop

Timing (Discontinuous Mode).

During discontinuous mode, the bias current of most

circuits are reduced. As a result, the total power supply

current during discontinuous mode is only about

450 μA, allowing the MIC28512-1 to achieve high

efficiency in light load applications.

4.3

pins to bypass the internal linear regulator.

4.4

Soft-Start

Soft-start reduces the power supply inrush current at

startup by controlling the output voltage rise time while

the output capacitor charges.

The MIC28511 implements an internal digital soft-start

0 to 100% in about 5 ms with 9.7 mV steps. This

controls the output voltage rate of rise at turn on,

minimizing inrush current and eliminating output

voltage overshoot. Once the soft-start cycle ends, the

related circuitry is disabled to reduce current

consumption.

4.5

Current Limit

low-side power MOSFET to sense overcurrent

conditions. In each switching cycle, the inductor current

is sensed by monitoring the low-side MOSFET during

with the power ground (PGND) after a blanking time of

compared with the low-side MOSFET voltage drop to

set the overcurrent trip level. The small capacitor

connected from the ILIM pin to PGND can be added to

filter the switching node ringing, allowing a better short

and the filter capacitor should be much less than the

minimum off time.

The overcurrent limit can be programmed by using

Equation 4-3:

EQUATION 4-3:

The peak-to-peak inductor current ripple is calculated

with Equation 4-4.

EQUATION 4-4:

temperature; therefore, it is recommended to use the

In case of hard short, the current-limit threshold 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 capacitor during soft-start is under the folded

short limit; otherwise the supply will go into hiccup

mode and may not be finishing the soft-start

successfully.

ESTIMATED ON-TIME

I

L

ZC

V

FB

V

REF

V

LSD

0

V

HSD

I

DISCONTINUOUS MODE STARTS

V

DISCONTINUOUS MODE.

R

ILIM

I

CLIM

0.5

–

I

LPP







 R

DS ON





V

CL

+

I

CL

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

=

Where:

Desired Current Limit

On-Resistance of Low-Side Power

MOSFET (40 mΩ typical)

Current-limit threshold.

14 mV (typical absolute value).

See the Electrical Characteristics table.

Current-limit source current.

70µA (typical). See the Electrical Char-

acteristics table.

Inductor Current Peak-to-Peak

Use Equation 4-4 to calculate the

inductor ripple current

I

LPP





V

OUT

V

IN MAX



V

OUT

–





V

IN MAX



f

SW



L



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

=