Micrel

MIC2003/MIC20013

October 2005

11

M9999-102605

Functional Description

Input and Output

circuitry driving the switch and the input (Source

Drain connection of the power MOSFET and supplies

power to the load. In a typical circuit, current flows from

When the switch is disabled, current will not flow to the

load, except for a small unavoidable leakage current of

more than a diode drop (~0.6V), while the switch is

disabled, current will flow from output to input via the

power MOSFET’s body diode. This effect can be used

to advantage when large bypass capacitors are placed

on MIC2003/2013’s’s output. When power to the switch

is removed, the output capacitor will be automatically

discharged.

consider using MIC2003/2013 or MIC2007/2017 in place

of MIC2003/2013. These MIC2000 family members are

equipped with a discharge FET to insure complete

Current Sensing and Limiting

MIC2003/2013 protects the system power supply and

load from damage by continuously monitoring current

through the on-chip power MOSFET. Load current is

monitored by means of a current mirror in parallel with

the power MOSFET switch. Current limiting is invoked

when the load exceeds an internally set over-current

threshold. When current limiting is activated the output

current is constrained to the limit value, and remains at

this level until either the load/fault is removed, the load’s

current requirement drops below the limiting value, or

the MIC2003/2013 goes into thermal shutdown.

Kickstart (MIC2013 only)

The MIC2013 is designed to allow momentary current

surges (Kickstart) before the onset of current limiting,

which permits dynamic loads, such as small disk drives

or portable printers to draw the energy needed to

overcome inertial loads without sacrificing system

safety. In this respect, the MIC2013 differs markedly

from MIC2003 and its peers, which immediately limit

load current, potentially starving the motor and causing

the appliance to stall or stutter.

During this delay period, typically 128 ms, a secondary

current limit is in effect. If the load demands a current in

excess the secondary limit, MIC2013 acts immediately

to restrict output current to the secondary limit for the

duration of the Kickstart period. After this time the

MIC2013 reverts to its normal current limit. An example

of Kickstart operation is shown below.

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Figure 3. Kickstart Operation

Picture Key:

A) MIC2013 is enabled into an excessive load (slew

rate limiting not visible at this time scale) The initial

current surge is limited by either the overall circuit

resistance and power supply compliance, or the

secondary current limit, whichever is less.

heating (effect exaggerated for emphasis).

C) Kickstart period.

D) Current limiting initiated. FAULT/ goes LOW.

for dead shorts).

F) Thermal shutdown followed by thermal cycling.

G) Excessive load released, normal load remains.

MIC2013 drops out of current limiting.

H) FAULT/ delay period followed by FAULT/ going

HIGH.

Slew Rate Control

Large capacitive loads can create significant current

surges when charged through a high-side switch such

as the MIC2003/2013. For this reason, MIC2003/2013

provides built-in slew rate control to limit the initial inrush

currents upon enabling the power MOSFET switch.

Slew rate control is active upon powering up, and upon

re-enabling the load. At shutdown, the discharge slew

rate is controlled by the external load and output

capacitor.