MIC2007 datasheet(15/17 Pages) MICREL

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Micrel

MIC2007/MIC2017

October 2005

M9999-102805

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 0 0 0 0 0 0 0 0 0

CSLEW (nF)

Typical Turn-on Times

vs. External C

SLEW

Capacitance

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

RISE

DELAY

Figure 8.

CSLEW’s effect on ILIMIT

An unavoidable consequence of adding CSLEW

capacitance is a reduction in the MIC2008/2018’s ability

to quickly limit current transients or surges. A

sufficiently large capacitance can prevent both the

primary and secondary current limits from acting in time

to prevent damage to the MIC2008/2018 or the system

from a short circuit fault. For this reason, the upper limit

on the value of CSLEW is 4nF.

Kickstart (MIC2017)

Kickstart allows brief current surges to pass to the load

before the onset of normal current limiting. This, in turn,

permits dynamic loads to draw bursts of energy without

sacrificing system safety.

Functionally, Kickstart is a forced override of the normal

current limiting function provided by the MIC2017. The

Kickstart period is governed by an internal timer which

allows current to pass unimpeded to the load for 128ms

and then normal (primary) current limiting goes into

action.

During Kickstart a secondary current limiting circuit is

monitoring output current to prevent damage to the

MIC2017. This is because a hard short, combined with

a robust power supply, can result in currents of many

tens of amperes. This secondary current limit is

nominally set at 4 Amps and reacts immediately and

independently of the Kickstart period. Once the Kickstart

timer has finished its count, the primary current limiting

circuit takes over and holds IOUT to its programmed limit

for as long as the excessive load persists.

Once the MIC2017 drops out of current limiting the

Kickstart timer initiates a lock-out period of 128ms such

that no further bursts of current above the primary

current limit, will be allowed until the lock-out period has

expired.

Kickstart may be over-ridden by the thermal protection

circuit and if sufficient internal heating occurs, Kickstart

will be terminated and IOUT

0. Upon cooling, if the

load is still present IOUT

ILIMIT, not IKICKSTART.

FAULT/

ENABLE

VOUT

IOUT

Time (ms)

100

200

300

400

500

600

Kickstart

Current Limiting

Load Removed

Figure 9. Kickstart Operation with Varying Load

Supply Filtering

A 0.1µF to 1µF bypass capacitor positioned close to the

VIN and GND pins of MIC2007/2017 is both good design

practice and required for proper operation of the

MIC2007/2017. This will control supply transients and

ringing. Without a bypass capacitor, large current surges

or an output short may cause sufficient ringing on VIN

(from supply lead inductance) to cause erratic operation

of the MIC2007/2017’s control circuitry. Good quality,

low ESR capacitors, such as Panasonic’s TE or ECJ

series, are suggested.

When bypassing with capacitors of 10µF and up, it is

good practice to place a smaller value capacitor in

parallel with the larger to handle the high frequency

components of any line transients. Values in the range

of 0.01µF to 0.1µF are recommended. Again, good

quality, low ESR capacitors should be chosen.

Power Dissipation

Power dissipation depends on several factors such as

the load, PCB layout, ambient temperature, and supply

voltage. Calculation of power dissipation can be

accomplished by the following equation:

(

OUT

DS(ON)

To relate this to junction temperature, the following

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