MIC2267 datasheet(13/19 Pages) MICREL

13 / 19 page

Micrel Inc.

MIC2267

January 2011

M9999-011711-A

LIMIT

Resistor values of between 750kΩ and 46kΩ should be

used to set the current limit between 0.1A and 1.6A. To

set the nominal LIMIT resistor value:

RLIMIT = 75kΩ/ILIMIT

In USB applications for example, two LIMIT resistors can

be used to switch between 1 unit load (100mA) and 5

unit loads (500mA).

Due to a minimum on time implemented during current

limit operation, there is lower limit to VOUT where input

current limit is regulated. When VOUT sees a near short

circuit at higher switching frequency, IIN will be higher

than the set ILIMIT. Circuit losses will tend to keep this to a

maximum of 250mA.

The limit can be found by:

()

200

)

(

−

⋅

OUTMIN

e.g., for VIN = 5v and FSW = 1MHz, VOUTMIN = 0.36V

COMP

As the MIC2267 uses a current mode control, the control

loop only requires a single pole/zero compensator to

optimise

stability

and

transient

response.

The

recommended values are 11kΩ and 15nF. An additional

680pF capacitor can also be added to reject switching

frequency noise.

Connect a resistor divider between the VOUT load

terminal connection and output ground reference

connection to minimize resistive voltage drops affecting

load regulation.

For most applications, R2 can be set to 10kΩ and R2

can be found by:

R1 = R2(VOUT/VREF – 1)

SLOPEC

Connect a resistor between this pin and AGND as close

to the MIC2267 as possible to reduce the possibility of

noise adding into the control loop.

As described in the theory of operation, the ideal slope

can be calculated as:

RSLOPEC = 2. FSW . L . 67700/VOUT

This value can be scaled towards ½ this value to ensure

stability up to and including 100% duty cycle.

FREQ

Connect a resistor between this pin and AGND as close

to the MIC2267 as possible to reduce the possibility of

switching noise causing frequency jitter.

Set the nominal switching frequency using:

FSW = 1/ (C.RFREQ)

Where nominal C = 10pF

INDUCTOR

The MIC2267 was designed to work with 3.3µH to 10µH

inductor values.

If a low ripple voltage output is a key design goal, then

larger value inductors will reduce switching ripple current

and output ripple voltage, but can also have larger DCR

values in small packages; which can reduce efficiency.

Inversely, if high efficiency is the key target, Lower value

inductors will increase switching ripple current and

therefore increase output ripple voltage, but will typically

have lower DCR values in small packages and can

improve efficiency.

As the MIC2267 uses input current limiting, care should

be taken that during a short circuit condition, the inductor

can operate with the power dissipated in it during this

fault condition. Helpfully, The MIC2267 switches the low

side driver off during a short circuit, which ensures most

of the power dissipation occurs within the IC. e.g.:

PFAULT = 5v x 1A = 5W

VNFET ≈ 0.7V

VL_DCR = IIN x DCR = 1A x 50mΩ = 0.05V

Power in the inductor DCR = 7% overall

dissipation.

PL_DCR = 0.36W

The MIC2267 thermal limit protection will therefore limit

the power in the inductor and protect from over

dissipation. Protection is further improved by designing a

low thermal resistance connection between the inductor

and the IC. To achieve this, a short, wide PCB trace

from the inductor to the SW pin is recommended.

Connecting the inductor close to the SW pin is also good

design practice as it minimizes the area available for

radiating switching frequency harmonics around the local

area of the switching regulator.

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