MIC2267 datasheet(11/19 Pages) MICREL

11 / 19 page

Micrel Inc.

MIC2267

January 2011

M9999-011711-A

SLOPEC

To guarantee stability in a current mode controller

operating at duty cycle >50%, a compensation ramp (m)

is required. This ramp ‘m’ is added to the inductor

current sense ramp (or alternatively, it can also be

subtracted from the error voltage which has the same

effect).

A resistor to ground from the SLOPEC pin sets the

amplitude of ‘m’ over the switching period.

Ideally, the magnitude of the compensation ramp (m) is

for many cases, set to ½ inductor discharge ramp (m2).

I.E. m= ½ m2

Where:

∝ Vo/L x FSW

and

∝ 67700/RSLOPEC

Therefore, for the ideal ½ m2 slope:

RSLOPEC = 2 x FSW x L x 67700/Vout

Where duty cycle can approach 100%, excess phase

shift in the loop can lead to a phenomenon called “sub-

harmonic oscillation”. Additional compensating slope ‘m’

may be required in these cases.

It can be shown that ensuring m = m2 will ensure

stability regardless of voltage loop gain for the case

where DC ~ 100%. Therefore, a value of RSLOPEC = FSW.x

L x 67700/Vout will ensure stability at the expense of

some line regulation.

AGND and PGND

Connect AGND to the quiet, signal reference points and

decouple closely to the IC. Connect PGND to the high

current carrying paths close to the COUT and CIN. Refer to

layout guide section later in the datasheet for more

detailed information.

FREQ

A resistor RFREQ from this pin to ground allows the

frequency of the MIC2267 to be programmed over the

range 400kHz to 1.5MHz.

Figure 3. MIC2267 FREQ Pin Internal Blocks

To set the frequency, this pin is forced to V1, the

resulting current through the resistor RFREQ sets the

internal reference current which charges a capacitor (C).

At the beginning of the clock cycle, C begins charging.

When it reaches V2, a clock pulse is generated and C is

discharged to ground; marking the beginning of the next

clock cycle.

This produces a clock frequency FSW of:

FSW = 1/(C x RFREQ)

Where C = 10pF

This is the center connection output of the P-channel

and N-channel Power MOSFETs. Connect this pin to the

Power inductor as close to the IC as possible. Refer to

the Layout Recommendations section for details on

placing the power inductor.

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