MIC26901 datasheet(21/30 Pages) MICREL

21 / 30 page

Micrel, Inc.

MIC26901

July 2011

M9999-071311-A

The process of sizing the ripple injection resistor and

capacitors is:

Step 1.

Select Cff to feed all output ripples into the

feedback pin and make sure the large time constant

assumption is satisfied. Typical choice of Cff is 1nF to

100nF if R1 and R2 are in kΩ range.

Step 2.

Select Rinj according to the expected feedback

voltage ripple using Equation 22:

ΔV

FB(pp)

div

−

Eq. 21

Then the value of Rinj is obtained as:

(

(R1//R2)

div

inj

−

Eq. 22

Step 3.

Select Cinj as 100nF, which could be considered

as short for a wide range of the frequencies.

Setting Output Voltage

The MIC26603 requires two resistors to set the output

voltage as shown in Figure 7.

The output voltage is determined by Equation 23:

)

OUT

Eq. 23

where VFB = 0.8V.

A typical value of R1 can be between 3kΩ and 10kΩ. If

R1 is too large, it may allow noise to be introduced into

the voltage feedback loop. If R1 is too small, it will

decrease the efficiency of the power supply, especially

at light loads. Once R1 is selected, R2 can be calculated

using:

OUT

−

Eq. 24

Figure 6. Voltage-Divider Configuration

In addition to the external ripple injection added at the

FB pin, internal ripple injection is added at the inverting

input of the comparator inside the MIC26901, as shown

in Figure 8. The inverting input voltage VINJ is clamped to

1.2V. As VOUT is increased, the swing of VINJ will be

clamped. The clamped VINJ reduces the line regulation

because it is reflected as a DC error on the FB terminal.

Therefore, the maximum output voltage of the MIC26901

should be limited to 5.5V to avoid this problem.

Figure 7. Internal Ripple Injection

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