MIC2102YML datasheet(28/37 Pages) MICREL

28 / 37 page

Micrel, Inc.

MIC2101/02

August 2012

M9999-080712-A

The output voltage is determined by the equation:

)

OUT







Eq. 26

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 in value, it will decrease the efficiency of the power

supply, especially at light loads. Once R1 is selected, R2

can be calculated using:

OUT







Eq. 27

Ripple Injection

The VFB ripple required for proper operation of the

MIC2101/02 gm amplifier and error comparator is 20mV

to 100mV. However, the output voltage ripple is

generally designed as 1% to 2% of the output voltage.

For a low output voltage, such as a 1V, the output

voltage ripple is only 10mV to 20mV, and the feedback

voltage ripple is less than 20mV. If the feedback voltage

ripple is so small that the gm amplifier and error

comparator can’t sense it, then the MIC2101/02 will lose

control and the output voltage is not regulated. In order

to have some amount of VFB ripple, a ripple injection

method is applied for low output voltage ripple

applications.

The applications are divided into three situations

according to the amount of the feedback voltage ripple:

1. Enough ripple at the feedback voltage due to the

large ESR of the output capacitors.

As shown in Figure 8a, the converter is stable

without any ripple injection. The feedback voltage

ripple is:

(pp)

FB(pp)

∆I

ESR

∆V

OUT







Eq. 28

where ∆IL(pp) is the peak-to-peak value of the

inductor current ripple.

2. Inadequate ripple at the feedback voltage due to the

small ESR of the output capacitors.

The output voltage ripple is fed into the FB pin

through a feedforward capacitor Cff in this situation,

as shown in Figure 8b. The typical Cff value is

between 1nF and 100nF. With the feedforward

capacitor, the feedback voltage ripple is very close

to the output voltage ripple:

(pp)

FB(pp)

∆I

ESR

∆V





Eq. 29

3. Virtually no ripple at the FB pin voltage due to the

very-low ESR of the output capacitors:

Figure 8a. Enough Ripple at FB

Figure 8b. Inadequate Ripple at FB

Figure 8c. Invisible Ripple at FB

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