MIC2126 datasheet(23/34 Pages) MICROCHIP

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DS20005459B-page 23

MIC2125/6

5.6

Output Voltage Setting

The MIC2125/26 requires two resistors to set the

output voltage, as shown in Figure 5-3.

FIGURE 5-3:

Voltage-Divider

Configuration.

The output voltage is determined by Equation 5-21:

EQUATION 5-21:

A typical value of R1 can be in the range of 3 kΩ and

15 kΩ. 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 Equation 5-22.

EQUATION 5-22:

5.7

Output Overvoltage Limit Setting

The output overvoltage limit should be typically 20%

higher than the nominal output voltage. Set the OVP

limit by connecting a resistor divider from the output to

ground as shown in Figure 5-4.

FIGURE 5-4:

OVP Voltage-Divider

Configuration.

Choose R2 in the range of 10 kΩ to 49.9 kΩ and

calculate R1 using Equation 5-23.

EQUATION 5-23:

5.8

Ripple Injection

The VFB ripple required for proper operation of the

MIC2125/6 gm amplifier and comparator is 20 mV to

100 mV. However, the output voltage ripple is generally

designed as 1% to 2% of the output voltage. For low

output voltages, such as a 1V, the output voltage ripple

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

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

small that the gm amplifier and comparator cannot

sense it, then the MIC2125/6 loses control and the

output voltage is not regulated. In order to have

sufficient VFB ripple, a ripple injection method should

be applied for low output voltage ripple applications.

The applications are divided into three situations

according to the amount of the feedback voltage ripple:

• Enough ripple at the feedback voltage due to the

large ESR of the output capacitors (Figure 5-5).

The converter is stable without any ripple

injection.

m AMP

REF

OUT

-------







Where:

VFB

0.6V



OUT

–

-----------------------------

OVP

REF

OVP

0.6

-------------

–

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