MIC28303 datasheet(26/38 Pages) MICROCHIP

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MIC28303

DS20005464A-page 26

 2016 Microchip Technology Inc.

5.5

Output Voltage Setting

Components

The MIC28303 requires two resistors to set the output

voltage, as shown in Figure 5-4:

FIGURE 5-4:

Voltage-Divider

Configuration.

The output voltage is determined by Equation 5-10:

EQUATION 5-10:

A typical value of R1 used on the standard evaluation

board is 10 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, R11 can be calculated using Equation 5-11:

EQUATION 5-11:

5.6

Ripple Injection

The VFB ripple required for proper operation of the

MIC28303 gm amplifier and error comparator is 20 mV

to 100 mV. 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 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 error

comparator cannot sense it, then the MIC28303 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. Table 2-2 summarizes the ripple injection

component values for ceramic output capacitor.

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):

FIGURE 5-5:

Enough Ripple at FB.

As shown in Figure 5-6, the converter is stable

without any ripple injection.

FIGURE 5-6:

Inadequate Ripple at FB.

The feedback voltage ripple is:

EQUATION 5-12:

• Inadequate ripple at the feedback voltage due to

the small ESR of the output capacitors, such is

the case with ceramic output capacitor.

The output voltage ripple is fed into the FB pin

through a feed-forward capacitor, Cff in this situation,

as shown in Figure 5-7. The typical Cff value is

between 1 nF and 100 nF.

m Amp

R11

REF

OUT

R11

----------







Where:

VFB

0.8V

R11



OUT

–

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

MIC28303

VOUT

R11

ESR

OUT

MIC28303

VOUT

R11

ESR

OUT

FB PP





R11

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

ESR

OUT



LPP







Where:

∆IL(PP)

Peak-to-Peak Value of the Inductor

Current Ripple

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