MIC28514 datasheet(21/34 Pages) MICROCHIP

21 / 34 page

 2017-2021 Microchip Technology Inc.

DS20005693G-page 21

MIC28514

5.8

Ripple Injection

The VFB ripple required for proper operation of the

MIC28514 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 1V, the output voltage ripple is

only 10 mV-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 MIC28514 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 5-2).

The converter is stable without any ripple

injection.

FIGURE 5-2:

Enough Ripple at FB.

The feedback voltage ripple is:

EQUATION 5-19:

Inadequate ripple at the feedback voltage due to

the small ESR of the output capacitors.

In this situation, the output voltage ripple is fed into

the FB pin through a Feed-Forward Capacitor, Cff,

as shown in Figure 5-3. The typical Cff value is

between 1 nF and 22 nF.

FIGURE 5-3:

Inadequate Ripple at FB.

With the feed-forward capacitor, the feedback

voltage ripple is very close to the output voltage

ripple.

EQUATION 5-20:

Virtually no ripple at the FB pin voltage due to

the very low-ESR of the output capacitors.

In this situation, the output voltage ripple is less

than 20 mV. Therefore, additional ripple is

injected into the FB pin from the Switching

Node, SW, via a resistor, RINJ, and a capacitor,

CINJ, as shown in Figure 5-4.

FIGURE 5-4:

Invisible Ripple at FB.

The injected ripple is:

EQUATION 5-21:

EQUATION 5-22:

In Equation 5-21 and Equation 5-22, it is assumed that

the time constant associated with Cff must be much

greater than the switching period:

EQUATION 5-23:

OUT

ESR

MIC28514

VFB PP





R1 R2

------------------- ESR

COUT



ILPP







Where:

ΔIL(PP) = Peak-to-Peak Value of the Inductor

Current Ripple

ESR

MIC28514

OUT

VFB(PP)  ESRCOUT  IL(PP)

COUT

ESR

MIC28514

CFF

CINJ

RINJ

VFB PP





VIN KDIV



–



fSW 



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



Where:

VIN

= Power Stage Input Voltage

= Duty Cycle

fSW = Switching Frequency

=(R1//R2//RINJ) x Cff

KDIV

R1//R2

RINJ R1//R2

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

fSW 



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



---1

MIC28514 датащи(PDF) 21 Page - Microchip Technology

MIC28514 датащи(HTML) 21 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

датащи скачать

ссылки URL