MIC45208-2 datasheet(21/36 Pages) MICROCHIP

21 / 36 page



2017 - 2022 Microchip Technology Inc. and its subsidiaries

DS20005603B-page 21

MIC45208-1/-2

The output voltage is determined by Equation 5-10:

EQUATION 5-10: OUTPUT VOLTAGE

DETERMINATION

A typical value of RFB1 used on the standard evaluation

board is 10 k. If RFB1 is too large, it may allow noise

to be introduced into the voltage feedback loop. If RFB1

is too small in value, it will decrease the efficiency of the

power supply, especially at light loads. Once RFB1 is

selected, RFB2 can be calculated using Equation 5-11:

EQUATION 5-11: CALCULATING RFB2

For fixed RFB1 = 10 k, the output voltage can be

selected by RFB2. Table 5-1 provides RFB2 values for

some common output voltages.

TABLE 5-1:

VOUT PROGRAMMING

RESISTOR LOOK-UP

5.5

Ripple Injection

The VFB ripple required for proper operation of the

MIC45208 gM amplifier and error comparator is 20 mV

to 100 mV. However, the output voltage ripple is gener-

ally too small to provide enough ripple amplitude at the

FB pin and this issue is more visible in lower output

voltage applications. If the feedback voltage ripple is so

small that the gM amplifier and error comparator cannot

sense it, then the MIC45208 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 two 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 5-4, the converter is stable

without any ripple injection.

FIGURE 5-4:

Enough Ripple at FB from

ESR.

The feedback voltage ripple is:

EQUATION 5-12: FEEDBACK VOLTAGE

RIPPLE

2. There is virtually inadequate or no ripple at the

FB pin voltage due to the very low-ESR of the

output capacitors; such is the case with the

ceramic output capacitor. In this case, the VFB

ripple waveform needs to be generated by

injecting a suitable signal. MIC45208 has provi-

sions to enable an internal series RC injection

network, RINJ and CINJ, as shown in Figure 5-5,

by connecting RIB to the FB pin. This network

injects a square wave current waveform into the

FB pin, which by means of integration across the

capacitor (C14), generates an appropriate

sawtooth FB ripple waveform.

FIGURE 5-5:

Internal Ripple Injection at

FB via RIB Pin.

RFB2

VOUT

OPEN

0.8V

40.2 k

1.0V

20 k

1.2V

11.5 k

1.5V

8.06 k

1.8V

4.75 k

2.5V

3.24 k

3.3V

1.91 k

5.0V

VOUT = VFB  1 +

RFB1

RFB2









Where:

VFB = 0.8V

RFB2 =

VFB  RFB1

VOUT – VFB

FB1

FB2

ESR

OUT

MIC45208



VFB(PP) 

RFB2

RFB1  RFB2



ESRCOUT  IL(PP)

Where:



IL(PP) = The peak-to-peak value of the inductor

current ripple

FB1

FB2

ESR

OUT

MIC45208

C14

RIB

RIA

INJ

MIC45208-2 Datasheet(PDF) 21 Page - Microchip Technology

MIC45208-2 Datasheet(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 35 36

Datasheet Download

Link URL