MIC2103 datasheet(25/36 Pages) MICREL

25 / 36 page

Micrel, Inc.

MIC2103/04

August 2012

M9999-080712-A

smaller than the ripple caused by the output capacitor

ESR. If low ESR capacitors, such as ceramic capacitors,

are selected as the output capacitors, a ripple injection

method should be applied to provide enough feedback

voltage ripple. Please refer to the “Ripple Injection”

subsection for more details.

The voltage rating of the capacitor should be twice the

output voltage for a tantalum and 20% greater for

aluminum electrolytic or OS-CON. The output capacitor

RMS current is calculated in Equation 21:

ΔI

L(PP)

(RMS)

COUT



(Eq. 21)

The power dissipated in the output capacitor is:

OUT

(RMS)

)

DISS(C

ESR





(Eq. 22)

Input Capacitor Selection

The input capacitor for the power stage input VIN should

be selected for ripple current rating and voltage rating.

Tantalum input capacitors may fail when subjected to

high inrush currents, caused by turning the input supply

on. A tantalum input capacitor’s voltage rating should be

at least two times the maximum input voltage to

maximize reliability. Aluminum electrolytic, OS-CON, and

multilayer polymer film capacitors can handle the higher

inrush currents without voltage de-rating. The input

voltage ripple will primarily depend on the input

capacitor’s ESR. The peak input current is equal to the

peak inductor current, so:

ΔVIN = IL(pk) × ESRCIN

(Eq. 23)

The input capacitor must be rated for the input current

ripple. The RMS value of input capacitor current is

determined at the maximum output current. Assuming

the peak-to-peak inductor current ripple is low:

OUT(max)

CIN(RMS)







(Eq. 24)

The power dissipated in the input capacitor is:

PDISS(CIN) = ICIN(RMS)

2 × ESRCIN

(Eq. 25)

Voltage Setting Components

The MIC2103 requires two resistors to set the output

voltage as shown in Figure 7:

Figure 7. Voltage-Divider Configuration

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

MIC2103/04 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 cannot sense it, then the MIC2103/04 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:

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