MIC2174 datasheet(15/24 Pages) MICREL

15 / 24 page

Micrel, Inc.

MIC2174

September 2009

M9999-090409-B

where:

D = duty cycle

COUT = output capacitance value

fSW = switching frequency

As described in the “Theory of Operation” subsection in

“Functional Description”, the MIC2174 requires at least

20mV peak-to-peak ripple at the FB pin to make the gm

amplifier and the error comparator to behavior properly.

Also, the output voltage ripple should be in phase with

the inductor current. Therefore the output voltage ripple

caused by the output capacitor COUT should be much

smaller than the ripple caused by the output capacitor

ESR. If low ESR capacitors are selected as the output

capacitors, such as ceramic capacitors, a ripple injection

method is applied to provide the enough FB voltage

ripples. 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 below:

ΔI

L(PP)

(RMS)

COUT

(21)

The power dissipated in the output capacitor is:

OUT

(RMS)

)

DISS(C

ESR

(22)

Input Capacitor Selection

The input capacitor for the power stage input VHSD

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

(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)

−

≈

(24)

The power dissipated in the input capacitor is:

PDISS(CIN) = ICIN(RMS)

2 × ESR

CIN

(25)

Voltage Setting Components

The MIC2174 requires two resistors to set the output

voltage as shown in Figure 5.

Figure 5. Voltage-Divider Configuration

The output voltage is determined by the equation:

)

REF

OUT

(26)

where, VREF = 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:

REF

OUT

REF

−

(27)

External Schottky Diode (Optional)

An external freewheeling diode, which is not necessary,

is used to keep the inductor current flow continuous

while both MOSFETs are turned off. This dead time

prevents current from flowing unimpeded through both

MOSFETs and is typically 30ns. The diode conducts

twice during each switching cycle. Although the average

current through this diode is small, the diode must be

able to handle the peak current.

OUT

D(avg)

30ns

(28)

The reverse voltage requirement of the diode is:

HSD

DIODE(rrm)

The power dissipated by the Schottky diode is:

D(avg)

DIODE

(29)

where, VF = forward voltage at the peak diode current.

The external Schottky diode is not necessary for the

circuit operation since the low-side MOSFET contains a

parasitic body diode. The external diode will improve

efficiency and decrease the high frequency noise. If the

MOSFET body diode is used, it must be rated to handle

the peak and average current. The body diode has a

relatively slow reverse recovery time and a relatively

high forward voltage drop. The power lost in the diode is

MIC2174 Scheda tecnica(PDF) 15 Page - Micrel Semiconductor

MIC2174 Scheda tecnica(HTML) 15 Page - Micrel Semiconductor

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

Scheda tecnica Scarica

Link URL