MIC2124YMM datasheet(14/24 Pages) MICREL

14 / 24 page

Micrel, Inc.

MIC2124

June 2010

M9999-060810-D

where:

D = duty cycle

COUT = output capacitance value

fSW = switching frequency

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

(20)

The power dissipated in the output capacitor is:

OUT

(RMS)

)

DISS(C

ESR

⋅

(21)

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:

L(pk)

ESR

ΔV

⋅

(22)

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)

(RMS)

CIN

−

⋅

≈

(23)

The power dissipated in the input capacitor is:

(RMS)

)

DISS(C

ESR

⋅

(24)

Voltage Setting Components

The MIC2124 requires two resistors to set the output

voltage as shown in Figure 5.

The output voltage is determined by the equation:

)

REF

OUT

⋅

(25)

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

−

⋅

(26)

Figure 5. Voltage-Divider Configuration

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

⋅

(27)

The reverse voltage requirement of the diode is:

HSD

DIODE(rrm)

The power dissipated by the Schottky diode is:

D(avg)

DIODE

(28)

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

proportional to the forward voltage drop of the diode. As

the high-side MOSFET starts to turn on, the body diode

becomes a short circuit for the reverse recovery period,

dissipating additional power. The diode recovery and the

circuit inductance will cause ringing during the high-side

MOSFET turn-on.

An external Schottky diode conducts at a lower forward

voltage preventing the body diode in the MOSFET from

turning on. The lower forward voltage drop dissipates

less power than the body diode. The lack of a reverse

recovery mechanism in a Schottky diode causes less

ringing and less power loss. Depending on the circuit

components and operating conditions, an external

Schottky diode will give a 0.5% to 1% improvement in

efficiency.

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