MIC28304 datasheet(26/39 Pages) MICREL

26 / 39 page

Micrel, Inc.

MIC28304

Application Information

Simplified Input Transient Circuitry

The 76V absolute maximum rating of MIC28304 allows

simplifying the transient voltage suppressor on the input

supply

side

which

very

common

industrial

applications. The input supply voltage VIN Figure 5 may

be operating at 12V input rail most of the time, but can

encounter noise spike of 60V for a short duration. By

using MIC28304, which has 76V absolute maximum

voltage rating, the input transient suppressor is not

needed. Which saves on component count, form factor,

and ultimately the system becomes less expensive.

Figure 5. Simplified Input Transient Circuitry

Setting the Switching Frequency

The MIC28304 switching frequency can be adjusted by

changing the value of resistor R19. The top resistor of

100k

Ω is internal to module and is connected between

VIN and FREQ pin, so the value of R19 sets the

switching

frequency.

The

switching

frequency

also

depends upon VIN, VOUT and load conditions.

Figure 6. Switching Frequency Adjustment

Equation 5 gives the estimated switching frequency:

Ω

100

ADJ

Eq. 5

Where:

fO = Switching frequency when R19 is open

For more precise setting, it is recommended to use

Figure 7:

Figure 7. Switching Frequency vs. R19

Output Capacitor Selection

The type of the output capacitor is usually determined by

the application and its equivalent series resistance

(ESR). Voltage and RMS current capability are two other

important factors for selecting the output capacitor.

Recommended capacitor types are MLCC, tantalum, low-

ESR aluminum electrolytic, OS-CON and POSCAP. The

output capacitor’s ESR is usually the main cause of the

output ripple. The MIC28304 requires ripple injection and

the output capacitor ESR effects the control loop from a

stability point of view.

100

200

300

400

500

600

700

800

10.00

100.00

1000.00

10000.00

R19 (k Ohm)

Switching Frequency

OUT = 5V

OUT = 2A

VIN =48V

VIN = 12V

March 25, 2014

Revision 1.1

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