MIC2295BML datasheet(10/13 Pages) MICREL

10 / 13 page

Micrel

MIC2295

July 2004

M9999-052402

(408) 955-1690

X7R

dielectric

ceramic

capacitors

are

recommended for designs with the MIC2295. Y5V

values may be used, but to offset their tolerance

over temperature, more capacitance is required. The

following table shows the recommended ceramic

(X5R) output capacitor value vs. output voltage.

Output Voltage

Recommended Output

Capacitance

<6V

10µF

<16V

4.7µF

<34V

2.2µF

Diode Selection

The MIC2295 requires an external diode for

operation. A Schottkey diode is recommended for

most applications due to their lower forward voltage

drop and reverse recovery time. Ensure the diode

selected can deliver the peak inductor current and

the maximum reverse voltage is rated greater than

the output voltage.

Input Capacitor

A minimum 1uF ceramic capacitor is recommended

for designing with the MIC2295. Increasing input

capacitance will improve performance and greater

noise immunity on the source. The input capacitor

should be as close as possible to the inductor and

the MIC2295, with short traces for good noise

performance.

Feedback Resistors

The MIC2295 utilizes a feedback pin to compare the

output to an internal reference. The output voltage is

adjusted by selecting the appropriate feedback

resistor values. The desired output voltage can be

calculated as follows;

OUT = VREF

+ 1

Where Vref is equal to 1.24V.

Duty-Cycle

The MIC2295 is a general-purpose step up DC-DC

converter. The maximum difference between the

input voltage and the output voltage is limited by the

maximum duty-cycle (Dmax) of the converter. In the

case of MIC2295, DMAX = 85%. The actual duty

cycle for a given application can be calculated as

follows:

D = 1

OUT

The actual duty-cycle, D, cannot surpass the

maximum rated duty-cycle, Dmax.

Output Voltage Setting

The following equation can be used to select the

feedback resistors R1 and R2 (see figure 1).

1 = R 2

OUT

1.24V

A high value of R2 can increase the whole system

efficiency, but the feedback pin input current (IFB) of

the gm operation amplifier will affect the output

voltage. An R2 value of xx KW is suitable for most

applications

Inductor Selection

In MIC2295, the switch current limit is 1.2A. The

selected inductor should handle at least 1.2A current

without saturating. The inductor should have a low

DC resistor to minimize power losses. The inductor’s

value can be 4.7uH to 10uH for most applications.

Capacitor Selection

Multi-layer ceramic capacitors are the best choice for

input and output capacitors. They offer extremely

low ESR, allowing very low ripple, and are available

in very small, cost effective packages. X5R

dielectrics are preferred. A 4.7uF to 10uF output

capacitor is suitable for most applications.

Diode Selection

For maximum efficiency, Schottky diode is

recommended for use with MIC2295. An optimal

component selection can be made by choosing the

appropriate reverse blocking voltage rating and the

average forward current rating for a given

application. For the case of maximum output voltage

(34V) and maximum output current capability, a 40V

/ 1A Schottky diode should be used.

Open-Circuit Protection

For MLF package option of MIC2295, there is an

output over-voltage protection function that clamps

the output to below 34V in fault conditions. Possible

fault conditions may include: if the device is

configured in a constant current mode of operation

and the load opens, or if in the standard application

the feedback resistors are disconnected from the

circuit. In these cases the FB pin will pull to ground,

causing the MIC2295 to switch with a high duty-

cycle. As a result, the output voltage will climb out of

regulation, causing the SW pin to exceed its

maximum voltage rating and possibly damaging the

IC and the external components.

To ensure the

highest level of safety, the MIC2295 has a dedicated

pin, OVP, to monitor and clamp the output voltage in

over-voltage conditions.

The OVP function is

offered in the 2mm x 2mm MLF-8L package option

only. To disable OVP function, tie the OVP pin to

ground

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