MIC2174 datasheet(20/27 Pages) MICREL

20 / 27 page

Micrel, Inc.

MIC2174/MIC2174C

September 2010

M9999-091310-C

Step 3.

Select Cinj as 100nF, which could be considered

as short for a wide range of the frequencies.

Once R1 is selected, R2 can be calculated using:

Setting Output Voltage

REF

OUT

REF

−

(42)

The MIC2174/MIC2174C requires two resistors to set

the output voltage as shown in Figure 8.

In addition to the external ripple injection added at the

FB pin, internal ripple injection is added at the inverting

input of the comparator inside the MIC2174, as shown in

Figure 9. The inverting input voltage VINJ is clamped to

1.2V. For applications with high VHSD and high VOUT, the

swing of VINJ will be clamped. The clamped VINJ reduces

the line regulation because it is reflected back as a DC

error on the FB terminal. Therefore, the maximum output

voltage of MIC2174 should be limited to 5.5V for up to

28V VHSD and 3.6V for VHSD higher than 28V. If a higher

output voltage is required, use the MIC2176 instead.

Figure 8. Voltage-Divider Configuration

The output voltage is determined by the equation:

)

REF

OUT

(41)

Figure 9. Internal Ripple Injection

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, it will decrease the efficiency of the

power supply, especially at light loads.

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