MIC2133 datasheet(21/50 Pages) MICROCHIP

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DS20006653B-page 21

MIC2133

4.4

Ripple Injection Circuit

Components Selection

Follow the steps below for selecting the ripple injection

circuit components if low-ESR output capacitors are

used. The below procedures provide a good starting

point for selecting the ripple injection components. It is

recommended that the final values be confirmed by

laboratory measurements.

Calculate the product of RINJ and CFF for a given

injected Feedback Ripple Voltage, ΔVFB, using

the equation below. Choose ΔVFB in the range

from 40 mV to 500 mV. A good starting point for

ΔVFB is 50 mV.

EQUATION 4-4:

Choose CFF in the range from 0.47 nF to 10 nF.

Calculate RINJ using the equation above.

Calculate the Top Feedback Resistor, RFB(TOP),

value using the equation below:

EQUATION 4-5:

Calculate the Bottom Feedback Resistor,

RFB(BOT), value using the equation below:

EQUATION 4-6:

Estimate the crossover frequency using the fol-

lowing equation. If fCO(EST) is above fSW/5,

lower the CFF value and repeat procedure 6.

EQUATION 4-7:

7a) Select CINJ using the below equation if fCO(EST)

calculated above meets Equation 5-11.

EQUATION 4-8:

Add a resistor in parallel with the soft start

capacitor, connected to the SS pin, if

CINJ > CFF × (RFB(TOP)/RFB(BOT)).

This ensures that there is no overshoot at the

end of the soft start. Use the equation below to

select the parallel resistor value.

EQUATION 4-9:

7b) Select CINJ using the below guidelines if

fCO(EST) is low (typically below fSW/15) when fCO

is limited by the minimum ΔVFB required in lower

VOUT applications. Assume fCO = fSW/10.

Calculate the maximum Equivalent Series

Resistance (ESR) of the output capacitor using

the following equation.

EQUATION 4-10:

Calculate the output capacitance using the fol-

lowing equation.

EQUATION 4-11:

RINJ CFF



5V 100ns



V

----------------------------

Where:

ΔVFB = Injected Feedback Ripple Voltage

RFB TOP







CFF 0.8



fLC



---------------------------------------------------------



Where:

fLC = LC Resonant Frequency = 1/(2 xx sqrt (L x COUT))

RFB BOT



RFB TOP



VOUT

VREF

------------- 1

–

----------------------------

Where:

VOUT = Target Output Voltage

VREF = Reference Voltage = 0.6V for MIC2133

fCO EST



RINJ CFF



 L



COUT



---------------------------------

VOUT 10



fSW

----------------------------



Where:

L = Inductance

COUT = Output Capacitance

VOUT = Output Voltage

fSW = Switching Frequency

CINJ

0.8 RINJ fCO EST





----------------------------------------------------



RSS

0.8V

ISS

-----------



Where:

ISS = Soft Start Current Source = 1.2 µA

ESRCOUT

V

OUT_TRANS

I

LOAD_STEP

----------------------------------



Where:

∆ILOAD_STEP = Magnitude of the Load Transient

∆VOUT_TRANS = Acceptable Output Voltage Deviation

during Load Transient

COUT

 f



ESRCOUT



-------------------------------------------------



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