MIC2132YML datasheet(21/48 Pages) MICROCHIP

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DS20006654B-page 21

MIC2132

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. Final values

should 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

of 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 above equation.

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

Equation 4-7. 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 con-

nected 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 Equation 4-9 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

Equation 4-10.

EQUATION 4-10:

Calculate the output capacitance using Equation 4-11.

EQUATION 4-11:

Calculate CINJ using Equation 4-12.

EQUATION 4-12:

Using too low a CINJ may result in oscillations at the

beginning of the soft start. These oscillations can be

reduced either by using a higher CINJ or COUT, by

reducing the feedback ripple.

Where:

ΔVFB = Injected Feedback Ripple Voltage

RINJ × CFF =

5V × 100 ns

VFB

Where:

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

RFB(TOP) ≥

2 × π × CFF × 0.8 × fLC

Where:

VOUT = Target Output Voltage

VREF = Reference Voltage = 0.6V for MIC2132

RFB(BOT) =

RFB(TOP)

VOUT

VREF

– 1

[]

Where:

L = Inductance

COUT = Output Capacitance

VOUT = Output Voltage

fSW = Switching Frequency

fCO(EST) =

RINJ × CFF

π × L × COUT

VOUT × 106

fSW

CINJ ≥

0.8 × RINJ × fCO(EST)

Where:

ISS = Soft Start Current Source = 1.2 µA

RSS ≥

0.8V

ISS

Where:

ΔILOAD_STEP = Magnitude of the Load Transient

ΔVOUT_TRANS = Acceptable Output Voltage Deviation

during Load Transient

ESRCOUT ≤

VOUT_TRANS

ILOAD_STEP

COUT ≥

π × fCO × ESRCOUT

CINJ = CFF ×

ESRCOUT

2 × π × fCO × L

VOUT

5V × 100 ns × fSW

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