MIC2174_10 Datasheet, PDF(18/27 Page) Micrel Semiconductor – Synchronous Buck Controller Featuring Adaptive On-Time Control 40V Input, 300kHz

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MIC2174_10 Datasheet, PDF (18/27 Pages) Micrel Semiconductor – Synchronous Buck Controller Featuring Adaptive On-Time Control 40V Input, 300kHz

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Micrel, Inc.

A proper snubber design requires the parasitic

inductance and capacitance be known. A method of

determining these values and calculating the damping

resistor value is outlined below.

1. Measure the ringing frequency at the switch node

which is determined by parasitic LP and CP. Define

this frequency as f1.

2. Add a capacitor CS (such as two times as big as the

COSS of the FET) from the switch node-to-ground

and measure the new ringing frequency. Define this

new (lower) frequency as f2. LP and CP can now be

solved using the values of f1, f2 and CS.

3. Add a resistor RS in series with CS to generate

critical damping.

Step 1: First measure the ringing frequency on the

switch node voltage when the high-side MOSFET turns

on. This ringing is characterized by the equation:

f1 = 2Ï

LP Ã CP

(28)

where CP and LP are the parasitic capacitance and

inductance.

Step 2: Add a capacitor, CS, in parallel with the

synchronous MOSFET, Q2. The capacitor value should

be approximately two times the COSS of Q2. Measure the

frequency of the switch node ringing, f2:

f2 =

(29)

2Ï Lp Ã (Cs + Cp)

Define fâ as:

f ' = f1

Combining the equations for f1, f2 and fâ to derive CP, the

parasitic capacitance:

(f' )2 â 1

(30)

LP is solved by re-arranging the equation for f1:

(2Ï)2

Ã CP

Ã (f1)2

(31)

MIC2174/MIC2174C

Step 3: Calculate the damping resistor.

Critical damping occurs at Q = 1:

Q = RS Ã

CP = 1

(32)

Solving for RS

RS =

(33)

Figure 6 shows the snubber in the circuit and the

damped switch node waveform. The snubber capacitor,

CS, is charged and discharged each switching cycle. The

energy stored in CS is dissipated by the snubber resistor,

RS, two times per switching period. This power is

calculated in Equation 34:

PSNUBBER = fSW Ã CS Ã VIN2

(34)

Ripple Injection

The VFB ripple required for proper operation of the

MIC2174/MIC2174C gm amplifier and error comparator

is 20mV to 100mV. However, the output voltage ripple is

generally designed as 1% to 2% of the output voltage.

For a low output voltage, such as a 1V output, the output

voltage ripple is only 10mV to 20mV, and the FB voltage

ripple is less than 20mV. If the FB voltage ripple is so

small that the gm amplifier and error comparator canât

sense it, then the MIC2174/MIC2174C will lose control

and the output voltage will not be regulated. In order to

have some amount of VFB voltage ripple, a ripple

injection method is applied for low output voltage ripple

applications.

The applications are divided into three situations

according to the amount of the FB voltage ripple:

1. Enough ripple at the FB voltage due to the large

ESR of the output capacitors.

As shown in Figure 7a, the converter is stable without

any ripple injection. The FB voltage ripple is:

ÎVFB(pp)

R1+ R2

ESR

COUT

Ã ÎIL (pp)

(35)

where ÎIL(pp) is the peak-to-peak value of the inductor

current ripple.

September 2010

M9999-091310-C

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