MIC2174 datasheet(16/24 Pages) MICREL

16 / 24 page

Micrel, Inc.

MIC2174

September 2009

M9999-090409-B

proportional to the forward voltage drop of the diode. As

the high-side MOSFET starts to turn on, the body diode

becomes a short circuit for the reverse recovery period,

dissipating additional power. The diode recovery and the

circuit inductance will cause ringing during the high-side

MOSFET turn-on.

An external Schottky diode conducts at a lower forward

voltage preventing the body diode in the MOSFET from

turning on. The lower forward voltage drop dissipates

less power than the body diode. The lack of a reverse

recovery mechanism in a Schottky diode causes less

ringing and less power loss. Depending on the circuit

components and operating conditions, an external

Schottky diode will give a 1/2% to 1% improvement in

efficiency.

Ripple Injection

The minimum FB voltage ripple requested by the

MIC2174 gm amplifier and error comparator is 20mV.

However, the output voltage ripple is generally designed

as 1% to 2% of the output voltage. For a low output

voltage, such as 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 could not sense it, the

MIC2174 will lose control and the output voltage is not

regulated. In order to have some amount of FB voltage

ripple, the 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 6a, the converter is stable without

any adding in this situation. The FB voltage ripple is:

(pp)

FB(pp)

ΔI

ESR

ΔV

OUT

(30)

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

current ripple.

2) Inadequate ripple at the FB voltage due to the small

ESR of the output capacitors.

The output voltage ripple is fed into the FB pin through a

feedforward capacitor Cff in this situation, as shown in

Figure 6b. The typical Cff value is between 1nF to 100nF.

With the feedforward capacitor, the FB voltage ripple is

very close to the output voltage ripple:

(pp)

FB(pp)

ΔI

ESR

ΔV

≈

(31)

3) Invisible ripple at the FB voltage due to the very low

ESR of the output capacitors.

Figure 6a. Enough Ripple at FB

Figure 6b. Inadequate Ripple at FB

Figure 6c. Invisible Ripple at FB

In this situation, the output voltage ripple is less than

20mV. Therefore, additional ripple is injected into the FB

pin from the switching node LX via a resistor Rinj and a

capacitor Cinj, as shown in Figure 6c. The injected ripple

is:

div

HSD

FB(pp)

ΔV

(32)

R1//R2

Rinj

R1//R2

K div

(33)

where

VHSD = Power stage input voltage at HSD pin

D = Duty Cycle

fSW = switching frequency

τ = (R1//R2//Rinj) × Cff

In the formula (32) and (33), it is assumed that the time

constant associated with Cff must be much greater than

the switching period:

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