 2016 Microchip Technology Inc.

DS20005669A-page 15

MIC4930

losses in the device. All but the inductor losses are

inherent to the device. In that case, inductor selection

becomes increasingly critical in efficiency calculations.

As the inductors are reduced in size, the DC resistance

(DCR) can become quite significant. The DCR losses

can be calculated as in Equation 5-3.

EQUATION 5-3:

From that, the loss in efficiency due to inductor DCR

Equation 5-4.

EQUATION 5-4:

5.5

External Ripple Injection

The MIC4930 control loop is ripple-based, and relies on

an internal ripple injection network to generate enough

ripple amplitude at the FB pin when negligible output

voltage ripple is present. The internal ripple injection

network is typically sufficient when recommended

amplitude should fall in the 20 mV to 100 mV range.

values are used, the amount of internal ripple injection

may not be sufficient for stable operation. In this case,

external

ripple

injection

is

needed.

This

is

between the SW and the FB pins, as shown in

FIGURE 5-1:

External Ripple Injection.

The injected ripple is:

EQUATION 5-5:

EQUATION 5-6:

In Equation 5-5 and Equation 5-6, it is assumed that

than the switching period.

EQUATION 5-7:

P

DCR

I

OUT

2

DCR



=

Efficiency Loss (%)

1

V

OUT

I

OUT



V

OUT

I

OUT

P

DCR

P

CORE

++



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





–

100



=

MIC4930

PGND AGND

SW

GND

GND

VOUT

FB

R1

R2

V

FB PP



V

IN

K

div



D



1

D

–





1

f

SW





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



=

K

div

R1//R2

R

INJ

R1//R2

+

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

=

1

f

SW





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

T



---

1

«

=

Where:

Power stage input voltage

D =

Switching frequency