MIC4950 datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel, Inc.

MIC4950

March 20, 2014

Revision 1.1

At the higher currents for which the MIC4950 is designed,

efficiency loss is dominated by MOSFET RDSON and

inductor losses. Higher input supply voltages will increase

the gate-to-source threshold on the internal MOSFETs,

thereby reducing the internal RDSON. This improves

efficiency by reducing DC losses in the device. All but the

inductor losses are inherent to the device. In this case,

inductor

selection

becomes

increasingly

critical

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 4.

DCR

OUT

DCR



 2

Eq. 4

From that, the loss in efficiency due to inductor DCR and

core losses (PCORE) can be calculated as in Equation 5.

100

(%)

Loss

Ef f iciency

CORE

DCR

OUT



































Eq. 5

External Ripple Injection

The MIC4950 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 R1-R2

and CF values are used. The FB ripple amplitude should

fall in the 20mV to 100mV range.

If significantly lower divider resistors and/or higher CF

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 accomplished

by connecting a series Rinj-Cinj circuit between the SW

and the FB pins, as shown in Figure 1.

Figure 1. External Ripple Injection

The injected ripple is calculated using Equation 6,







div

FB(pp)

ΔV

Eq. 6

with Kdiv given by Equation 7

R1//R2

inj

div





Eq. 7

and:

VIN = Power stage input voltage

D = VOUT/VIN = Duty cycle

fSW = Switching frequency

= (R1//R2//R

inj) × CF

In Equations 6 and 7, it is assumed that the time constant

associated with CF must be much greater than the

switching period:











Eq. 8

MIC4950 Datasheet(PDF) 13 Page - Micrel Semiconductor

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