MIC2168AYMM datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel, Inc.

MIC2168A

A typical value of R1 can be between 3k

Ω and 10kΩ. If

R1 is too large, it may allow noise to be introduced into

the voltage feedback loop. If R1 is too small, in value, it

will decrease the efficiency of the power supply,

especially at light loads. Once R1 is selected, R2 can be

calculated using:

REF

−

External Schottky Diode

An external freewheeling diode is used to keep the

inductor current flow continuous while both MOSFETs

are turned off. This dead time prevents current from

flowing unimpeded through both MOSFETs and is

typically 15ns. The diode conducts twice during each

switching cycle. Although the average current through

this diode is small, the diode must be able to handle the

peak current.

The reverse voltage requirement of the diode is:

VDIODE(rms) = VIN

The power dissipated by the Schottky diode is:

PDIODE = ID(avg) × VF

where:

VF = forward voltage at the peak diode current

The external Schottky diode, D1, is not necessary for

circuit operation since the low-side MOSFET contains a

parasitic body diode. The external diode will improve

efficiency and decrease high frequency noise. If the

MOSFET body diode is used, it must be rated to handle

the peak and average current. The body diode has a

relatively slow reverse recovery time and a relatively

high forward voltage drop. The power lost in the diode is

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

2% to

1% improvement in efficiency.

Feedback Loop Compensation

The MIC2168A controller comes with an internal

transconductance error amplifier used for compensating

the voltage feedback loop by placing a capacitor (C1) in

series with a resistor (R1) and another capacitor C2 in

parallel from the COMP pin to ground. See “Functional

Diagram.”

Power Stage

The power stage of a voltage mode controller has an

inductor,

L1,

with

its

winding

resistance

(DCR)

connected to the output capacitor, COUT, with its

electrical series resistance (ESR) as shown in Figure 3.

Figure 3. The Output LC Filter in a Voltage-Mode

Buck Converter

The transfer function G(s), for such a system is:













−

ESR

DCR

ESR

(S)

Plotting this transfer function with the following assumed

values (L = 2

μH, DCR = 0.009Ω, C

OUT = 1000

μF, ESR =

0.025

Ω) gives lot of insight as to why one needs to

compensate the loop by adding resistor and capacitors

on the COMP pin. Figures 4 and 5 show the gain curve

and phase curve for the above transfer function.

April 22, 2015

Revision 3.0

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