11

FN9003.3

February 11, 2005

50

µA at full output load current, and the nominal trip point for

overcurrent detection is 165% of that value, or 82.5

µA.

4m

The overcurrent trip point would be 165% of 25A, or ~ 41A

overcurrent trip point, but it is suggested to stay within

±25%

of nominal.

Current Balancing

The detected currents are also used to balance the phase

currents.

Each phase’s current is compared to the average of all

phase currents, and the difference is used to create an offset

in that phase’s PWM comparator. The offset is in a direction

to reduce the imbalance.

The balancing circuit can not make up for a difference in

Figures 8 and 9 show the inductor current of a two phase

system without and with current balancing.

Inductor Current

The inductor current in each phase of a multi-phase Buck

converter has two components. There is a current equal to

The sawtooth component is dependent on the size of the

inductors, the switching frequency of each phase, and the

values of the input and output voltage. Ignoring secondary

effects, such as series resistance, the peak to peak value of

the sawtooth current can be described by:

L= value of the inductor

L= 1.3

µH,

through Q1 and from ground through Q2. The ISL6554

samples the on-state voltage drop across each Q2 transistor

to indicate the inductor current in that phase. The voltage

turned OFF and Q2 is turned on. Because of the sawtooth

current component, the sampled current is different from the

average current per phase. Neglecting secondary effects,

Where:

n = the number of channels

Example: Using the previously given conditions, and

For

n = 4

= 25.49A

FIGURE 7. SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM SHOWING CURRENT AND VOLTAGE SAMPLING

CURRENT

SENSING

COMPARATOR

PWM

CIRCUIT

AVERAGING

CURRENT

FROM

OTHER

CHANNELS

SAWTOOTH

GENERATOR

+

DIFFERENCE

+

CORRECTION

ERROR

AMPLIFIER

FB

COMP

REFERENCE

TO OTHER

CHANNELS

ISEN

Q1

Q2

COMPARATOR

REFERENCE

TO OVER

CURRENT

TRIP

PHASE

INDUCTOR

CURRENT(S)

FROM

OTHER

CHANNELS

PWM

DAC

ISL6554

ONLY ONE OUTPUT

HIP6601

-

-

STAGE SHOWN

SENSING

+

-

+

-

+

-

ISL6554