11

With a high dv/dt load transient, typical of high performance

microprocessors, the largest deviations in output voltage

occur at the leading and trailing edges of the load transient. In

order to fully utilize the output-voltage tolerance range, the

output voltage is positioned in the upper half of the range

when the output is unloaded and in the lower half of the range

when the controller is under full load. This droop

compensation allows larger transient voltage deviations and

thus reduces the size and cost of the output filter components.

the normal full load current 50

resistor (or at a different full load current if adjusted as under

Current Balancing

The detected currents are also used to balance the phase

currents.

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

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

reduced.

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 ISL6553

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,

= total load current

n = the number of channels

Example: Using the previously given conditions, and

n= 2

0

5

10

15

20

25

FIGURE 8. TWO CHANNEL multi-phase SYSTEM WITH

CURRENT BALANCING DISABLED

0

5

10

15

20

25

FIGURE 9. TWO CHANNEL multi-phase SYSTEM WITH

CURRENT BALANCING ENABLED

ISL6553