ISL6552

FN4918 Rev 2.00

Page 12 of 18

July 2004

Current Sensing and Balancing

Overview

The ISL6552 samples the on-state voltage drop across each

synchronous rectifier FET, Q2, as an indication of the inductor

current in that phase, see Figure 7. Neglecting AC effects (to

be discussed later), the voltage drop across Q2 is simply

depending on how many phases are in use.

The voltage at Q2’s drain, the PHASE node, is applied to the

ISEN pin. This pin is held at virtual ground, so the current

functions:

1. Detection of an over-current condition

2. Reduce the regulator output voltage with increasing load

current (droop)

with the current(s) detected in the other 1, 2, or 3 channels.

The averaged current is compared with a trimmed, internally

generated current, and used to detect an over-current

condition.

50

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

over-current detection is 165% of that value, or 82.5

A.

The over-current trip point would be 165% of 25A, or ~41A per

current trip point, but it is suggested to stay within

25% of

nominal.

resistors is also steered to the FB pin. There is no DC return

the FB pin. This is the desired “droop” voltage used to maintain

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

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

ISL6552

ONLY ONE OUTPUT

HIP6601

-

-

STAGE SHOWN

SENSING

+

-

+

-

+

-