ISL6553

FN4931 Rev 1.00

Page 6 of 15

August 2004

Operation

Figure 1 shows a simplified diagram of the voltage regulation

and current control loops. Both voltage and current feedback

are used to precisely regulate voltage and tightly control output

loop comprises the Error Amplifier, Comparators, gate drivers

and output MOSFETs. The Error Amplifier is essentially

connected as a voltage follower that has as an input, the

Programmable Reference DAC and an output that is the

CORE voltage.

Voltage Loop

the inverting input of the error amplifier. This signal can drive

the error amplifier output either high or low, depending upon

the CORE voltage. Low CORE voltage makes the amplifier

output move towards a higher output voltage level. Amplifier

output voltage is applied to the positive inputs of the

comparators via the correction summing networks. Out-of-

phase sawtooth signals are applied to the two comparators

inverting inputs. Increasing error amplifier voltage results in

increased comparator output duty cycle. This increased duty

cycle signal is passed through the PWM circuit with no phase

reversal and on to the HIP6601, again with no phase reversal

for gate drive to the upper MOSFETs, Q1 and Q3. Increased

duty cycle or ON time for the MOSFET transistors results in

increased output voltage to compensate for the low output

voltage sensed.

Current Loop

The current control loop works in a similar fashion to the

voltage control loop, but with current control information

applied individually to each channel’s comparator. The

information used for this control is the voltage that is developed

they are conducting. A single resistor converts and scales the

voltage across the MOSFETs to a current that is applied to the

current sensing circuit within the ISL6553. Output from these

sensing circuits is applied to the current averaging circuit. Each

PWM channel receives the difference current signal from the

summing circuit that compares the average sensed current to

the individual channel current. When a power channel’s current

is greater than the average current, the signal applied via the

FIGURE 1. SIMPLIFIED BLOCK DIAGRAM OF THE ISL6553 VOLTAGE AND CURRENT CONTROL LOOPS FOR A TWO POWER

CHANNEL REGULATOR

CURRENT

SENSING

COMPARATOR

PWM

CIRCUIT

+

+

CORRECTION

ERROR

AMPLIFIER

FB

REFERENCE

ISEN1

Q3

Q4

L2

PHASE

PWM1

DAC

ISL6553

HIP6601

-

Q1

Q2

L1

PHASE

HIP6601

CURRENT

SENSING

COMPARATOR

PWM

CIRCUIT

CORRECTION

PWM2

-

I AVERAGE

+

+

+

-

PROGRAMMABLE

ISEN2

-

-

-

-

+

+

CURRENT

AVERAGING

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