24

FN6802.2

August 31, 2010

the power stage, since the DC resistance of the current

sense element may be changed according to the operational

resistance of the current sense element at the all operational

temperature.

In certain circumstances, it may be necessary to adjust the

value of one or more ISEN resistors. When the components

of one or more channels are inhibited from effectively

dissipating their heat so that the affected channels run hotter

affected phases (see the section entitled “Voltage

desired decrease in temperature rise in order to cause

proportionally less current to flow in the hotter phase, as

shown in Equation 28:

In Equation 28, make sure that

rise above the ambient temperature, and

temperature rise above the ambient temperature. While a

single adjustment according to Equation 28 is usually

two or more times to achieve optimal thermal balance

between all channels.

Compensation

The ISL6334D converter can be accurately modeled as a

voltage-mode regulator with two poles at the L-C resonant

frequency and a zero at the ESR frequency. A type III

controller, as shown in Figure 17, provides the necessary

compensation.

compensated system. Choose a frequency high enough to

assure adequate transient performance but not higher than

1/3 of the switching frequency. The type III compensator has

added noise rejection or to assure adequate attenuation at

the error-amplifier high-order pole and zero frequencies. A

cause problems with too much phase shift below the system

bandwidth.

In the solutions to the compensation equations, there is a

single degree of freedom. For the solutions presented in

The remaining compensation components are then selected.

In Equation 29, L is the per-channel filter inductance divided

by the number of active channels; C is the sum total of all

output capacitors; ESR is the equivalent-series resistance of

peak-to-peak sawtooth signal amplitude, typically 1.5V.

Output Filter Design

The output inductors and the output capacitor bank together

to form a low-pass filter responsible for smoothing the

pulsating voltage at the phase nodes. The output filter also

must provide the transient energy until the regulator can

respond. Because it has a low bandwidth compared to the

switching frequency, the output filter necessarily limits the

system transient response. The output capacitor must

supply or sink load current while the current in the output

inductors increases or decreases to meet the demand.

In high-speed converters, the output capacitor bank is usually

the most costly (and often the largest) part of the circuit.

Output filter design begins with minimizing the cost of this part

of the circuit. The critical load parameters in choosing the

output capacitors are the maximum size of the load step,

ΔI;

the load-current slew rate, di/dt; and the maximum allowable

output-voltage deviation under transient loading,

Capacitors are characterized according to their capacitance,

ESR, and ESL (equivalent series inductance).

At the beginning of the load transient, the output capacitors

supply all of the transient current. The output voltage will initially

deviate by an amount approximated by the voltage drop across

the ESL. As the load current increases, the voltage drop across

the ESR increases linearly until the load current reaches its final

ΔT

2

ΔT

1

----------

=

(EQ. 28)

FIGURE 17. COMPENSATION CIRCUIT

ISL6333

COMP

FB

VSEN

⋅

1

–

⋅⋅

⋅

()

⋅

2

π

⋅

LC

⋅

() R

⋅⋅

⋅

⋅

⋅

-----------------------------------------------------------------------------------------------------

=

⎝⎠

⎛⎞

⋅

2

⋅⋅

⋅ ⋅ ⋅

⋅

1

–

⋅⋅

⋅

()

⋅

-----------------------------------------------------------------------------------------

=

CESR

⋅

LC

⋅

C ESR

⋅

–

--------------------------------------------

⋅

=

LC

⋅

C ESR

⋅

–

--------------------------------------------

=

2

π

⋅

LC

⋅

() R

⋅⋅

⋅

⋅

⋅

-----------------------------------------------------------------------------------------------------

=

(EQ. 29)

ISL6334D