CS5308 datasheet(13/31 Pages) ONSEMI

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CS5308

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operation. The inductor current provides a portion of the

PWM ramp through the Current Sense Amplifier. The PWM

cycle ends when the sum of the current ramp, the “partial”

internal ramp voltage signal and Offset exceed the level of

the COMP pin. At T1 the output current increases and the

output voltage sags. The next PWM cycle begins and the

cycle continues longer than previously while the current

signal increases enough to make up for the lower voltage at

the VFB pin and the cycle ends at T2. After T2 the output

voltage remains lower than at light load and the average

current signal level (CSn output) is raised so that the sum of

the current and voltage signal is the same as with the original

load. In a closed loop system the COMP pin would move

higher to restore the output voltage to the original level.

SWNODE

VFB (VOUT)

Internal Ramp

CSA Out w/

Exaggerated

Delays

COMP−Offset

CSA Out + Ramp + CSREF

Figure 10. Open Loop Operation

Figure 11. Enhanced V2 Control Employing Lossless Inductive Current Sensing and Internal Ramp

SWNODE

RCSn

RLn

CSn

CSA

COn

CSREF

VOUT

(VCORE)

“Fast−Feedback”

Connection

PWM

COMP

To F/F

Reset

Channel

Start−Up

Offset

E.A.

DAC

Out

VFB

COMP

Internal Ramp

n = 1 or 2

CCSn

− +

Inductive Current Sensing

For lossless sensing, current can be sensed across the

inductor as shown in Figure 11. In the diagram, L is the

output inductance and RL is the inherent inductor resistance.

To compensate the current sense signal, the values of RCSn

and CCSn are chosen so that L/RL = RCSn • CCSn. If this

criteria is met, the current sense signal will be the same shape

as the inductor current and the voltage signal at CSn will

represent the instantaneous value of inductor current. Also,

the circuit can be analyzed as if a sense resistor of value RL

was used as a sense resistor (RS).

When choosing or designing inductors for use with

inductive sensing, tolerances and temperature effects should

be considered. Cores with a low permeability material or a

large gap will usually have minimal inductance change with

temperature and load. Copper magnet wire has a

temperature coefficient of 0.39% per °C. The increase in

winding resistance at higher temperatures should be

considered when setting the ILIM threshold. If a more

accurate current sense is required than inductive sensing can

provide, current can be sensed through a resistor as shown

in Figure 9.

Current Sharing Accuracy

Printed circuit board (PCB) traces that carry inductor

current can be used as part of the current sense resistance

depending on where the current sense signal is picked off.

For accurate current sharing, the current sense inputs should

sense the current at relatively the same point for each phase

and the connection to the CSREF pin should be made so that

no phase is favored. In some cases, especially with inductive

sensing, resistance of the PCB can be useful for increasing

the current sense resistance. The total current sense

resistance used for calculations must include any PCB trace

resistance between the CSn input and the CSREF input that

carries inductor current.

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