CS5307GDWR24 datasheet(13/24 Pages) ONSEMI

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CS5307

http://onsemi.com

Figure 16. Enhanced V2 Control Employing Lossless Inductive Current Sensing and Internal Ramp

−

SWNODE

RCSx

RLx

CSx

CSA

COx

CSREF

VOUT

(VCORE)

“Fast−Feedback”

Connection

−

PWM

COMP

To F/F

Reset

Channel

Start−Up

Offset

−

E.A.

DAC

Out

VFB

COMP

Internal Ramp

x = 1, 2, 3, or 4

CCSx

Inductive Current Sensing

For lossless sensing, current can be sensed across the

inductor as shown in Figure 16. In the diagram, L is the output

inductance and RL is the inherent inductor resistance. To

compensate the current sense signal, the values of RCSx and

CCSx are chosen so that L/RL = RCSx ⋅ CCSx. If this criteria

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

inductor current and the voltage signal at CSx will represent

the instantaneous value of inductor current. Also, the circuit

can be analyzed as if a sense resistor of value RL was used.

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 OCSET 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 14.

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 CSx input and the CSREF input that

carries inductor current.

Current Sense Amplifier (CSA) input mismatch and the

value of the current sense component will determine the

accuracy of the current sharing between phases. The worst

case CSA input mismatch is ±10 mV and will typically be

within 4.0 mV. The difference in peak currents between

phases will be the CSA input mismatch divided by the

current sense resistance. If all current sense components are

of equal resistance, a 3.0 mV mismatch with a 2.0 mΩ sense

resistance will produce a 1.5 A difference in current between

phases.

External Ramp Size and Current Sensing

The internal ramp allows flexibility in setting the current

sense time constant. Typically, the current sense RCSx ⋅ CCSx

time constant should be equal to or slightly slower than the

inductor’s time constant. If RC is chosen to be smaller

(faster) than L/RL, the AC or transient portion of the current

sensing signal will be scaled larger than the DC portion. This

will provide a larger steady−state ramp, but circuit

performance will be affected and must be evaluated

carefully. The current signal will overshoot during transients

and settle at the rate determined by RCSx ⋅ CCSx. It will

eventually settle to the correct DC level, but the error will

decay with the time constant of RCSx ⋅ CCSx. If this error is

excessive, it will affect transient response, adaptive

positioning and current limit. During a positive current

transient, the COMP pin will be required to undershoot in

response to the current signal in order to maintain the output

voltage. Similarly, the VDRP signal will overshoot which

will produce too much transient droop in the output voltage.

The single−phase pulse−by−pulse overcurrent protection

will trip earlier than it would if compensated correctly and

hiccup−mode current limit will have a lower threshold for

fast rising step loads than for slowly rising output currents.

The waveforms in Figure 17 show a simulation of the

current sense signal and the actual inductor current during a

positive step in load current with values of L = 500 nH, RL

= 1.6 mΩ, RCSx = 20 kΩ and CCSx = .01 μF. In this case, ideal

current signal compensation would require RCSx to be 31 kΩ.

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