NCP5331 datasheet(16/38 Pages) ONSEMI

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NCP5331

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SWNODE

VFB (VOUT)

Internal Ramp

CSA Out w/

Exaggerated

Delays

COMP-Of fset

CSA Out + Ramp + CSREF

Figure 16. Open Loop Operation

microseconds of a transient before the feedback loop has

repositioned the COMP pin.

The peak output current can be calculated from

IOUT,PEAK

(VCOMP * VCORE * Offset)

(RSx

@ GCSA)

Figure 16 shows the step response of the COMP pin at a

fixed level. Before time T1 the converter is in normal steady

state 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

internal ramp voltage and Startup OFFSET exceed the

voltage level of the COMP pin. At T1 the output current

increases and the output voltage sags. The next PWM cycle

begins and this PWM cycle continues longer than

previously. As a result, 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 (CSx

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.

Inductive Current Sensing

For lossless sensing, current can be sensed across the

output inductor as shown in Figure 15. In the diagram, Lx is

the output inductance and RLx is the inherent inductor

resistance. To compensate the current sense signal, the

values of RSx and CSx are chosen so that Lx/RLx = RSx

⋅

CSx. 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 RLx was used as a sense resistor (RSx).

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 overcurrent (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 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 connected.

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 Current Sense Amplifier input mismatch is

±5.0 mV

and will typically be within

±3.0 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 m

Ω total sense resistance will produce a 1.5 A

difference in current between phases.

External Ramp Size and Current Sensing

The internal ramp allows flexibility of current sense time

constant. Typically, the current sense RSxCSx time constant

should be equal to or slower than the inductor’s time

constant. If the RC time constant 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 (i.e. transient response) will be affected and

must be evaluated carefully. The current signal will

overshoot during transients and settle at the rate determined

by RSx

⋅ CSx. It will eventually settle to the correct dc level,

but the error will decay with the time constant of RSx

⋅ CSx.

If this error is excessive it will effect 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 and will produce too much transient droop in the

output voltage. Also, the hiccup mode current limit will have

a lower threshold for fast rise step loads than for slowly

rising output currents.

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