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NX2415

TWO PHASE SYNCHRONOUS PWM CONTROLLER WITH INTEGRATED FET DRIVER AND DIFFERENTIAL CURRENT SENSE

Microsemi Corporation 

NX2415

Inductor Current Sensing

VIN

Control &

Driver

iL

L

DCR

Current

Sensing

Amplifier

Rs

Rs

Cs

VS_IL

racy during the transient if droop function is required.

The illustration is shown in the following figure.

Overshoot caused

VOUT

by inductor

VS_IL----Voltage accross

nonlinearity

the sensing

capacitor Cs

iL--- inductor current

Figure 8 - Inductor current sensing using RC network.

The inductor current can be sensed through a RC

network as shown above. The advantage of the RC net-

work is the lossless comparing with a resistor in series

with output inductor.

The selection of the resistor sensing network is

chosen by the following equation:

RS

× CS

=

L

DCR

...(18)

If the above equation is satisfied, the voltage across

the sensing capacitor Cs will be equal to the inductor

current times DCR of inductor for all frequency domain.

VS _ IL = DCR × iL

If the sensing capacitor is chosen

CS = 1µF

CS must be X7R or COG ceramic capacitor.

The sensing resistor is calculated as

RS

=

L

DCR × CS

For example, for 0.68uH inductor with 1.4mΩ

DCR, we have

RS

=

0.68µH

1.4mΩ ×1µF

=

486Ω

In most of cases, the selection of sensing resis-

tor based on the above equation will be sufficient. How-

ever, for some inductor such as toroid coiled inductor

with micrometal, even the product of sensing resistor

and capacitor is perfectly match with L/DCR, the voltage

across the capacitor still has overshoot due to the

nonlinearity of inductor. This will affect the droop accu-

Output voltage

with droop function

Droop misbehavoir

caused by

overshoot of VS_IL

Figure 9 - Droop accuracy affected by the nonlinearity

of inductor.

In this case, the sensing resistor has to be chosen

RS

≥

L

DCR × CS

to compensate the overshoot. This selection only af-

fects the small signal mode of current loop. For DC ac-

curacy, there is no effect since the DC voltage across

the sensing capacitor will equal to the DCR times induc-

tor current at DC load no matter what Rs is. In this ex-

ample, Rs=620Ω.

R

S

value

is

preferred

to

be

less

than

400Ω

in

NX2415's application, therefore we need to reiterate the

calculation, choose C 2.2uF instead. R value is finally

S

S

chosen as 301Ω .

Powe dissipation of Rs resistor is calculated as

followed:

PD (RS )

=

(VIN

− VOUT )2

RS

×D

+

V2

OUT

RS

× (1 − D)

= (12V − 1.2V)2 × 0.1 + (1.2V)2 × (1 − 0.1)

301Ω

301Ω

= 0.04W

The power rating of Rs should be over 0.04W.

Rev.4.8

14

05/06/08

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