IR3522

Page 11

V3.01

Figure 5 PWM Operating Waveforms

Lossless Average Inductor Current Sensing

Inductor current can be sensed by connecting a series RC network in parallel with the inductor and measuring the

voltage across the capacitor, as shown in Figure 6. The equation of this sensing network is,

(

)

.

1

1

)

(

1

1

)

(

)

(

CS

CS

L

L

L

CS

CS

L

C

C

sR

R

L

s

R

s

i

C

sR

s

v

s

v

+

+

=

+

=

Usually, the resistor Rcs and capacitor Ccs are chosen so that the RC time constant equals the time constant of the

across Ccs is proportional to the current through L, and the sense circuit can be treated as if only a sense resistor

current, but affects the AC component of the inductor current.

Figure 6 Inductor Current Sensing and Current Sense Amplifier

The advantage of sensing the inductor current versus high side or low side sensing is that actual output current

being delivered to the load is obtained rather than peak or sampled information about the switch currents. The

output voltage can be positioned to meet a load line based on real time information. Except for a sense resistor in

series with the inductor, this is the only sense method that can support a single cycle transient response. Other

methods provide no information during either load increase (low side sensing) or load decrease (high side sensing).

L

Current

Sense Amp

CSOUT

c

PHASE IC

CLOCK

PULSE

VDAC

EAIN

PWMRMP

GATEL

GATEH

DUTY CYCLE DECREASE

DUE TO VIN INCREASE

(FEED-FORWARD)

DUTY CYCLE INCREASE

DUE TO LOAD

INCREASE

STEADY-STATE

OPERATION

STEADY-STATE

OPERATION

DUTY CYCLE DECREASE DUE TO LOAD

DECREASE (BODY BRAKING) OR FAULT

(VCC UV, OCP, VID FAULT)