IR3529
Page 17 of 22 February 12, 2010
DESIGN PROCEDURES - IR3529
Inductor Current Sensing Capacitor C
CS
and Resistor R
CS
The DC resistance of the inductor is utilized to sense the inductor current. Usually the resistor R
CS
and capacitor
C
CS
in parallel with the inductor are chosen to match the time constant of the inductor, and therefore the voltage
across the capacitor C
CS
represents the inductor current. If the two time constants are not the same, the AC
component of the capacitor voltage is different from that of the real inductor current. The time constant mismatch
does not affect the average current sharing among the multiple phases, but does affect the current signal IOUT as
well as the output voltage during the load current transient if adaptive voltage positioning is adopted.
Measure the inductance L and the inductor DC resistance R
L
. Pre-select the capacitor C
CS
and calculate R
CS
as
follows.
CS
L
CS
C
RL
R =
(1)
Bootstrap Capacitor C
BST
Depending on the duty cycle and gate drive current of the phase IC, a capacitor in the range of 0.1uF to 1uF is
needed for the bootstrap circuit.
Decoupling Capacitors for Phase IC
A 0.1uF-1uF decoupling capacitor is required at the VCCL pin.
Current Share Loop Compensation
The internal compensation of current share loop ensures that crossover frequency of the current share loop is at
least one decade lower than that of the voltage loop so that the interaction between the two loops is eliminated.
The crossover frequency of current share loop
is approximately 8 kHz
Cycle-by-Cycle Over Current Protection
Cycle-by-cycle over current protection helps to improve the transient response at load repetition rates closer to
the switching frequency of the VR. The over current threshold is programmed with an external resistor (R
OCSET
)
connected to the OCSET pin with a sink current of 200 µA. The OCSET comparator monitors the voltage across
the on-resistance (R
DS, ON
) of the high-side MOSFET and terminates the high side pulse if the sensed voltage
reaches the over current threshold. A capacitor (C
OCSET
) is used to reduce noise coupling into the OCSET pin of
the IC.
R
OCSET
=
µ
200
,*__ onRdsphaseperIload
where Iload_per_phase is the maximum current per phase which you do not want to exceed.
