ir3092 International Rectifier Corp., ir3092 Datasheet - Page 22

• Manufacturer Part Number: ir3092
• Description: 2 Phase Opteron, Athlon, Or Vr10.x Control Ic
• Manufacturer: International Rectifier Corp.

A resistor R
difference between V
programmed by the same external resistor that programs the oscillator frequency, R
FB pin.
The voltage at the VDRP pin is an average of both phase Current Sense Amplifiers and represents the sum of the VDAC
voltage and the average inductor current of all the phases. The VDRP pin is connected to the FB pin through the Adaptive
Voltage Positioning Resistor R
required output impedance of the converter. R
required output impedance of the converter.
Lossless Average Inductor Current Sensing
Inductor current can be sensed by connecting a series resistor and a capacitor network in parallel with the inductor and
measuring the voltage across the capacitor. The equation of the sensing network is,
Usually the resistor Rcs and capacitor Ccs are chosen so that the time constant of Rcs and Ccs equals the time constant
of the inductor which is the inductance L over the inductor DCR. If the two time constants match, the voltage across Ccs
is proportional to the current through L, and the sense circuit can be treated as if only a sense resistor with the value of
R
AC component of the inductor current.
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).
An additional problem associated with peak or valley current mode control for voltage positioning is that they suffer from
peak-to-average errors. These errors will show in many ways but one example is the effect of frequency variation. If the
frequency of a particular unit is 10% low, the peak to peak inductor current will be 10% larger and the output impedance
of the converter will drop by about 10%. Variations in inductance, current sense amplifier bandwidth, PWM prop delay,
any added slope compensation, input voltage, and output voltage are all additional sources of peak-to-average errors.
The DC resistance of the inductor is utilized to sense the inductor current. Usually the resistor R
parallel with the inductor are chosen to match the time constant of the inductor, and therefore the voltage across the
capacitor C
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 affects the AC component of the inductor current as well as the
output voltage during the load current transient if adaptive voltage positioning is adopted.
L
was used. The mismatch of the time constants does not affect the measurement of inductor DC current, but affects the
Page 22 of 37
CS
FB
between FB pin and the converter output is used to create output voltage offset V
represents the inductor current. If the two time constants are not the same, the AC component of the
DAC
voltage and output voltage at no load condition. An internal current source whose value is
DRP.
Adaptive voltage positioning lowers the converter voltage by R
v
R
C
DRP
(
s
R
)
FB
FB
v
R
L
FB
V
(
and R
s
O
)
2
I
_
1
ROSC
NLOFST
R

L
R
DRP
sR
O
1
23
S
are determined by (9) and (10) respectively, where R
C
5 .
S
i
L
(
s
)
1
R

L
sR

S
sL
(10)
C
(9)
S
ROSC
, pumps current I
CS
06/25/04
O_NLOFST
O
and capacitor C
*I
O,
where R
IR3092
ROSC
which is the
out of the
O
O
is the
is the
CS
in