IR3500MPBF International Rectifier, IR3500MPBF Datasheet - Page 12

• Manufacturer Part Number: IR3500MPBF
• Description: IC XPHASE3 CONTROL 32-MLPQ
• Manufacturer: International Rectifier
• Series: XPhase3™r

Specifications of IR3500MPBF

• Applications: Processor
• Current - Supply: 6.5mA
• Voltage - Supply: 4.75 V ~ 7.5 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 32-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 (R
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
inductor DC current, but affects the 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.
Current Sense Amplifier
A high speed differential current sense amplifier is located in the phase IC, as shown in Figure 7. Its gain is
nominally 32.5 and the 3850 ppm/ºC increase in inductor DCR should be compensated in the voltage loop feedback
path.
The current sense amplifier can accept positive differential input up to 50mV and negative up to -10mV before
clipping. The output of the current sense amplifier is summed with the DAC voltage and sent to the control IC and
other phases through an on-chip 3K
tied together and the voltage on the share bus represents the average current through all the inductors and is used
by the control IC for voltage positioning and current limit protection. The input offset of this amplifier is calibrated to
+/- 1mV in order to reduce the current sense error.
The input offset voltage is the primary source of error for the current share loop. In order to achieve very small input
offset error and superior current sharing performance, the current sense amplifier continuously calibrates itself. This
calibration algorithm creates ripple on ISHARE bus with a frequency of fsw / 896 in a multiphase architecture.
Page 12 of 47
Figure 7 - Inductor Current Sensing and Current Sense Amplifier
L
was used. The mismatch of the time constants does not affect the measurement of
v
C
CSOUT
resistor connected to the ISHARE pin. The ISHARE pins of all the phases are
(
s
)
=
v
Current
Sense Amp
L
(
s
)
1
i
+
L
sR
1
CS
L
R
C
CS
CS
v
=
L
R
C
v
i
L
c
CS
L
CS
(
s
)
1
+
R
sR
V
L
O
+
CS
C
sL
C
O
CS
L
). If the two time constants match, the
May 18, 2009
IR3500