ISL6327A Datasheet, PDF (12/29 Pages) Intersil Corporation – Enhanced 6-Phase PWM Controller with 8-Bit VID Code and Differential Inductor DCR or Resistor Current Sensing

ISL6327A
V_IN
ISL6609
PWM(n)
IL(s)
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
ISL6327A INTERNAL CIRCUIT
I_n
R_ISEN(n)
(PTC)
CURRENT
SENSE
ISEN-(n)
+
-
ISEN+(n)
C_T
I_SEN
=
I_L
---^D----^C-----^R-----
R_ISEN
FIGURE 3. DCR SENSING CONFIGURATION
If the RC network components are selected such that the RC
time constant (= R*C) matches the inductor time constant
(= L/DCR), the voltage across the capacitor V_C is equal to
the voltage drop across the DCR, i.e., proportional to the
channel current.
With the internal low-offset current amplifier, the capacitor
voltage V_C is replicated across the sense resistor R_ISEN.
Therefore the current out of ISEN+ pin, I_SEN, is proportional
to the inductor current.
Because of the internal filter at ISEN- pin, one capacitor C_T
is needed to match the time delay between the ISEN- and
ISEN+ signals. Select the proper C_T to keep the time
constant of R_ISEN and C_T (R_ISEN x C_T) close to 27ns.
Equation 6 shows that the ratio of the channel current to the
sensed current I_SEN is driven by the value of the sense
resistor and the DCR of the inductor.
I_SEN
=
I_L
⋅
---^D----^C-----^R-----
R_ISEN
(EQ. 6)
RESISTIVE SENSING
For accurate current sense, a dedicated current-sense
resistor R_SENSE in series with each output inductor can
serve as the current sense element (see Figure 4). This
technique is more accurate, but reduces overall converter
efficiency due to the additional power loss on the current
sense element R_SENSE.
The same capacitor C_T is needed to match the time delay
between ISEN- and ISEN+ signals. Select the proper C_T to
keep the time constant of R_ISEN and C_T (R_ISEN x C_T) close
to 27ns.
Equation 7 shows the ratio of the channel current to the
sensed current I_SEN.
I_SEN
=
I_L
⋅
R_{-----S----E----N----S----E--}
R_ISEN
(EQ. 7)
IL
L
R_SENSE V_OUT
C_OUT
ISL6327A INTERNAL CIRCUIT
I_n
R_ISEN(n)
CURRENT
SENSE
ISEN-(n)
+
-
ISEN+(n)
C_T
ISEN
=
IL
-R----S----E-----N-----S----E---
RISEN
FIGURE 4. SENSE RESISTOR IN SERIES WITH INDUCTORS
The inductor DCR value will increase as the temperature
increases. Therefore the sensed current will increase as the
temperature of the current sense element increases. In order
to compensate the temperature effect on the sensed current
signal, a Positive Temperature Coefficient (PTC) resistor can
be selected for the sense resistor R_ISEN, or the integrated
temperature compensation function of ISL6327A should be
utilized. The integrated temperature compensation function
is described in “Temperature Compensation” on page 21.
Channel-Current Balance
The sensed current I_n from each active channel are summed
together and divided by the number of active channels. The
resulting average current I_AVG provides a measure of the
total load current. Channel current balance is achieved by
comparing the sensed current of each channel to the
average current to make an appropriate adjustment to the
PWM duty cycle of each channel with Intersil’s patented
current-balance method.
Channel current balance is essential in achieving the
thermal advantage of multiphase operation. With good
current balance, the power loss is equally dissipated over
multiple devices and a greater area.
Voltage Regulation
The compensation network shown in Figure 5 assures that
the steady-state error in the output voltage is limited only to
the error in the reference voltage (output of the DAC) and
12
FN6833.0
February 17, 2009