ISL6326B Datasheet, PDF (12/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision DCR Differential Current Sensing

ISL6326B
pages 4 and 5). Equation 3 is provided to assist in selecting
the correct resistor value.
R_T
=
2----.--5---X-----1---0----¹_---⁰_-
F_SW
(EQ. 3)
where F_SW is the switching frequency of each phase.
Current Sensing
ISL6326B senses the current continuously for fast response.
ISL6326B supports inductor DCR sensing, or resistive
sensing techniques. The associated channel current sense
amplifier uses the ISEN inputs to reproduce a signal
proportional to the inductor current, I_L. The sense current,
I_SEN, is proportional to the inductor current. The sensed
current is used for current balance, load-line regulation, and
overcurrent protection.
The internal circuitry, shown in Figures 3, and 4, represents
one channel of an N-channel converter. This circuitry is
repeated for each channel in the converter, but may not be
active depending on the status of the PWM3 and PWM4
pins, as described in the PWM Operation section.
INDUCTOR DCR SENSING
An inductor’s winding is characteristic of a distributed
resistance as measured by the DCR (Direct Current
Resistance) parameter. Consider the inductor DCR as a
separate lumped quantity, as shown in Figure 3. The
channel current I_L, flowing through the inductor, will also
pass through the DCR. Equation 4 shows the s-domain
equivalent voltage across the inductor V_L.
V_L = I_L ⋅ (s ⋅ L + DCR)
(EQ. 4)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 3.
The voltage on the capacitor V_C, can be shown to be
proportional to the channel current I_L, see Equation 5.
⎝⎛^s
⋅
------^L-------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
V_C
=
--------------------------------------------------------------------
(s ⋅ RC + 1)
(EQ. 5)
If the R-C 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.
V_IN
ISL6605
PWM(n)
IL(s)
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
ISL6326B 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
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.
12
FN9286.0
April 21, 2006