ISL6366 Datasheet, PDF (17/44 Pages) Intersil Corporation – Dual 6-Phase + 1-Phase PWM Controller for VR12/IMVP7 Applications

ISL6366
Current Sensing
The ISL6366 senses current continuously for fast response. The
ISL6366 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 5-6 and 9-10, represents
VR1’s channel or one channel of the VR0 output, respectively. For
VR0 output, the ISEN± circuitry is repeated for each channel, but
may not be active depending on the status of the PWM[6:2] pins,
as described in “PWM and PSI# Operation” on page 15. The input
bias current of the current sensing amplifier is typically 60nA;
less than 8.34kΩ input impedance (0.5mV offset) is preferred to
minimized the offset error, i.e., a larger C value as needed.
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 5. The channel current I_L, flowing through the
inductor, will also pass through the DCR. Equation 5 shows the
s-domain equivalent voltage across the inductor V_L.
V_L(s) = I_L ⋅ (s ⋅ L + DCR)
(EQ. 5)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 5.
V_IN
IL(s)
ISL6596
L
DCR
INDUCTOR
V_L
V_OUT
C_OUT
V_C(s)
PWMS
R
C
ISL6366
I_n
R_ISEN(n)
CURRENT
SENSE
+
ISENS-
-
10.5 ISENS+
C_T
I_SEN
=
I_L
------------^D----^C----^R-------------
10.5 + R_ISEN
FIGURE 5. DCR SENSING CONFIGURATION FOR VR1
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L. See Equation 6.
V_C(s)
=
⎛
⎝
s
⋅
-----^L------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
----------------------------------------------------------------
(s ⋅ RC + 1)
(EQ. 6)
If the R-C network components are selected such that the RC time
constant matches the inductor time constant (R*C = 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 the ISENS+ pin, I_SEN, is proportional
to the inductor current.
Because of the internal filter at the ISENS- pin, one capacitor, C_T,
is needed to match the time delay between the ISENS- and
ISENS+ 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 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. 7)
RESISTIVE SENSING
For more accurate current sensing, a dedicated current-sense
resistor R_SENSE in series with each output inductor can serve as the
current sense element (see Figure 7). This technique however
reduces overall converter efficiency due to the additional power loss
on the current sense element R_SENSE.
IL
ISL6366
L
R
R_SEN ESL
R_SENSE
V_R
V_C(s)
V_OUT
C_OUT
C
R_ISEN(n)
I_n
CURRENT
SENSE
ISENS-
+
10.5
-
ISENS+
C_T
I_SEN
=
I_L
-----------^R----^S---^E---^N------------
10.5 + R_ISEN
FIGURE 6. SENSE RESISTOR IN SERIES WITH INDUCTOR FOR
VR1
A current sensing resistor has a distributed parasitic inductance,
known as ESL (equivalent series inductance, typically less than
1nH) parameter. Consider the ESL as a separate lumped
quantity, as shown in Figure 7. The channel current I_L, flowing
through the inductor, will also pass through the ESL. Equation 8
shows the s-domain equivalent voltage across the resistor V_R.
V_R(s) = I_L ⋅ (s ⋅ ESL + R_SEN)
(EQ. 8)
17
FN6964.0
January 3, 2011