ISL6364 Datasheet, PDF (17/44 Pages) Intersil Corporation – Dual 4-Phase + 1-Phase PWM Controller for VR12/IMVP7 Applications

ISL6364
V_IN
ISL6596
ISL6364
PWMS
I_n
IL(s)
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
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 6). This technique however
reduces overall converter efficiency due to the additional power loss
on the current sense element R_SENSE.
ISL6364
L
R
IL
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 6. 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)
A simple R-C network across the current sense resistor extracts
the R_SEN voltage, as shown in Figure 6.
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L. See Equation 9.
V_C(s) = -^⎝⎛--^s----^â‹----^R---E------^S--^S----^E----^L₍--^N----_s---+-_â‹---_R-¹---_C-^⎠⎞----^â‹_+---⁽--_1R----₎-^S---^E---^N-----^â‹---^I--^L---⁾
(EQ. 9)
If the R-C network components are selected such that the RC
time constant matches the ESL-R_SEN time constant
(R*C = ESL/R_SEN), the voltage across the capacitor V_C is equal to
the voltage drop across the R_SEN, i.e., proportional to the channel
current. As an example, a typical 1mΩ sense resistor can use
R = 348 and C = 820pF for the matching. Figures 7 and 8 show
the sensed waveforms without and with matching RC when using
resistive sense.
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.
FIGURE 7. VOLTAGE ACROSS R WITHOUT RC
17
FN6861.0
December 22, 2010