ISL6334AR5368 Datasheet, PDF (15/31 Pages) Intersil Corporation – VR11.1, 4-Phase PWM Controller with Light Load Efficiency Enhancement and Load Current Monitoring Features

ISL6334AR5368
pass through the DCR. Equation 4 shows the S-domain
equivalent voltage across the inductor V_L.
V_L(s) = I_L ⋅ (s ⋅ L + DCR)
(EQ. 4)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 4.
V_IN
IL(s)
ISL6596
L
DCR
INDUCTOR
V_L
V_OUT
C_OUT
V_C(s)
PWM(n)
R
C
ISL6334AR5368 INTERNAL CIRCUIT
I_n
R_ISEN(n)
CURRENT
SENSE
ISEN-(n)
+
-
ISEN+(n)
C_T
I_SEN
=
I_L
---^D----^C-----^R-----
R_ISEN
FIGURE 4. DCR SENSING CONFIGURATION
The voltage on the capacitor V_C, can be shown to be
proportional to the channel current I_L. See Equation 5.
V_C(s)
=
⎛
⎝
s
⋅
------^L-------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
--------------------₍--_s-----_â‹---_R----_C------+-----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.
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 5). 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)
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 ISL6334AR5368
should be utilized. The integrated temperature compensation
function is described in “External Temperature Compensation”
on page 24.
IL
L
R_SENSE V_OUT
C_OUT
ISL6334AR5368 INTERNAL CIRCUIT
R_ISEN(n)
I_n
CURRENT
SENSE
ISEN-(n)
+
-
ISEN+(n)
C_T
ISEN
=
IL
-R----S----E-----N-----S----E---
RISEN
FIGURE 5. SENSE RESISTOR IN SERIES WITH INDUCTORS
Channel-Current Balance
The sensed current I_n from each active channel is 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.
15
FN6839.2
September 7, 2010