ISL6334ACRZ-T Datasheet, PDF (15/31 Pages) Intersil Corporation – VR11.1, 4-Phase PWM Controller with Light Load Efficiency Enhancement and Load Current Monitoring Features

ISL6334, ISL6334A
V_IN
ISL6596
PWM(n)
IL(s)
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
ISL6334, ISL6334A 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 ⋅ 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.
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)
The current sense circuitry operates in a very similar manner
for negative current feedback, where inductor current is
flowing from the output of the regulator to the PHASE node,
opposite of flow pictured in Figures 4 and 5. However, the
range of proper operation with negative current sensing has
a limitation. The worst-case peak-to-peak inductor ripple
current should be kept less than 70% of the OCP trip point
(internal mirrored current = ~70µA). Care should be taken to
avoid operation with negative current feedback exceeding
this threshold, as this may lead to momentary loss of current
balance between phases and disruption of normal circuit
operation. Note that the negative current can especially
affect coupled inductor designs, where the effective
inductance is the leakage between the two channels, much
lower than the specified mutual inductance (LM). To limit the
impact, a higher R_ISEN value (1.5x to 2x) can be used to
reduce the effective negative current seen by the controller
in coupled inductor designs. Refer to Intersil's application
note, AN1268 for a detailed coupled inductor discussion and
ripple current calculation.
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 ISL6334, ISL6334A
should be utilized. The integrated temperature compensation
function is described in “External Temperature Compensation”
on page 25.
15
FN6482.2
February 1, 2013