ISL6336B Datasheet, PDF (14/31 Pages) Intersil Corporation – 6-Phase PWM Controller with Light Load Efficiency Enhancement and Current Monitoring

ISL6336B
Inductors” on page 6). 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.
Equation 3 also applies for connecting FS to VCC or GND.
Figure 3 shows the relationship between R_T and F_SW,
according to Equation 3.
1000
900
800
700
600
500
400
300
200
100
20 30 40 50 60 70 80 90 100
R_T (kΩ)
FIGURE 3. SWITCHING FREQUENCY vs R_T
Current Sensing
The ISL6336B senses current continuously for fast response.
The ISL6336B supports inductor DCR sensing, or resistor
sensing techniques. The associated channel current sense
amplifier uses the ISEN inputs to reproduce a signal
proportional to the inductor current, I_L. The sensed current,
I_SEN, is used for current balance, load-line regulation, and the
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 PWM2, PWM3,
PWM4, PWM5, PWM6 pins, “PWM and PSI# Operation” on
page 12.
The input bias current of the current sensing amplifier is
typically 60nA; less than 5kΩ input impedance is preferred to
minimize the offset error.
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 4. 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 4.
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-----_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 the ISEN- pin, a 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)
V_IN
ISL6609
PWM(n)
IL(s)
L _DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
ISL6336B
INTERNAL CIRCUIT
I_n
R_ISEN(n)
(PTC)
CURRENT
SENSE
ISEN-(n)
+
-
ISEN+(n)
C_T
ISEN
=
IL
----^D----^C-----^R------
RISEN
FIGURE 4. DCR SENSING CONFIGURATION
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
14
FN6696.2
August 31, 2010