ISL6326 Datasheet, PDF (12/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision rDS ON or DCR Differential Current Sensing

ISL6326
by the resistor between the FS pin and ground. The PWM
signals command the MOSFET driver to turn on/off the
channel MOSFETs.
For 4-channel operation, the channel firing order is 4-3-2-1:
PWM3 pulse happens 1/4 of a cycle after PWM4, PWM2
output follows another 1/4 of a cycle after PWM3, and
PWM1 delays another 1/4 of a cycle after PWM2. For
3-channel operation, the channel firing order is 3-2-1.
Connecting PWM4 to VCC selects three channel operation
and the pulse times are spaced in 1/3 cycle increments. If
PWM3 is connected to VCC, two channel operation is
selected and the PWM2 pulse happens 1/2 of a cycle after
PWM pulse.
Switching Frequency
Switching frequency is determined by the selection of the
frequency-setting resistor, R_T, which is connected from FS
pin to GND (see the figures labeled Typical Applications on
page 4 and page 5). 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.
Current Sensing
ISL6326 senses the current continuously for fast response.
ISL6326 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 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 PWM3 and PWM4
pins, as described in the “PWM Operation” on page 11.
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 3. 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 RC network across the inductor extracts the DCR
voltage, as shown in Figure 3.
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 RC 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.
V_IN
IL(s)
ISL6605
L
DCR
INDUCTOR
V_L
V_OUT
C_OUT
V_C(s)
PWM(n)
R
C
ISL6326 INTERNAL CIRCUIT
I_n
R_ISEN(n)
(PTC)
CURRENT
SENSE
ISEN-(n)
+
-
ISEN+(n)
C_T
ISEN
=
IL
----D----C-----R------
RISEN
FIGURE 3. DCR SENSING CONFIGURATION
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)
12
FN9262.1
May 5, 2008