ISL6316_06 Datasheet, PDF (13/29 Pages) Intersil Corporation – Enhanced 4-Phase PWM Controller with 6-Bit VID Code Capable of Precision rDS(ON) or DCR Differential Current Sensing for VR10 Application

ISL6316
PWM Operation
The timing of each channel is set by the number of active
channels. The default channel setting for the ISL6316 is four.
The switching cycle is defined as the time between PWM
pulse termination signals of each channel. The pulse
termination signal is an internally generated clock signal
which triggers the falling edge of PWM signal. The cycle time
of the pulse termination signal is the inverse of the switching
frequency set by the resistor between the FS pin and ground.
Each cycle begins when the clock signal commands the
channel PWM signal to go low. 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 terminates 1/4 of a cycle after PWM4, PWM2
output follows another 1/4 of a cycle after PWM3, and PWM1
terminates 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-termination times are spaced in 1/3 cycle
increments. If PWM3 is connected to VCC, two channel
operation is selected and the PWM2 pulse terminates 1/2 of a
cycle later.
Once a PWM signal transitions low, it is held low for a
minimum of 1/3 cycle. This forced off time is required to
ensure an accurate current sample. Current sensing is
described in the next section. After the forced off time expires,
the PWM output is enabled. The PWM output state is driven
by the position of the error amplifier output signal, V_COMP,
minus the current correction signal relative to the sawtooth
ramp as illustrated in Figure 7. When the modified V_COMP
voltage crosses the sawtooth ramp, the PWM output
transitions high. The MOSFET driver detects the change in
state of the PWM signal and turns off the synchronous
MOSFET and turns on the upper MOSFET. The PWM signal
will remain high until the pulse termination signal marks the
beginning of the next cycle by triggering the PWM signal low.
Current Sampling
During the forced off-time following a PWM transition low, the
associated channel current sense amplifier uses the ISEN
inputs to reproduce a signal proportional to the inductor
current, I_L. This current gets sampled starting 1/6 period after
each PWM goes low and continuously gets sampled for 1/3
period, or until the PWM goes high, whichever comes first. No
matter the current sense method, the sense current, I_SEN, is
simply a scaled version of the inductor current. Coincident
with the falling edge of the PWM signal, the sample and hold
circuitry samples the sensed current signal I_SEN, as illustrated
in Figure 3.
Therefore, the sample current, I_n, is proportional to the output
current and held for one switching cycle. The sample current
is used for current balance, load-line regulation, and
overcurrent protection.
I_L
PWM
I_SEN
0.5Tsw
SAMPLE CURRENT, I_n
SWITCHING PERIOD
TIME
FIGURE 3. SAMPLE AND HOLD TIMING
Current Sensing
The ISL6316 supports inductor DCR sensing, MOSFET
r_DS(ON) sensing, or resistive sensing techniques. The internal
circuitry, shown in Figures 4, 5, and 6, 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 section.
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 3 shows the s-domain equivalent voltage
across the inductor V_L.
V_L = I_L ⋅ (s ⋅ L + DCR)
(EQ. 3)
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 4.
⎛
⎝
s
⋅
------^L-------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
V_C = --------------------₍--_s-----_â‹---_R----_C------+-----₁----₎-------------------
(EQ. 4)
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.
13
FN9227.1
December 12, 2006