ISL6316 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
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
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.
V_C = -^--^s-----^â‹---^{D----------C-L}-------₍^-R----_s-----+-_â‹---_R-¹---_C^-----^â‹-+--⁽--^D-₁----₎^C-----^R------^â‹---^I--^L---⁾
(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.
V_IN
ISL6605
PWM(n)
IL(s)
L
DCR
INDUCTOR
V_L
V_C(s)
R
C
V_OUT
C_OUT
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
ISL6316 INTERNAL CIRCUIT
I_n
SAMPLE
&
HOLD
+
-
I_SEN
=
I_L
---^D----^C-----^R-----
R_ISEN
ISEN-(n)
ISEN+(n)
R_ISEN(n)
(PTC)
FIGURE 4. DCR SENSING CONFIGURATION
13
FN9227.0
August 31, 2005