isl6306 Intersil Corporation, isl6306 Datasheet - Page 14

• Manufacturer Part Number: isl6306
• Description: 4-phase Pwm Controller With 8-bit Dac Code Capable Of Precision Rds On Or Dcr Differential Current Sensing
• Manufacturer: Intersil Corporation

V
sawtooth ramp as illustrated in Figure 7. When the modified
V
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
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
inductor current. Coincident with the falling edge of the PWM
signal, the sample and hold circuitry samples the sensed
current signal I
Therefore, the sample current, I
output current and held for one switching cycle. The sample
current is used for current balance, load-line regulation, and
overcurrent protection.
Current Sensing
The ISL6306 supports inductor DCR sensing, MOSFET
r
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.
DS(ON)
COMP
COMP
, minus the current correction signal relative to the
voltage crosses the sawtooth ramp, the PWM output
sensing, or resistive sensing techniques. The
FIGURE 3. SAMPLE AND HOLD TIMING
SEN
L
. This current gets sampled starting
, as illustrated in Figure 3.
SEN
PWM
0.5Tsw
, is simply a scaled version of the
SWITCHING PERIOD
14
I
L
TIME
SAMPLE CURRENT, I
n
, is proportional to the
I
SEN
n
ISL6306
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
pass through the DCR. Equation 3 shows the s-domain
equivalent voltage across the inductor V
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 4.
The voltage on the capacitor V
proportional to the channel current I
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
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
Therefore the current out of ISEN+ pin, I
to the inductor current.
V
V
C
L
=
=
ISL6306 INTERNAL CIRCUIT
⎛
⎝
-------------------------------------------------------------------- -
SAMPLE
I
L
s
HOLD
⋅
⋅
FIGURE 4. DCR SENSING CONFIGURATION
C
&
(
-------------
DCR
I
n
I
s L
SEN
is replicated across the sense resistor R
L
⋅
ISL6605
(
s RC
+
+
⋅
=
DCR
1
L
PWM(n)
⎞
⎠
I
, flowing through the inductor, will also
L
⋅
+
----------------- -
R
(
DCR
DCR I
)
1
ISEN
)
+
-
⋅
V
L
IN
)
C
, can be shown to be
ISEN-(n)
ISEN+(n)
INDUCTOR
R
L
L
, see Equation 4.
I
L
s ( )
V
L
L
SEN
V
.
DCR
C
-
C
(s)
, is proportional
-
R
(PTC)
December 8, 2006
ISEN(n)
C
V
OUT
ISEN
OUT
(EQ. 3)
(EQ. 4)
FN9226.1
C
.
is