ISL6559 Intersil Corporation, ISL6559 Datasheet - Page 8

• Manufacturer Part Number: ISL6559
• Description: Multi-Phase PWM Controller
• Manufacturer: Intersil Corporation

across the R
conducting. The resulting current into the ISEN pin is
proportional to the channel current, I
then sampled and held after sufficient settling time every
switching cycle. The sampled current, I
channel-current balance, load-line regulation, overcurrent
protection, and module current sharing. From Figure 3, the
following equation for I
where I
If R
sense resistor in series with the lower MOSFET source can
serve as a sense element. The circuitry shown in Figure 3
represents channel n of an N-channel converter. This
circuitry is repeated for each channel in the converter, but
may not be active depending upon the status of the PWM3
and PWM4 pins as described in the previous section.
Channel-Current Balance
The sampled current, I
gauge both overall load current and the relative channel
current carried in each leg of the converter. The individual
sample currents are summed and divided by the number of
I
n
SAMPLE
V
HOLD
FIGURE 3. INTERNAL AND EXTERNAL CURRENT-SENSING
=
FIGURE 4. CHANNEL-1 PWM FUNCTION AND CURRENT-
ISL6559 INTERNAL CIRCUIT
DS(ON)
COMP
I
&
I
n
L
L
r
----------------------
DS ON
R
I
SEN
is the channel current.
ISEN
(
sensing is not desired, an independent current-
DS(ON)
NOTE: *CHANNELS 3 and 4 are OPTIONAL.
CIRCUITRY
BALANCE ADJUSTMENT
=
)
+
I
ER
I
L
-
+
f(jω)
r
------------------------- -
I
1
+
DS ON
R
-
ISEN
of the lower MOSFET while it is
-
(
n
n
I
AVG
, from each active channel is used to
is derived:
SAWTOOTH SIGNAL
)
÷ N
8
ISEN(n)
EXTERNAL CIRCUIT
CHANNEL N
LOWER MOSFET
+
-
R
L
ISEN
. The ISEN current is
n
, is used for
V
Σ
IN
+
CHANNEL N
UPPER MOSFET
-
I L r DS ON
I
L
PWM1
I
I
I
(
4
3
2
(EQ. 3)
*
*
)
ISL6559
active channels. The resulting average current, I
provides a measure of the total load current demand on the
converter and the appropriate level of channel current. Using
Figures 3 and 4, the average current is defined as
I
I
where N is the number of active channels and I
total load current.
The average current is then subtracted from the individual
channel sample currents. The resulting error current, I
then filtered before it adjusts V
signal is compared to a sawtooth ramp signal and produces
a pulse width which corrects for any unbalance and drives
the error current toward zero. Figure 4 illustrates Intersil’s
patented current-balance method as implemented on
channel-1 of a multi-phase converter.
Two considerations designers face are MOSFET selection
and inductor design. Both are significantly improved when
channel currents track at any load level. The need for
complex drive schemes for multiple MOSFETs, exotic
magnetic materials, and expensive heat sinks is avoided.
Resulting in a cost-effective and easy to implement solution
relative to single-phase conversion. Channel-current
balance insures the thermal advantage of multi-phase
conversion is realized. Heat dissipation is spread over
multiple channels and a greater area than single phase
approaches.
In some circumstances, it may be necessary to deliberately
design some channel-current unbalance into the system. In
a highly compact design, one or two channels may be able
to cool more effectively than the other(s) due to nearby air
flow or heat sinking components. The other channel(s) may
have more difficulty cooling with comparatively less air flow
and heat sinking. The hotter channels may also be located
close to other heat-generating components tending to drive
their temperature even higher. In these cases, the proper
selection of the current sense resistors (R
introduces channel current unbalance into the system.
Increasing the value of R
decreasing it in the hotter channels moves all channels into
thermal balance at the expense of current balance.
Voltage Regulation
The output of the error amplifier, V
sawtooth waveform to modulate the pulse width of the PWM
signals. The PWM signals control the timing of the Intersil
MOSFET drivers and regulate the converter output to the
specified reference voltage. Three distinct inputs to the error
amplifier determine the voltage level of V
and external circuitry which control voltage regulation is
illustrated in Figure 5.
AVG
AVG
=
=
I
----------------------------------
I
------------ -
1
OUT
N
+
I
2
N
r
----------------------
+
DS ON
R
…I
ISEN
(
N
)
ISEN
in the cooler channels and
COMP
COMP
. The modified V
, is compared to the
COMP
ISEN
. The internal
OUT
in Figure 3)
AVG
is the
,
COMP
(EQ. 4)
ER
, is