ISL6534 Datasheet, PDF (11/26 Pages) Intersil Corporation – Dual PWM with Linear

ISL6534
This causes VOUT2 to track VOUT1 at one-half its value.
Connect VOUT2 to FB2 (through the compensation resistor).
A buffered copy of REFIN is provided on REFOUT.
For Independent mode operation on OUT2 (Figure 3), a 3.3V
reference is provided on VREF which can be used directly, or
divided down for REFIN. A resistor divider from VOUT2 to
FB2 sets the output voltage.
Operational Modes
Table 1 shows how to select the various modes and phasing
between the two switching regulators.
TABLE 1. MODE AND PHASE SELECTION
MODE
DDR
DDR
Independent
Independent
EN_SS2 REFOUT PWM1/2
CH1/2
VCC
VCC
0 deg EN1/SS1 enables
CH1 and CH2
VCC
Open 90 deg
“
SS2 cap VCC
0 deg EN1/SS1 for Ch1;
EN2/SS2 for Ch2
SS2 cap Open 180 deg
“
DDR mode is chosen by connecting the SS2/EN2 pin to
VCC (5V). In this mode, SS1/EN1 is used to enable and soft-
start both OUT1 and OUT2 (note that only a single 30µA
current source is charging a single soft-start capacitor). In
addition, VOUT1 (usually divided by 2) can be used as the
REFIN for OUT2. VOUT1 is often used as VIN2 (especially
when the VOUT2 current is low enough) although it is not
necessary. And OUT2 does allow both sinking and sourcing
of current for the DDR.
For Independent mode, SS2/EN2 is not connected to VCC.
Instead it is connected to a soft-start capacitor to GND,
similar to SS1/EN1. The capacitors will ramp each output
independently, and each can be turned off by pulling its
SS/EN pin to GND; releasing will start a new soft-start ramp.
SS3/EN3 is also independent of the first two. As explained
earlier, one capacitor can be shared by more than one
SS/EN pin.
To select the Phase shift between Channel 1 and 2, the
REFOUT pin is used. Tie it to the VCC pin to get 0 degrees
in either mode (which means both switchers are in phase). In
this case, the REFOUT pin is not available for use
elsewhere; the buffer is disabled. Leave REFOUT open
(driven to whatever voltage is supplied at REFIN) and it
selects 90 degrees in the DDR mode, or 180 degrees in
Independent mode; REFOUT can be used as a reference in
this case. The advantage of Phase shift is to keep the
switching current spikes from lining up to create even higher
noise, or interaction between the channels; it also reduces
the RMS current through the input capacitors, allowing fewer
caps to be employed. However, depending on the VOUT to
VIN ratios of both, there is no guarantee that opposite edges
might not line up, depending on the duty cycles; so the user
should check for that possibility.
Figure 4 shows the phases. The rising edge of LGATE1
(LG1) and LGATE2 (LG2) is fixed; the phase difference is
relative to the rising edges. The falling edge of each is the
variable one (determined by the duty cycle). LG1 is shown
with a pulse width shorter than LG2; this is just an arbitrary
example, and it does not affect the rising edges.
LG1
LG2 (0 deg)
LG2 (90 deg)
LG2 (180 deg)
0
90 180 270 0
FIGURE 4. PHASE OF LG2 WITH RESPECT TO RISING EDGE
OF LG1
Output Regulation
The basic PWM regulator voltage is usually set up as
follows: FB and the internal reference are the two inputs to
the error amplifier, which are forced to be equal. The output
voltage is externally divided down to the FB pin, to equal the
reference. In the ISL6534, VOUT1 uses an internal 0.6V
reference; VOUT2 uses an external REFIN pin for the
reference. There are many variations of the above,
especially when the modes (Independent or DDR) are also
considered. Below are some of the cases that can be used,
along with the advantages or disadvantages of each.
The following figures show the compensation circuit for
VOUT1 and VOUT2; they include a full Type 3 compensation
network. Also shown is the resistor divider for REFIN.
Several notes:
1. The labeling of the resistors may not match other
diagrams; they should be used just for the equations
included.
2. The VREF pin (nominal 3.3V) is assumed here, but any
other appropriate fixed voltage reference can be used as
REFIN for OUT2.
3. One percent (or better) resistors are typically used for
these resistor dividers; the overall system accuracy
depends directly upon them. Exact ratios are not always
possible, due to the limited values of standard resistors
available; these errors must also be added to the
tolerance.
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FN9134.1