ISL6432 Datasheet, PDF (7/12 Pages) Intersil Corporation – PWM and Triple Linear Power Controller for Home Gateway Applications

ISL6432
There may be a second restriction on the size of the
resistors used to set the linear regulators’ output voltage
based on ACPI functionality. Read the ‘ACPI
Implementation’ section under ‘Application Guidelines’ to
see if this additional constraint concerns your application.To
ensure the parallel combination of the feedback resistors
equals a certain chosen value, R_FB, use the following
equations:
R_S
=
V_{-----O----U----T--}
V_FB
×
R_FB
R_P
=
----^R-----^S-----^×-----^V----^F---^B------ ,
V_OUT – V_FB
where
V_OUT - the desired output voltage,
V_FB - feedback (reference) voltage, 0.8V.
Application Guidelines
Soft-Start Interval
The soft-start function controls the output voltages rate of rise
to limit the current surge at start-up. The soft-start function is
integrated on the chip and the soft-start interval is thus fixed.
Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. The voltage
spikes can degrade efficiency, radiate noise into the circuit,
and lead to device overvoltage stress. Careful component
layout and printed circuit design minimizes the voltage
spikes in the converter. Consider, as an example, the turn-
off transition of the upper PWM MOSFET. Prior to turn-off,
the upper MOSFET was carrying the full load current.
During the turn-off, current stops flowing in the upper
MOSFET and is picked up by the lower MOSFET or
Schottky diode. Any inductance in the switched current
path generates a large voltage spike during the switching
interval. Careful component selection, tight layout of the
critical components, and short, wide circuit traces minimize
the magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using a ISL6432 controller. The switching power
components are the most critical because they switch large
amounts of energy, and as such, they tend to generate
equally large amounts of noise. The critical small signal
components are those connected to sensitive nodes or
those supplying critical bypass current.
The power components and the controller IC should be
placed first. Locate the input capacitors, especially the high-
frequency ceramic decoupling capacitors, close to the power
switches. Locate the output inductor and output capacitors
between the MOSFETs and the load. Locate the PWM
controller close to the MOSFETs.
+5V_IN
L_IN
+
C_IN
V_OUT2
+
C_OUT2
+12V
C_VCC
VCC GND
OCSET
C_OCSET
R_OCSET
UGATE
PHASE
Q1
L_OUT
V_OUT1
DRIVE2
Q3
LGATE
Q2
C_OUT1⁺
CR1
V_OUT3
+
C_OUT3
Q4
ISL6432
V_OUT4
DRIVE3 DRIVE4
+
C_OUT4
PGND
Q5
+3.3V_IN
KEY
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT OR POWER PLANE LAYER
VIA CONNECTION TO GROUND PLANE
FIGURE 4. PRINTED CIRCUIT BOARD POWER PLANES AND
ISLANDS
The critical small signal components include the bypass
capacitor for VCC and the feedback resistors. Locate these
components close to their connecting pins on the control IC.
A multi-layer printed circuit board is recommended. Figure 4
shows the connections of the critical components in the
converter. Note that the capacitors C_IN and C_OUT each
represent numerous physical capacitors. Dedicate one solid
layer for a ground plane and make all critical component
ground connections with vias to this layer. Dedicate another
solid layer as a power plane and break this plane into smaller
islands of common voltage levels. The power plane should
support the input power and output power nodes. Use copper
filled polygons on the top and bottom circuit layers for the
PHASE nodes, but do not unnecessarily oversize these
particular islands. Since the PHASE nodes are subjected to
very high dV/dt voltages, the stray capacitor formed between
these islands and the surrounding circuitry will tend to couple
switching noise. Use the remaining printed circuit layers for
small signal wiring. The wiring traces from the control IC to the
MOSFET gate and source should be sized to carry 2A peak
currents.
PWM Controller Feedback Compensation
The PWM controller uses voltage-mode control for output
regulation. This section highlights the design consideration
for a PWM voltage-mode controller. Apply the methods and
considerations only to the PWM controller.
Figure 5 highlights the voltage-mode control loop for a
synchronous-rectified buck converter. The output voltage
(V_OUT) is regulated to the Reference voltage level, 0.8V. The
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