ISL6563 Intersil Corporation, ISL6563 Datasheet - Page 16

• Manufacturer Part Number: ISL6563
• Description: Two-Phase Multi-Phase Buck PWM Controller with Integrated MOSFET Drivers
• Manufacturer: Intersil Corporation

bulk capacitors allows them to supply the increased current
with less output voltage deviation.
The ESR of the bulk capacitors is also responsible for the
majority of the output-voltage ripple. As the bulk capacitors
sink and source the inductor ac ripple current, a voltage
develops across the bulk-capacitor ESR equal to I
once the output capacitors are selected and a maximum
allowable ripple voltage, V
analysis of the available output voltage budget, the following
equation can be used to determine a lower limit on the
output inductance.
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
∆V
While the previous equation addresses the leading edge, the
following equation gives the upper limit on L for cases where
the trailing edge of the current transient causes a greater
output voltage deviation than the leading edge.
Normally, the trailing edge dictates the selection of L, since
duty cycles are usually less than 50%. Nevertheless, both
inequalities should be evaluated, and L should be selected
based on the lower of the two results. In all equations in this
paragraph, L is the per-channel inductance and C is the total
output bulk capacitance.
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. These voltage
spikes can degrade efficiency, radiate noise into the circuit
and lead to device over-voltage stress. Careful component
layout and printed circuit design minimizes the voltage
spikes in the converter. Consider, as an example, the turnoff
transition of the upper PWM MOSFET. Prior to turnoff, the
upper MOSFET was carrying channel current. During the
turnoff, current stops flowing in the upper MOSFET and is
picked up by the lower MOSFET. 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.
L
L
L
≥
≤
≤
MAX
ESR
4 C V
--------------------------------
2.5 C
---------------- -
(
∆I
⋅
(
⋅
)
. This places an upper limit on inductance.
∆I
2
⋅
⋅
(
---------------------------------------------------------------- -
)
⋅
V
2
OUT
(
f
IN
∆V
S
⋅
–
MAX
V
⋅
2 V
(
IN
∆V
⋅
⋅
–
OUT
MAX
V
∆I ESR
PP MAX
⋅
) V
(
–
⋅
∆I ESR
PP(MAX)
OUT
⋅
)
)
16
⋅
(
V
IN
)
, is determined from an
–
V
O
)
PP
. Thus,
ISL6563
There are two sets of critical components in a DC-DC
converter using a ISL6563 controller. The power
components are the most critical because they switch large
amounts of energy. Next are small signal components that
connect to sensitive nodes or supply critical bypassing
current and signal coupling.
Note that as the ISL6563 does not allow external adjustment
of the channel-to-channel current balancing (current
information is multiplexed across a single R
is important to have a symmetrical layout, preferably with the
controller equidistantly located from the two power trains it
controls. Equally important are the gate drive lines (UGATE,
LGATE, PHASE): since they drive the power train MOSFETs
using short, high current pulses, it is important to size them
accordingly and reduce their overall impedance. Equidistant
placement of the controller to the two power trains also helps
keeping these traces equally long (equal impedances,
resulting in similar driving of both sets of MOSFETs).
The power components should be placed first. Locate the
input capacitors close to the power switches. Minimize the
length of the connections between the input capacitors, C
and the power switches. Locate the output inductors and
output capacitors between the MOSFETs and the load.
Locate the high-frequency decoupling capacitors (ceramic)
as close as practicable to the decoupling target, making use
of the shortest connection paths to any internal planes, such
as vias to GND immediately next, or even onto the capacitor
solder pad.
The critical small components include the bypass capacitors
for VCC and PVCC. Locate the bypass capacitors, C
close to the device. It is especially important to locate the
components associated with the feedback circuit close to
their respective controller pins, since they belong to a high-
impedance circuit loop, sensitive to EMI pick-up. It is
important to place the R
terminal of the ISL6563.
A multi-layer printed circuit board is recommended. Figure 9
shows the connections of the critical components for one
output channel of the converter. Note that capacitors C
and C
capacitors. Dedicate one solid layer, usually the one
underneath the component side of the board, 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. Keep the metal runs from the PHASE terminal
to inductor L
input power and output power nodes. Use copper filled
polygons on the top and bottom circuit layers for the phase
nodes. Use the remaining printed circuit layers for small signal
wiring. The wiring traces from the IC to the MOSFETs’ gates
and sources should be sized to carry at least one ampere of
current (0.02” to 0.05”).
xxOUT
OUT
could each represent numerous physical
short. The power plane should support the
ISEN
resistor close to the respective
ISEN
resistor), it
BP
xxIN
,
IN
,