9
FN9056.11
September 29, 2015
For an equation for the ripple current see the section under
component guidelines titled “Output Inductor Selection” on
page 11.
A small ceramic capacitor should be placed in parallel with
R
OCSET
to smooth the voltage across
R
OCSET
in the
presence of switching noise on the input voltage.
Current Sinking
The ISL6527, ISL6527A incorporate a MOSFET
shoot-through protection method, which allows a converter
to sink current as well as source current. Care should be
exercised when designing a converter with the ISL6527,
ISL6527A when it is known that the converter may sink
current.
When the converter is sinking current, it is behaving as a
boost converter that is regulating its input voltage. This
means that the converter is boosting current into the input
rail of the regulator. If there is nowhere for this current to go,
such as to other distributed loads on the rail or through a
voltage limiting protection device, the capacitance on this rail
will absorb the current. This situation will allow the voltage
level of the input rail to increase. If the voltage level of the rail
is boosted to a level that exceeds the maximum voltage
rating of any components attached to the input rail, then
those components may experience an irreversible failure or
experience stress that may shorten their lifespan. Ensuring
that there is a path for the current to flow other than the
capacitance on the rail will prevent this failure mode.
External Reference
The ISL6527, ISL6527A allow the designer to determine the
reference voltage that is used. This allows the ISL6527,
ISL6527A to be used in many specialized applications, such
as the V
TT
termination voltage in a DDR Memory power
supply, which must track the V
DDQ
voltage by 50%. Care
must be taken to insure that this voltage does not exceed
1.5V.
Application Guidelines
Layout Considerations
Layout is very important in high frequency switching
converter design. With power devices switching efficiently at
300kHz or 600kHz, the resulting current transitions from one
device to another cause 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 board design
minimizes the voltage spikes in the converters.
As an example, consider the turn-off transition of the PWM
MOSFET. Prior to turn-off, the MOSFET is carrying the full
load current. During turn-off, current stops flowing in the
MOSFET and is picked up by the lower MOSFET. Any
parasitic 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 traces minimizes the
magnitude of voltage spikes.
There are two sets of critical components in a DC/DC
converter using the ISL6527, ISL6527A. The switching
components are the most critical because they switch large
amounts of energy, and therefore tend to generate large
amounts of noise. Next, are the small signal components,
which connect to sensitive nodes or supply critical bypass
current and signal coupling.
A multi-layer printed circuit board is recommended. Figure 4
shows the connections of the critical components in the
converter. Note that capacitors C
IN
and C
OUT
could each
represent numerous physical capacitors. Dedicate one solid
layer, usually a middle layer of the PC 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 terminals to the output inductor short. 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. Use the remaining printed
circuit layers for small signal wiring. The wiring traces from
the GATE pins to the MOSFET gates should be kept short
and wide enough to easily handle the 1A of drive current.
The switching components should be placed close to the
ISL6527, ISL6527A first. Minimize the length of the
connections between the input capacitors, C
IN
, and the power
switches by placing them nearby. Position both the ceramic
and bulk input capacitors as close to the upper MOSFET drain
as possible. Position the output inductor and output capacitors
between the upper MOSFET and lower MOSFET and the
load.
The critical small signal components include any bypass
capacitors, feedback components, and compensation
components. Position the bypass capacitor, C
BP
, close to
the VCC pin with a via directly to the ground plane. Place the
PWM converter compensation components close to the FB
and COMP pins. The feedback resistors for both regulators
should also be located as close as possible to the relevant
FB pin with vias tied straight to the ground plane as required.
Feedback Compensation
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. The
error amplifier (Error Amp) output (V
E/A
) is compared with
the oscillator (OSC) triangular wave to provide a
pulse-width modulated (PWM) wave with an amplitude of
V
IN
at the PHASE node. The PWM wave is smoothed by the
output filter (L
O
and C
O
).
ISL6527, ISL6527A
