9
FN9073.9
October 21, 2015
normal operating load range, find the r
OCSET
resistor from
Equation 3 with:
1. The maximum r
DS(ON)
at the highest junction
temperature.
2. The minimum I
OCSET
from the specification table.
3. Determine I
PEAK
for, I
PEAK
> I
OUT(MAX)
+ (I/2)
where I is the output inductor ripple current.
For an equation for the ripple current see the section under
Component Selection 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.
When the controller enters hiccup mode the differential
voltage across the error amplifier forces the COMP pin to rail
HIGH to approximately 5V. When the controller begins a new
soft-start sequence out of hiccup mode the COMP pin will
need to discharge down to approximately 1.2V near the
beginning of the PWM ramp in order to start up correctly. To
ensure the controller can discharge the COMP pin fast
enough the R and C from COMP to FB must not have too
high a time constant. For time constant recommendations
refer to the section “Feedback Compensation” on page 10.
Current Sinking
The ISL6406 incorporates 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 ISL6406 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.
Application Guidelines
Layout Considerations
Layout is very important in high frequency switching
converter design. With power devices switching, 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 overvoltage 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 ISL6406. 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 6
shows the connections of the critical components in the
converter. Note that capacitors C
IN
and C
OUT
could each
represent numerous physical capacitors.
V
OUT
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
L
OUT
C
OUT
C
IN
+3.3V V
IN
KEY
COMP
ISL6406
UGATE
R4
R
2
C
BP
FB
GND
CPVOUT
FIGURE 6. PRINTED CIRCUIT BOARD POWER PLANES
AND ISLANDS
R
1
BOOT
C
2
VIA CONNECTION TO GROUND PLANE
LOAD
Q1
C
BOOT
PHASE
D1
R
3
C
3
C
1
Q2
LGATE
PHASE
VCC
C
VCC
ISL6406
