ISL70001SRH Datasheet, PDF (13/16 Pages) Intersil Corporation – Radiation Hardened and SEE Hardened 6A Synchronous Buck Regulator with Integrated MOSFETs

ISL70001SRH
inductance should satisfy Equation 14 to insure this
requirement is met.
L_OUT
≥
----------------------------------⁴----^.-³----²----^μ---^H-------------------------------------
NumberofLXxPinsConnected
(EQ. 14)
Input Capacitor Selection
Input capacitors are responsible for sourcing the AC
component of the input current flowing into the
switching power devices. Their RMS current capacity
must be sufficient to handle the AC component of the
current drawn by the switching power devices which is
related to duty cycle. The maximum RMS current
required by the regulator is closely approximated by
Equation 15.
IRMSMAX =
V_{-----O-----U-----T--}
VIN
×
⎛
⎜
⎝
IO
U
TMA
2
X
+
--¹----
12
×
⎛
⎜
⎝
V_{-----I--N------}–_----V_{----O------U----T--}
L_OUT × fs
×
V_{----V-O---I--UN-----T--⎠⎟⎞}
2_⎞
⎟
⎠
(EQ. 15)
The important parameters to consider when selecting
an input capacitor are the voltage rating and the RMS
ripple current rating. For reliable operation, select
capacitors with voltage ratings at least 1.5x greater
than the maximum input voltage. The capacitor RMS
ripple current rating should be higher than the largest
RMS ripple current required by the circuit.
Ceramic capacitors with X7R dielectric are
recommended. Alternately, a combination of low ESR
solid tantalum capacitors and ceramic capacitors with
X7R dielectric may be used. The ISL70001SRH
requires a minimum effective input capacitance of
100µF for stable operation.
PCB Design
PCB design is critical to high-frequency switching
regulator performance. Careful component placement
and trace routing are necessary to reduce voltage
spikes and minimize undesirable voltage drops.
Selection of a suitable thermal interface material is
also required for optimum heat dissipation and to
provide lead strain relief.
PCB Plane Allocation
Four layers of two ounce copper are recommended.
Layer 2 should be a dedicated ground plane with all
critical component ground connections made with vias
to this layer. Layer 3 should be a dedicated power
plane split between the input and output power rails.
Layers 1 and 4 should be used primarily for signals,
but can also provide additional power and ground
islands as required.
PCB Component Placement
Components should be placed as close as possible to
the IC to minimize stray inductance and resistance.
Prioritize the placement of bypass capacitors on the
pins of the IC in the order shown: REF, SS, AVDD,
DVDD, PVINx (high frequency capacitors), EN, PGOOD,
PVINx (bulk capacitors).
Locate the output voltage resistive divider as close as
possible to the FB pin of the IC. The top leg of the
divider should connect directly to the POL (Point Of
Load) and the bottom leg of the divider should connect
directly to AGND. The junction of the resistive divider
should connect directly to the FB pin.
Locate a Schottky clamp diode as close as possible to
the LXx and PGNDx pins of the IC. A small series R-C
snubber connected from the LXx pins to the PGNDx
pins may be used to damp high frequency ringing on
the LXx pins if desired.
PCB Layout
Use a small island of copper to connect the LXx pins of
the IC to the output inductor on layers 1 and 4. Void
the copper on layers 2 and 3 adjacent to the island to
minimize capacitive coupling to the power and ground
planes. Place most of the island of layer 4 to minimize
the amount of copper that must be voided from the
ground plane (layer 2).
Keep all other signal traces as short as possible.
For an example layout refer to ANxxxx.
Thermal Management
For optimum thermal performance, place a pattern of
vias on the top layer of the PCB directly underneath
the IC. Connect the vias to the ground plane on layer
2, which serves as a heatsink. To insure good thermal
contact, thermal interface material such as a Sil-Pad or
thermally conductive epoxy should be used to fill the
gap between the vias and the bottom of the IC.
Lead Strain Relief
Use of a Sil-Pad or a thin layer of thermally conductive
epoxy will raise the bottom of the IC from the PCB
surface so that a slight bend can be added to the leads
of the IC for strain relief.
13
FN6947.0
December 15, 2009