ISL70003ASEH Datasheet, PDF (28/36 Pages) Intersil Corporation – Radiation and SEE Tolerant 3V to 13.2V, 9A Buck Regulator

ISL70003ASEH
High Current Protection Clamp
When using the ISL70003ASEH to output >6A it is necessary to
implement a LX to PGND Schottky diode clamp to prevent
damage to the lower power FET devices. The MBRS320T3G diode
is used on the ISL70003ASEHEV1Z evaluation platform.
Derating Current Capability
Most space programs issue specific derating guidelines for parts,
but these guidelines take the pedigree of the part into account.
For instance, a device built to MIL-PRF-38535, such as the
ISL70003ASEH, is already heavily derated from a current density
standpoint. However, a mil-temp or commercial IC that is
up-screened for use in space applications may need additional
current derating to ensure reliable operation because it was not
built to the same standards as the ISL70003ASEH.
FIGURE 56. CURRENT vs TEMPERATURE
Figure 56 shows the wear out maximum average output current
of the ISL70003ASEH with respect to junction temperature for
0.1% failure at 100k hours of operation. This plot takes into
account the worst-case current share mismatch in the power
blocks and the current density requirement of MIL-PRF-38535
(< 2 x 10⁵ A/cm²). The plot clearly shows that the
ISL70003ASEH can handle 7A at +150°C from a worst-case
current density standpoint, but the part is rated to 6A. Therefore,
no further current derating of the ISL70003ASEH is needed.
General Design Guide
This design guide is intended to provide a high-level explanation
of the steps necessary to design the power stage and feedback
compensation network of a single phase power converter. It is
assumed that the reader is familiar with many of the basic skills
and techniques in switch mode power supply design. In addition
to this guide, Intersil provides an evaluation board that includes
schematic, bills of materials and board layout.
Output Inductor Selection
The output inductor is selected to minimize the converter’s
response time to a load transient and meet steady state output
voltage ripple requirements. The inductor value determines the
converter’s inductor ripple current and the output voltage ripple
is a function of the inductor ripple current. The output voltage
ripple and the inductor ripple current are approximated by using
Equation 13:
I = ^V------^I---_f^N--_S-----^–-_W-----^V---_---^O----_L--^U------^T---  -^V---_V-^O---_I-^U-_N---^T--
V_OUT = I  ESR
(EQ. 13)
Increasing the value of inductance reduces the ripple current and
output voltage ripple. However, the large inductance values
reduce the converter’s response time to a load transient.
One of the parameters limiting the converter’s response to a load
transient is the time required to change the inductor current. The
response time is the time required to slew the inductor current
from an initial current value to the transient current level. During
this interval the difference between the inductor current and the
transient current level must be supplied by the output capacitor.
Minimizing the response time can minimize the output
capacitance required.
The response time to a transient is different for the application of
load and the removal of load. Equation 14, gives the approximate
response time interval for application and removal of a transient
load.
t_RISE =
L x I_TRAN
V_IN - V_OUT
t_FALL =
L x I_TRAN
V_OUT
(EQ. 14)
Where I_TRAN is the transient load current step, t_RISE is the
response time to the application of load, and t_FALL is the
response time to the removal of load. The worst case response
time can be either at the application or removal of load. Be sure
to check both Equations 13 and 14 at the minimum and
maximum output levels for the worst case response time.
Output Capacitor Selection
An output capacitor is required to filter the inductor current and
supply the load transient current. The filtering requirements are a
function of the switching frequency and the ripple current. The
load transient requirements are a function of the slew rate (di/dt)
and the magnitude of the transient load current. These
requirements are generally met with a mix of capacitors and
careful layout.
High-frequency capacitors initially supply the transient and slow
the current load rate seen by the bulk capacitors. The bulk filter
capacitor values are generally determined by the ESR (Effective
Series Resistance) and voltage rating requirements rather than
actual capacitance requirements.
High-frequency decoupling capacitors should be placed as close
to the power pins of the load as physically possible. Be careful
not to add inductance in the circuit board wiring that could
cancel the usefulness of these low inductance components.
The shape of the output voltage waveform during a load transient
that represents the worst case loading conditions will ultimately
determine the number of output capacitors and their type. When
this load transient is applied to the converter, most of the energy
required by the load is initially delivered from the output
capacitors. This is due to the finite amount of time required for
the inductor current to slew up to the level of the output current
required by the load. This phenomenon results in a temporary dip
in the output voltage. At the very edge of the transient, the
Equivalent Series Inductance (ESL) of each capacitor induces a
spike that adds on top of the existing voltage drop due to the
Equivalent Series Resistance (ESR).
Submit Document Feedback ₂₈
FN8746.0
August 5, 2015