ISL78235R5668 Datasheet, PDF (16/20 Pages) Intersil Corporation – Automotive 5A Synchronous Buck Regulator

ISL78235R5668
Tie SS to SGND for internal soft-start (1ms typical). Connect a
capacitor from SS to SGND to adjust the soft-start time. This
capacitor, along with an internal 2.1µA current source, sets the
soft-start interval of the converter, t_SS as shown by Equation 2.
C_SSF = 3.1  t_SSs
(EQ. 2)
C_SS must be less than 33nF to insure proper soft-start reset after
fault condition.
Enable
The Enable (EN) input allows the user to control the turning on or
off of the regulator for purposes such as power-up sequencing or
minimizing power dissipation when the output is not needed.
When the regulator is enabled, there is typically a 600µs delay
for waking up the bandgap reference and then the soft start-up
begins.
Discharge Mode (Soft-Stop)
When a transition to shutdown mode occurs, (EN low or fault
condition) or the V_IN UVLO is set, the output is discharged to GND
through an internal 100Ω switch on the PHASE pin.
Power MOSFETs
The power MOSFETs are optimized for highest efficiency. The
ON-resistance for the PFET is typically 35mΩ and the
ON-resistance for the NFET is typically 11mΩ.
100% Duty Cycle
The ISL78235R5668 features a 100% duty cycle operation to
maximize the battery operation life and provide very low dropout
down to the minimum operating voltage. When the battery
voltage drops to a level that the ISL78235R5668 can no longer
maintain the regulation at the output, the regulator completely
turns on the PFET. The maximum dropout voltage under the
100% duty cycle operation is the product of the load current and
the ON-resistance of the PFET.
Thermal Shutdown
The ISL78235R5668 has built-in over-temperature thermal
protection. When the internal temperature reaches +150°C, the
regulator completely shuts down. As the temperature drops to
+125°C, the ISL78235R5668 resumes operation after a
soft-start cycle.
Applications Information
8A Pulsed Load Operation
The ISL78235R5668 is intended for 5A continuous output
current operation. However, the peak current limit threshold of
the overcurrent comparator is set to allow up to 8A of load
current without going into Overcurrent Protection (OCP) for
automotive point-of-load regulation applications.
It is not recommended to operate the ISL78235R5668 above 5A
continuous operation. The peak current limit set point is intended
to allow pulse load currents that do not exceed 100µs and 10%
duty cycle for applications that require high peak transient
current while the nominal load is 5A or less. The inductor value
must be chosen accordingly such that the peak ripple current
does not trigger the current limit threshold (12A typical). The
overcurrent protection circuitry prevents continuous operation in
an overcurrent condition. The overcurrent protection circuitry is
not active until the output current exceeds the current limit
threshold. Because the current limit threshold is much higher
than 5A, a fault condition that causes the output current to
nominally be above 5A but below the current limit threshold is
not protected by the IC and such operating condition should be
avoided.
For applications where the load transient has a very fast di/dt
and large amplitude, the input decoupling capacitance must
provide the instantaneous current for the buck regulator. As a
result, there will be undershoot of the input voltage during the
transient duration. The voltage at the VDD pin of the
ISL78235R5668 must stay above the UVLO threshold for
continuous operation. If the input voltage crosses the UVLO
threshold, a power on reset is initiated. It is necessary to have
adequate input capacitance to support the application load
transient condition.
Output Inductor and Capacitor Selection
To consider steady state and transient operation, the
ISL78235R5668 typically uses a 0.33µH to 0.78µH output
inductor. Higher or lower inductor values can be used to optimize
the total converter system performance. For example, for a
higher output voltage 3.3V application, in order to decrease the
inductor current ripple and output voltage ripple, the output
inductor value can be increased. It is recommended to set the
ripple inductor current to approximately 30% of the maximum
output current for optimized performance. The inductor ripple
current can be expressed as shown in Equation 3:
I = ^V-----^O----^U----^T----_L-^----_{}---¹-_f--_S--–--_W---^V-------V--^O------^I--^U--^N-------^{T-----}-
(EQ. 3)
The inductor’s saturation current rating needs to be larger than
the positive peak current limit specified in the electrical
specification table. The ISL78235R5668 has a typical peak
current limit of 12A. The inductor saturation current needs to be
over 12A for proper operation.
The ISL78235R5668 uses an internal compensation network for
regulator stability and the output capacitor value is dependent on
the output voltage. The recommended ceramic capacitors are
low ESR X7R rated or better. The recommended minimum output
capacitor values are shown in Table 2 on page 5.
Table 2 shows the minimum output capacitor value is given for
the different output voltages to make sure that the whole
converter system is stable. Additional output capacitance should
be added for better performance in applications where high load
transient or low output ripple is required. It is recommended to
check the system level performance along with the simulation
model.
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FN8769.1
January 21, 2016