ISL78201 Datasheet, PDF (15/23 Pages) Intersil Corporation – 40V 2.5A Regulator with Integrated High-side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78201
state, once the AUXVCC pin voltage is over the AUX LDO Switch-
over Rising Threshold, the MAIN LDO is shut off and the
AUXILIARY LDO is activated to bias VCC. Since the AUXVCC pin
voltage is lower than input voltage V_IN, the internal LDO dropout
voltage and the consequent power loss is reduced. This feature
brings substantial efficiency improvements in light load range
especially at high input voltage applications.
When the voltage at AUXVCC falls below the AUX LDO Switch-over
Falling Threshold, the AUXILIARY LDO is shut off and the MAIN LDO
is re-activated to bias VCC. At the OV/UV fault events, the IC also
switch over back from AUXILIARY LDO to MAIN LDO.
The AUXVCC switchover function is offered in buck configuration.
It is not offered in boost configuration when the AUXVCC pin is
used to monitor the boost output voltage for OVP.
Input Voltage
With the part switching, the operating ISL78201 input voltage
must be under 40V. This recommendation allows for short
voltage ringing spikes (within a couple of ns time range) due to
part switching while not exceeding 44V as Absolute Maximum
Ratings.
The lowest IC operating input voltage (VIN pin) depends on VCC
voltage and the Rising and Falling V_CC POR Threshold in
Electrical Specifications table on page 6. At IC start-up when V_CC
is just over rising POR threshold, there is no switching yet before
the soft-start starts. So the IC minimum start-up voltage on VIN
pin is 3.05V (MAX of Rising V_CC POR). When the soft-start is
initiated, the regulator is switching and the dropout voltage
across the internal LDO increases due to driving current. Thus the
IC VIN pin shutdown voltage is related to driving current and V_CC
POR falling threshold. The internal upper side MOSFET has
typical 10nC gate drive. For a typical example of synchronous
buck with 4nC lower MOSFET gate drive and 500kHz switching
frequency, the driving current is 7mA total causing 70mV drop
across internal LDO under 3V Vin. Then the IC shut down voltage
on VIN pin is 2.87V (2.8V+0.07V). In practical design, extra room
should be taken into account with concerns of voltage spikes at
VIN.
With boost buck configuration, the input voltage range can be
expanded further down to 2.5V or lower depending on the boost
stage voltage drop upon maximum duty cycle. Since the boost
output voltage is connected to VIN pin as the buck inputs, after
the IC starts up, the IC will keep operating and switching as long
as the boost output voltage can keep the VCC voltage higher than
falling threshold. Refer to “Boost Converter Operation” on page 15 for
more details.
Output Voltage
The output voltage can be programmed down to 0.8V by a
resistor divider from V_OUT to FB. For Buck, the maximum
achievable voltage is (V_IN * D_MAX - V_DROP), where V_DROP is the
voltage drop in the power path including mainly the MOSFET
r_DS(ON) and inductor DCR. The maximum duty cycle D_MAX is
decided by (1 - Fs * t_MIN(OFF)).
Output Current
With the high-side MOSFET integrated, the maximum current
ISL78201 can support is decided by the package and many
operating conditions including input voltage, output voltage, duty
cycle, switching frequency and temperature, etc. From the
thermal perspective, the die temperature shouldn’t be above
+125°C with the power loss dissipated inside of the IC.
Figures 14 through 16 show the thermal performance of this
part operating in buck at different conditions. The part can
output 2.5A under typical buck application condition V_IN 8~36V,
V_O 5V, 500kHz, still air and +85°C ambient conditions. The
output current should be derated under any conditions causing
the die temperature to exceed +125°C.
Figure 14 shows a 5V, 2A output application over V_IN range
under +105°C ambient temperature with 100 CFM air flow.
Figure 15 shows 2A applications under +25°C still air conditions.
Different V_OUT (5V, 9V, 12V, 20V) applications thermal data are
shown over V_IN range at +25°C and still air. The temperature rise
data in this figure can be used to estimate the die temperature at
different ambient temperatures under various operating
conditions. Note: More temperature rise is expected at higher
ambient temperatures due to more conduction loss caused by
r_DS(ON) increase.
Figure 16 shows thermal performance under various output
currents and input voltages. It shows the temperature rise trend
with load and V_IN changes.
Basically, the die temperature equals the sum of ambient
temperature and the temperature rise resulting from power
dissipated from the IC package with a certain junction to
ambient thermal impedance _JA. The power dissipated in the IC
is related to the MOSFET switching loss, conduction loss and the
internal LDO loss. Besides the load, these losses are also related
to input voltage, output voltage, duty cycle, switching frequency
and temperature. With the exposed pad at the bottom, the heat
of the IC mainly goes through the bottom pad and _JA is greatly
reduced. The _JA is highly related to layout and air flow
conditions. In layout, multiple vias (20) are strongly
recommended in the IC bottom pad. In addition, the bottom pad
with its vias should be placed in ground copper plane with an
area as large as possible connected through multiple layers.
The _JA can be reduced further with air flow.
For applications with high output current and bad operating
conditions (compact board size, high ambient temperature, etc.),
synchronous buck is highly recommended since the external low-
side MOSFET generates smaller heat than external low-side
power diode. This helps to reduce PCB temperature rise around
the ISL78201 and less junction temperature rise.
Boost Converter Operation
The Typical Application Schematic III on page 5 shows the
circuits where the boost works as a pre-stage to provide input to
the following Buck stage. This is for applications when the input
voltage could drop to a very low voltage in some constants (in
some battery powered systems as an example), causing the
output voltage drops out of regulation. The boost converter can
be enabled to boost the input voltage up to keep the output
voltage in regulation. When the system input voltage recovers
back to normal, the boost stage is disabled while only the buck
stage is switching.
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March 31, 2015