ISL78200_14 Datasheet, PDF (14/22 Pages) Intersil Corporation – 2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78200
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 ISL78200 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. At IC start-up when VCC 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 VCC 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 14 for
more details.
Output Voltage
The ISL78200 output voltage can be programmed down to 0.8V
by a resistor divider from V_OUT to FB. 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
ISL78200 can support is decided by the package and many
operating conditions including input voltage, output voltage, duty
cycle, switching frequency and temperature, etc.
First: The maximum output current is limited by the maximum
OC threshold that is 4.18A (TYP).
Second: From the thermal perspective, the die temperature
shouldn’t be above +125°C with the power loss dissipated inside
of the IC. Figures 12 through 14 show the thermal performance
of this part operating at different conditions. The part can output
2.5A under typical 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 12 shows a 5V, 2A output application over V_IN range under
+105°C ambient temperature with 100 CFM air flow.
Figure 13 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 14 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. Refer to Figure 12
for the thermal performance with 100 CFM air flow.
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.
EXT_BOOST pin is used to set boost mode and monitor the boost
input voltage. At IC start-up before soft-start, the controller will
latch in boost mode when the voltage on this pin is above
200mV; it will latch in synchronous buck mode if voltage on this
pin is below 200mV. In boost mode, the low-side driver output
PWM has the same PWM signal with the buck regulator.
In boost mode, the EXT_BOOST pin is used to monitor boost input
voltage to turn on and turn off the boost PWM. The AUXVCC pin is
used to monitor the boost output voltage to turn on and turn off
the boost PWM.
14
FN7641.2
December 24, 2013