ISL78200 Datasheet, PDF (14/19 Pages) Intersil Corporation – 2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78200
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 DC output current is 5A limited by the
package.
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 θ_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 θ_JA is greatly
reduced. The θ_JA is highly related to layout and air flow
conditions. In layout, multiple vias (≥15) 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 θ_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
output 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.
Referring to Figure 28, a resistor divider from boost input voltage
to the EXT_BOOST pin is used to detect the boost input voltage.
When the voltage on the EXT_BOOST pin is below 0.8V, the boost
PWM is enabled with a fixed 500µs soft-start when the boost
duty cycle increases from t_MINON*Fs to ~50% and a 3µA sinking
current is enabled at the EXT_BOOST pin for hysteresis purposes.
When the voltage on the EXT_BOOST pin recovers to above 0.8V,
the boost PWM is disabled immediately. Use Equation 3 to
calculate the upper resistor R_UP (R1 in Figure 28) for a desired
hysteresis VHYS at boost input voltage
R_UP[MΩ] = ₃-^V---_[-^H--_μ--^Y--_A--^S--_]
(EQ. 3)
Use Equation 4 to calculate the lower resistor R_LOW (R2 in
Figure 28) according to a desired boost enable threshold.
R_LOW
=
---^R----^U----^P-----^â‹---⁰---^.--⁸----
VFTH – 0.8
(EQ. 4)
where VFTH is the desired falling threshold on boost input
voltage to turn on the boost, 3µA is the hysteresis current, and
0.8V is the reference voltage to be compared.
Note the boost start-up threshold has to be selected in a way that
the buck is operating well at close loop before boost start-up.
Otherwise, large inrush current at boost start-up could occur at
boost input due to the buck loop saturation.
Similarly, a resistor divider from boost output voltage to the
AUXVCC pin is used to detect the boost output voltage. When the
voltage on AUXVCC pin is below 0.8V, the boost PWM is enabled
with a fixed 500µs soft-start, and a 3µA sinking current is
enabled at AUXVCC pin for hysteresis purpose. When the voltage
on the AUXVCC pin recovers to above 0.8V, the boost PWM is
disabled immediately. Use Equation 3 to calculate the upper
resistor R_UP (R3 in Figure 28) according to a desired hysteresis
VHY at boost output voltage. Use Equation 4 to calculate the
lower resistor R_LOW (R4 in Figure 28) according to a desired
boost enable threshold at boost output.
14
FN7641.0
September 22, 2011