ISL78233 Datasheet, PDF (14/18 Pages) Intersil Corporation – 3A and 4A Compact Synchronous Buck Regulators

ISL78233, ISL78234
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.
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 or the VIN UVLO is
set, the outputs discharge to GND through an internal 100Ω
switch.
Power MOSFETs
The power MOSFETs are optimized for best efficiency. The
ON-resistance for the P-FET is typically 35mΩ and the
ON-resistance for the N-FET is typically 11mΩ.
100% Duty Cycle
The ISL78233, ISL78234 features 100% duty cycle operation to
maximize the battery life. When the battery voltage drops to a
level that the ISL78233, ISL78234 can no longer maintain the
regulation at the output, the regulator completely turns on the
P-FET. The maximum dropout voltage under the 100% duty-cycle
operation is the product of the load current and the
ON-resistance of the P-FET.
Thermal Shutdown
The ISL78233, ISL78234 has built-in thermal protection. When the
internal temperature reaches +150°C, the regulator is completely
shut down. As the temperature drops to +125°C, the ISL78233,
ISL78234 resumes operation by stepping through the soft-start.
Applications Information
Output Inductor and Capacitor Selection
To consider steady state and transient operations, the ISL78233,
ISL78234 typically uses a 1.0µH output inductor. The higher or
lower inductor value can be used to optimize the total converter
system performance. For example, for 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-----^---_L--^{}--¹-_----_f^–--_S---^V--V-------I--^O--^N------^{}-
(EQ. 3)
The inductor’s saturation current rating needs to be at least
larger than the peak current. The ISL78233, ISL78234 protects
the typical peak current 4.9A/6.7A. The saturation current needs
to be over 7A for maximum output current application.
The ISL78233, ISL78234 uses an internal compensation
network and the output capacitor value is dependent on the
output voltage. The recommended ceramic capacitor is X5R or
X7R. The recommended X5R or X7R minimum output capacitor
values are shown in Table 1 on page 3.
In Table 1, 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.
Output Voltage Selection
The output voltage of the regulator can be programmed via an
external resistor divider that is used to scale the output voltage
relative to the internal reference voltage, and feed it back to the
inverting input of the error amplifier (see Figure 2 on page 3).
The output voltage programming resistor, R₂, will depend on the
value chosen for the feedback resistor and the desired output
voltage of the regulator. The value for the feedback resistor, R₃,
is typically between 10kΩ and 100kΩ, as shown in Equation 4.
R₂ = R_{3V}---^V--_F--^O--_B-- – 1_
(EQ. 4)
If the output voltage desired is 0.6V, then R₃ is left unpopulated
and R₂ is shorted. There is a leakage current from VIN to PHASE.
It is recommended to preload the output with 10µA minimum.
For better performance, add 15pF in parallel with R₂ (200kΩ).
Check loop analysis before use in application.
Input Capacitor Selection
The main functions for the input capacitor are to provide
decoupling of the parasitic inductance and to provide a filtering
function to prevent the switching current flowing back to the
battery rail. At least two 22µF X5R or X7R ceramic capacitors are
a good starting point for the input capacitor selection.
Loop Compensation Design
When COMP is not connected to VDD, COMP pin is active for
external loop compensation. The ISL78233, ISL78234 uses
constant frequency peak current mode control architecture to
achieve a fast loop transient response. An accurate current
sensing pilot device in parallel with the upper MOSFET is used for
peak current control signal and overcurrent protection. The
inductor is not considered as a state variable since its peak
current is constant, and the system becomes a single order
system. It is much easier to design a type II compensator to
stabilize the loop than to implement voltage mode control. Peak
current mode control has an inherent input voltage feed-forward
function to achieve good line regulation. Figure 35 on page 15
shows the small signal model of the synchronous buck regulator.
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FN8359.5
April 23, 2015