ISL91127IR Datasheet, PDF (9/12 Pages) Intersil Corporation – High Efficiency Buck-Boost Regulator with 4.5A Switches

ISL91127IR
voltage is lower than approximately 20% of the target output
voltage, switching frequency is reduced to a fraction of the
normal switching frequency to aid in producing low duty cycles
necessary to avoid input inrush current spikes. Once the output
voltage exceeds 20% of the target voltage, switching frequency is
increased to its nominal value.
When the target output voltage is higher than the input voltage,
there will be a transition from buck mode to boost mode during
the soft-start sequence. At the time of this transition, the ramp
rate of the reference voltage is decreased, such that the output
voltage slew rate is decreased. This provides a slower output
voltage slew rate.
The V_OUT ramp time is not constant for all operating conditions.
Soft-start into boost mode will take longer than soft-start into
buck mode. The total soft-start time into buck operating mode is
typically 2ms, whereas the typical soft-start time into boost
mode operating mode is typically 3ms. Increasing the load
current will increase these typical soft-start times.
Short-Circuit Protection
The ISL91127IR provides short-circuit protection by monitoring the
feedback voltage. When feedback voltage is sensed to be lower
than a certain threshold, the PWM oscillator frequency is reduced in
order to protect the device from damage. The P-channel MOSFET
peak current limit remains active during this state.
Thermal Shutdown
A built-in thermal protection feature protects the ISL91127IR, if the
die temperature reaches +155°C (typical). At this die temperature,
the regulator is completely shut down. The die temperature
continues to be monitored in this thermal shutdown mode. When
the die temperature falls to +125°C (typical), the device will resume
normal operation. When exiting thermal shutdown, the
ISL91127IR will execute its soft-start sequence.
Buck-Boost Conversion Topology
The ISL91127IR operates in either buck or boost mode. When
operating in conditions where PVIN is close to VOUT, ISL91127IR
alternates between buck and boost mode as necessary to
provide a regulated output voltage.
Figure 19 shows a simplified diagram of the internal switches
and external inductor.
L₁
LX1
LX2
PVIN
SWITCH A
SWITCH D
VOUT
SWITCH B
SWITCH C
FIGURE 19. BUCK-BOOST TOPOLOGY
PWM Operation
In Buck PWM mode, Switch D is continuously closed and
Switch C is continuously open. Switches A and B operate as a
synchronous buck converter when in this mode.
In Boost PWM mode, Switch A remains closed and Switch B
remains open. Switches C and D operate as a synchronous boost
converter when in this mode.
PFM Operation
During PFM operation in buck mode, Switch D is continuously
closed and Switch C is continuously open. Switches A and B
operate in discontinuous mode during PFM operation. During
PFM operation in boost mode, the ISL91127IR closes Switch A
and Switch C to ramp up the current in the inductor. When the
inductor current reaches a certain threshold, the device turns off
Switches A and C, then turns on Switches B and D. With Switches
B and D closed, output voltage increases as the inductor current
ramps down.
In most operating conditions, there will be multiple PFM pulses
to charge up the output capacitor. These pulses continue until
V_OUT has achieved the upper threshold of the PFM hysteretic
controller. Switching then stops, and remains stopped until V_OUT
decays to the lower threshold of the hysteretic PFM controller.
Operation with V_IN Close to V_OUT
When the output voltage is close to the input voltage, the
ISL91127IR will rapidly and smoothly switch from boost to buck
mode as needed to maintain the regulated output voltage. This
behavior provides excellent efficiency and very low output
voltage ripple.
Output Voltage Programming
The ISL91127IR is available in fixed and adjustable output
voltage versions. To use the fixed output version, the VOUT pin
must be connected directly to FB.
In the adjustable output voltage version (ISL91127IIAZ), an
external resistor divider is required to program the output
voltage. The FB pin has very low input leakage current, so it is
possible to use large value resistors (e.g., R₁ = 1MΩ and
R₂ = 324kΩ for V_OUT = 3.3V) in the resistor divider connected to
the FB input.
Applications Information
Component Selection
The fixed-output version of ISL91127IR requires only three
external power components to implement the buck-boost
converter: an inductor, an input capacitor and an output
capacitor.
The adjustable output version of ISL91127IR requires three
additional components to program the output voltage, as shown
in Figure 20. Two external resistors program the output voltage,
and a small capacitor is added to improve stability and response.
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FN8859.1
August 3, 2016