ISL91110IR
9
FN8709.0
April 17, 2015
Submit Document Feedback
Functional Description
Functional Overview
Refer to the “Block Diagram” on page 2. The ISL91110IR
implements a complete buck boost switching regulator, with
PWM controller, internal switches, references, protection circuitry
and control inputs.
The PWM controller automatically switches between buck and
boost modes as necessary to maintain a steady output voltage
with changing input voltages and dynamic external loads.
Internal Supply and References
Referring to the “Block Diagram” on page 2, the ISL91110IR
provides four power input pins. The PVIN pin supplies input power
to the DC/DC converter, while the VIN pin provides operating
voltage source required for stable V
REF
generation. Separate
ground pins (SGND and PGND) are provided to avoid problems
caused by ground shift due to the high switching currents.
Enable Input
The device is enabled by asserting the EN pin HIGH. Driving EN
LOW invokes a power-down mode, where most internal device
functions are disabled.
Soft Discharge
When the device is disabled by driving EN LOW, an internal resistor
between VOUT and SGND is activated to slowly discharge the
output capacitor. This internal resistor has a typical 120Ω
resistance.
POR Sequence and Soft-start
Asserting the EN pin HIGH allows the device to power-up. A
number of events occur during the start-up sequence. The
internal voltage reference powers up and stabilizes. The device
then starts to operate. There is a typical 1ms delay between
assertion of the EN pin and the start of switching regulator
soft-start ramp.
The soft-start feature minimizes output voltage overshoot and
input in-rush currents. During soft-start, the reference voltage is
ramped to provide a ramping V
OUT
voltage. While the output
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 in-rush 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 ISL91110IR 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 ISL91110IR, 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
ISL91110IR will execute its soft-start sequence.
Buck-Boost Conversion Topology
The ISL91110IR operates in either buck or boost mode. When
operating in conditions where PVIN is close to VOUT, ISL91110IR
alternates between buck and boost mode as necessary to
provide a regulated output voltage.
Figure 21
shows a simplified diagram of the internal switches
and external inductor.
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 ISL91110IR 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.
FIGURE 21. BUCK BOOST TOPOLOGY
PVIN
VOUT
SWITCH A SWITCH D
SWITCH B SWITCH C
LX1 LX2
L
1
