ISL91110

8

FN8434.4

February 5, 2016

Submit Document Feedback

Functional Description

Functional Overview

Refer to the “Block Diagram” on page 2. The ISL91110

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 ISL91110

provides four power input pins. The PVIN pin supplies input power

to the DC/DC converter, while the VIN pin provides operating

ground pins (GND 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 GND 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 operating. 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 inrush currents. During soft-start, the reference voltage is

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.

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.

Overcurrent Protection

The ISL91110 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.

When the current in the P-Channel MOSFET is sensed to reach the

current limit for 16 consecutive switching cycles, the internal

protection circuit is triggered, and switching is stopped for

approximately 40ms. The device then performs a soft-start cycle. If

the external output overcurrent condition exists after the soft-start

cycle, the device will again detect 16 consecutive switching cycles

reaching the peak current threshold and turns off for 40ms. The

process will repeat as long as the external overcurrent condition is

present. This behavior is called “hiccup mode”.

LX2 (2V/DIV)

LX1 (2V/DIV)

20ms/DIV

10µs/DIV