ISL8012

FN6616 Rev 2.00

Page 13 of 17

December 1, 2011

Mode Control

The ISL8012 has a MODE pin that controls the operation mode.

When the MODE pin is driven to low or shorted to ground, the

regulator operates in a forced PWM mode. The forced PWM

mode remains the fixed PWM frequency at light load instead of

entering the skip mode.

Overcurrent Protection

The overcurrent protection is realized by monitoring the CSA

output with the OCP comparator, as shown in Figure 2. The current

sensing circuit has a gain of 285mV/A, from the P-MOSFET current

to the CSA output. When the CSA output reaches 1V, which is

equivalent to 2.9A for the switch current (0.15V offset), the OCP

comparator is tripped to turn off the P-MOSFET immediately. The

overcurrent function protects the switching converter from a

shorted output by monitoring the current flowing through the

upper MOSFET.

Upon detection of overcurrent condition, the upper MOSFET will

be immediately turned off and will not be turned on again until

the next switching cycle.

Short-Circuit Protection

The short-circuit protection SCP comparator monitors the VFB pin

voltage for output short-circuit protection. When the VFB is lower

than 0.2V, the SCP comparator forces the PWM oscillator

frequency to drop to minimum value to reduce the power

dissipation. This comparator is effective during start-up or an

output short-circuit event.

RSI/PG Function

When powering up, the open-collector Power-Good output holds

active-HI reset signal RSI is issued, PG goes to low immediately

and holds for the same period of time after RSI comes back to

LOW. The output voltage is unaffected. Please refer to the timing

diagram in Figure 34. When the function is not used, connect RSI

The PG output also serves as a 1ms delayed Power-Good signal

directly (or indirectly through a resistor) connected to Ground for

PG to be actively monitoring the output voltage.

UVLO

When the input voltage is below the undervoltage lock-out (UVLO)

threshold, the regulator is disabled.

Soft Start-Up

The soft-start-up reduces the in-rush current during the start-up.

The soft-start block outputs a ramp reference to the input of the

error amplifier. This voltage ramp limits the inductor current as

well as the output voltage speed so that the output voltage rises

in a controlled fashion. When VFB is less than 0.2V at the

beginning of the soft-start, the switching frequency is reduced to

1/3 of the nominal value so that the output can start up

smoothly at light load condition. During soft-start, the IC operates

in the SKIP mode to support pre-biased output condition.

Enable

The enable (EN) input allows the user to control the turning on or

off 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 100



switch.

Power MOSFETs

The power MOSFETs are optimized for best efficiency. The

ON-resistance for the P-MOSFET is typically 120m

 and the

ON-resistance for the N-MOSFET is typically 110m

.

100% Duty Cycle

The ISL8012 features 100% duty cycle operation to maximize

the battery life. When the battery voltage drops to a level that the

ISL8012 can no longer maintain the regulation at the output, the

regulator completely turns on the P-MOSFET. The maximum

dropout voltage under the 100% duty-cycle operation is the

product of the load current and the ON-resistance of the

P-MOSFET.

Thermal Shut-Down

The ISL8012 has built-in thermal protection. When the internal

temperature reaches +140°C, the regulator is completely shut

down. As the temperature drops to +115°C, the ISL8012 resumes

operation by stepping through the soft-start.

Applications Information

Output Inductor and Capacitor Selection

To consider steady state and transient operations, ISL8012

typically uses a 2.2µH 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

approximately 30% of the maximum output current for optimized

MIN

25ns

1ms

PG

RSI

FIGURE 34. RSI AND PG TIMING DIAGRAM

1ms