ISL8012(2008) 查看數據表（PDF） - Intersil
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ISL8012 Datasheet PDF : 15 Pages
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ISL8012
The regulator resumes normal PWM mode operation when
the output voltage drops 1.5% below the nominal voltage.
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 low for about 1ms after VO reaches the preset voltage.
When the 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 to ground and leave the
pull-up resister R4 open at the PG pin.
The PG output also serves as a 1ms delayed Power-Good
signal when the pull-up resister R1 is installed. The RSI pin
needs to be directly (or indirectly through a resistor)
connected to Ground for PG to be actively monitoring the
output voltage.
VO
MIN
RSI
25ns
1ms
1ms
PG
FIGURE 34. RSI AND PG TIMING DIAGRAM
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 100Ω switch.
Power MOSFETs
The power MOSFETs are optimized for best efficiency. The
ON-resistance for the P-MOSFET is typically 120mΩ and
the ON-resistance for the N-MOSFET is typically 110mΩ.
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
13
FN6616.0
March 11, 2008
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