isl8009a Intersil Corporation, isl8009a Datasheet - Page 10

• Manufacturer Part Number: isl8009a
• Description: 1.5a Low Quiescent Current 1.6mhz High Efficiency Synchronous Buck Regulator
• Manufacturer: Intersil Corporation

Once the skip mode is entered, the pulse modulation starts
being controlled by the SKIP comparator shown in Figure 25.
Each pulse cycle is still synchronized by the PWM clock. The
P-MOSFET is turned on at the clock and turned off when its
current reaches 20% of the current limit value (0.2V at the
CSA output). As the average inductor current in each cycle
is higher than the average current of the load, the output
voltage rises cycle over cycle. When the output voltage
reaches 1.5% above the nominal voltage, the P-MOSFET is
turned off immediately and the inductor current is fully
discharged to zero and stays at zero. The output voltage
reduces gradually due to the load current discharging the
output capacitor. When the output voltage drops to the
nominal voltage, the P-MOSFET will be turned on again at
the clock, repeating the previous operations.
The regulator resumes normal PWM mode operation when
the output voltage drops 1.5% below the nominal voltage.
Mode Control
The ISL8009A has a SKIP pin that controls the operation
mode. When the SKIP 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 25. The
current sensing circuit has a gain of 0.4V/A, from the
N-MOSFET current to the CSA output. When the CSA
output reaches 0.8V, (which is equivalent to 2A for the switch
current) the OCP comparator is tripped to turn off the
P-MOSFET immediately.
Short-Circuit Protection
A 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 1/3 of the normal operation
value. This comparator is effective during start-up or an
output short-circuit event.
RSI/POR Function
When powering up, the open-collector Power-On-Reset
output holds low for about 1ms after VO reaches the preset
voltage. When the active-HI reset signal RSI is issued, POR
goes to low immediately and holds for the same period of
time after RSI comes back to LOW. The output voltage is
unaffected (refer to Figure 28). When the function is not
used, connect RSI to ground and leave the pull-up resistor,
R
The POR output also serves as a 1ms delayed Power-Good
signal when the pull-up resistor, R
needs to be directly (or indirectly through resistor, R
connected to ground for this to function properly.
4
, open at the POR pin.
10
4,
is installed. The RSI pin
5
)
ISL8009A
.
UVLO
When the input voltage is below the undervoltage lock out
(UVLO) threshold, the regulator is disabled.
Soft-Start-Up
The soft start-up eliminates the in-rush current during the
start-up. The soft-start block outputs a ramp reference to
both the voltage loop and the current loop. The two ramps
limit the inductor current rising speed as well as the output
voltage speed so that the output voltage rises in a controlled
fashion. At the very beginning of the start-up, the output
voltage is less than 0.2V; hence the PWM operating
frequency is 1/3 of the normal frequency.
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:.
Duty Cycle
The ISL8009A features duty cycle operation to maximize the
battery life. When the battery voltage drops to a level that the
ISL8009A can no longer maintain the regulation at the
output, the regulator completely turns on the P-MOSFET.
The maximum drop out voltage under the duty-cycle
operation is the product of the load current and the
ON-resistance of the P-MOSFET.
Enable
The Enable (EN) input allows the user to control the turning on
or off of 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, then the
soft-start-up begins. When the regulator is disabled, the
P-MOSFET and the N-MOSFET are turned off immediately.
The 100: soft discharge resistor from LX to GDN is
activated and pulls the output to 0V.
Thermal Shutdown
The ISL8009A has built-in thermal protection. When the
internal temperature reaches +140°C, the regulator is
completely shutdown. As the temperature drops to +120°C,
the ISL8009A resumes operation by stepping through a
soft-start-up.
POR
RSI
V
O
FIGURE 28. RSI AND POR TIMING DIAGRAM
2ms
25ns
MIN
2ms
February 13, 2008
FN6656.0