ISL6273 Datasheet, PDF (10/12 Pages) Intersil Corporation – 1.2A Low Quiescent Current 1.5MHz High Efficiency Synchronous Buck Regulator

ISL6273
Once the skip mode is entered, the pulse modulation starts
being controlled by the SKIP comparator shown in Figure 17.
Each pulse cycle is still synchronized by the PWM clock. The
N-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. Then 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 ISL6273 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 17. 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 1V, which is equivalent to 2.5A 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 FB
pin voltage for output short-circuit protection. When the FB 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.
POR Signal
The ISL6273 offers a power-on reset (POR) signal for
resetting the microprocessor at the power up. When the
output voltage is not within a power-good window, the POR
pin outputs an open-drain low signal to reset the
microprocessor. The output voltage is monitored through the
FB pin. For the fixed output voltage versions, the monitoring
node is the center of the resistive voltage divider. For the
adjustable version, the FB pin voltage is monitored directly.
When the voltage of the monitored node is within the window
of 0.736V and 0.864V, a power-good signal is issued to turn
off the open-drain POR pin. The rising edge of the POR
output is delayed by 200ms.
RSI Signal
The RSI signal is a reset input control for the POR signal.
The power-good signal is gated by the RSI signal, as shown
in Figure 17. When the RSI is high, the POR signal will
remain low, independent on the power good signal.
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 inrush 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.
Figure 10 shows the start-up waveforms.
Power MOSFETs
The power MOSFETs are optimized for best efficiency. The
on resistance for the P-MOSFET is typically150mΩ and the
on resistance for the N-MOSFET is typically 150mΩ.
100% Duty Cycle
The ISL6273 features 100% duty cycle operation to
maximize the battery life. When the battery voltage drops to
a level that the ISL6273 can no longer maintain the
regulation at the output, the regulator completely turns on
the P-MOSFET. The maximum drop out voltage under the
100% duty-cycle operation is the product of the load current
and the on resistance of the P-MOSFET.
Enable
The enable (EN) input allows user to control the turning on
or off the regulator for purposes such as power-up
sequencing. The the regulator is enabled, there is typically a
300µs delay for waking up the bandgap reference. Then the
soft start-up begins. When the regulator is disabled, the
P-MOSFET is turned off immediately and the N-MOSFET is
turned on.
Thermal Shut Down
The ISL6273 has built-in thermal protection. When the
internal temperature reaches 150°C, the regulator is
completely shut down. As the temperature drops to 130°C,
the ISL6273 resumes operation by stepping through a soft
start-up.
VCC By-Passing
The VCC is voltage is the supply to the internal control circuit
and is derived from the PVIN pin. An internal 10Ω resistor
connects the two pins and also serves as an filtering resistor.
An external 0.1µF ceramic capacitor is recommended to by-
pass the VCC supply.
10
FN9256.0
March 7, 2006