PS1541
OPERATION DESCRIPTION
Under Voltage Lockout
The PS1541 is a high efficiency, fixed frequency, peak
current mode boost regulator. It has the architecture to
regulate the voltage at the feedback pin, so that a
regulated fixed current is achieved. The power MOS is
turned ON through the control circuitry, at the start of
each oscillator cycle and thus the charging phase is
initiated. The error amplifier, consisting of a voltage
comparator and current sense amplifier, is basically, a
PWM comparator. The voltage comparator amplifies the
difference between the reference and feed back voltage.
When the output of the current sense amplifier, reaches
the output of the voltage comparator, the POWER MOS is
turned OFF and thus the charging phase is terminated.
To prevent sub-harmonic oscillations at duty cycles
greater than 50 percent, a stabilizing ramp is added to
the current sense signal. In this way the peak current
level keeps the output in regulation. The PS1541 has
internal soft start mechanism, to limit the inrush current
at startup and to limit the amount of overshoot on the
output, also. An internal blanking time is provided during
start up, to prevent the start of switching before all the
circuitry become ready for operation. The current limit is
increased by a fourth every 60µs giving a total soft start
time of 240µs.
When the input supply goes too low (below 2.2V)
the PS1541 produces an internal UVLO (under
voltage lockout) signal that shuts down the chip.
This mechanism protects the chip from producing
false logic due to low input supply.
Thermal Overload Protection
Thermal-overload protection limits total power
dissipation in the PS1541. When the junction
temperature exceeds Tj= +145°C, the thermal
sensor signals the shutdown logic and turning off
most of the internal circuitry. The thermal sensor
turns internal circuitry on again after the IC’s
junction temperature drops by 20°C. The regulator
then starts functioning in the required mode based
on the supply voltage.
Thermal-Overload protection is designed to protect
the PS1541 in the event of a fault condition. For
continual operation, do not exceed the absolute
maximum junction temperature rating of Tj =
+125°C.
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. Use 12kΩ for the low side
resistor R2 of the voltage divider. Determine the
high-side resistor R1 by the equation:
R1= R2(VOUT - VFB)/VFB
Where VOUT is the output voltage.
Current Limit
The PS1541 includes a current limiter. It monitors
the peak current through the inductor and controls
gate of the power device.
Enable Input
The PS1541 features an active-high Enable input
(EN) pin that allows on/off control of the regulator.
The PS1541 bias current reduces to less than
microampere when it is shutdown. The Enable input
is TTL/CMOS compatible.
July 2007
www.picsemi.com
© 2007 Power IC. All Rights Reserved.
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