PS0020
OPERATION DESCRIPTION
The PS0020 is
a
compact, high-efficiency,
True Shutdown Mode
synchronous boost converter in a 6-leads SOT-26
package designed for space-restricted applications.
The parts are available with adjustable output
voltages ranging from 2.5V to 5.0V. They are able
to start up with input voltages as low as 0.8V and
operate with input voltages down to 0.5V. With its
internal synchronous rectifier and low on-resistance
of the internal NMOS switch, the devices maintain
high efficiency over a wide range of load current.
As shown in the Block Diagram, the PS0020
consists of accurate band gap core, error amplifier,
start-up oscillator and control logic unit along with
PMOS and NMOS switches. With current mode PWM
control, the PS0020 has ultra-fast line and improved
load regulation. Moreover, the PS0020 provides real
shutdown circuitry, which disconnects the output
from the input in shutdown and results in discharge
of the output to ground.
The PS0020 is designed to allow the true shutdown
by managing the body diode of the PMOS switch.
As the PMOS switch is turned off and there is no
conductive path through the body of the PMOS, the
output is allowed to go to zero during shutdown,
drawing zero current from the input.
Output Voltage Selection
The PS0020A features a user-adjustable output
through an external feedback network. A voltage
divider from Vout to ground programs the output
voltage via FB from 2.5V to 5V using the following
equation:
Vout=Vref*[1+(R1/R2)]
Where, Vref=1V
Thermal Overload Protection
Thermal-overload protection limits total power
dissipation in the PS0020. When the junction
temperature exceeds TJ = +145°C, the thermal
sensor signals the shutdown logic and turns off
most of the internal circuitry. The thermal sensor
turns internal circuitry on again after the IC’s
junction temperature drops by 15°C.
Start-Up Mode:
The PS0020 will start up typically at 0.8V. When it
is turned-on, the device gets its start up bias from
Vin. A start-up oscillator, which runs typically at 100
KHz, brings the output voltage high enough so that
Vout exceeds Vin. Once Vout exceeds Vin, internal
bias switches from Vin to Vout by an internal bias-
select circuit. Thus, once started (i.e. Vout exceeds
Vin), internal circuit bias is completely independent
of Vin. The start-up oscillator runs at 66% duty
cycle around 100KHz. Once Vout exceeds Vin
(typically 1.9V), the start-up oscillator is disabled
and the normal fixed Ton PWM operation takes
over.
Thermal-Overload protection is designed to protect
the PS0020 in the event of a fault condition. For
continual operation, do not exceed the absolute
maximum junction temperature rating of TJ =
+125°C.
Current Limit and Short Circuit Protection
The PS0020 includes a current limiter that monitors
the peak inductor current through the NMOS switch
and turns the NOMS switch off when the inductor
peak current exceeds 300mA. Because of its true
shutdown feature, the IC is allowed to be short-
circuited. The thermal shutdown turns off the
regulator in case of excessive heating, when the die
temperature reaches 145°C.
Internal PMOS and NMOS switches
The PS0020 features a 0.45Ω NMOS switch and a
0.6Ω PMOS switch. In normal operation these
switches are alternatively turned-on and thus
initiate charging of the inductor from Vin and then
discharging of it to the output capacitor and the
load. However, between the event of one switch
turning-off and the other turning-on, a dead time is
provided to avoid huge in-rush current from output
to ground via switches. In the dead time, both
switches remain off and the inductor discharges via
body diode of the PMOS switch to the output.
Version 1.1
July 6, 2007
www.picsemi.com
© 2007 Power IC. All Rights Reserved.
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