PS0021 Datasheet, PDF (9/12 Pages) Power IC Ltd. – High Efficiency PFM Step-up DC/DC Converter

PS0021
OPERATION
The PS0021 is a compact, high-efficiency, synchronous
boost converter in a SOT-26 package designed for space-
restricted applications. The part is available with
adjustable output voltages ranging from 2.5V to 5.0V. It
is 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 device maintains high
efficiency over a wide range of load current. As shown in
the Block Diagram, the PS0021 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 PS0021 has ultra-fast line
and improved load regulation. Moreover, the PS0021
provides real shutdown circuitry which disconnects the
output from the input during shutdown and results in the
discharge of the output to ground.
Start-Up Mode
The PS0021 starts 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 650 KHz, takes the
output voltage high enough so that V_OUT exceeds V_IN.
Once V_OUT exceeds V_IN, the internal bias switches from
V_IN to V_OUT by an internal bias-select circuit. Thus, once
started (i.e. V_OUT exceeds V_IN), the internal circuit bias is
completely independent of V_IN. The start-up oscillator
runs at 66% duty cycle at around 650KHz. Once V_OUT
exceeds V_IN (typically 1.9V), the start-up oscillator is
disabled and the normal fixed T_ON PWM operation takes
over.
Internal PMOS and NMOS Switches
The PS0021 features a 0.45Ω NMOS switch and a 0.6Ω
PMOS switch. In normal operation, these switches are
alternatively turned on and thus initiate the 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 a huge in-rush current
from the output to ground via switches. During the dead
time, both switches remain off and the inductor
discharges via the body diode of the PMOS switch to the
output.
Thermal Overload Protection
Thermal overload protection limits the total power
dissipation in the PS0021. When the junction temperature
exceeds T_J = +145°C, the thermal sensor signals the
shutdown logic and turns off most of the internal
circuitry. The thermal sensor turns the internal circuitry
on again after the IC’s junction temperature drops by
15°C.
Thermal overload protection is designed to protect the
PS0021 in the event of a fault condition. For continuous
operation, do not exceed the absolute maximum junction
temperature rating of T_J = +125°C.
Current Limit Protection
The PS0021 includes a current limiter that monitors the
peak inductor current through the NMOS switch and
turns the NMOS switch off when the inductor peak
current exceeds 150mA.
Light Load Operation
The PS0021 provides improved light load efficiency. The
internal zero current comparator monitors the inductor
current to the load and shuts off the PMOS switch once
this current reduces to some low value (typically 50mA).
This prevents the inductor current from reversing its
polarity, avoiding back-charging and thus improving the
efficiency at light loads. At that time, both the NMOS and
PMOS switches remain off. However, normal operation
starts again when the output voltage falls below the
regulation point. The IC automatically skips pulses at light
load, thus providing better efficiency.
July 2007
www.picsemi.com
9
© 2007 Power IC. All Rights Reserved.