AND8028 Datasheet, PDF (1/4 Pages) ON Semiconductor – Precision Sub-One Volt 1.7 Ampere Output LDO

AND8028/D
Precision Sub-One Volt
1.7 Ampere Output LDO
Jason Hansen
ON Semiconductor
INTRODUCTION
The following Application Note is a description of a 0.9
volt 1.7 ampere output LDO with an explanation of each
circuit element. The dropout voltage is 230 millivolts for
1.7 amperes or 34 millivolts for light loads. Depending
upon system design, the minimum input voltage can be less
than 1.4 volts.
SYSTEM DESIGN
For an accurate output voltage, a precision voltage
reference must be used for the feedback network. A device
should not be used if the output voltage is within a 20%
margin from the reference voltage. Since the output voltage
is below 1.0 volts, the traditional TL431 and TLV431
programmable precision references cannot be used. Instead
the NCP100 is selected with its 0.7 volt reference. With a
low reference level, 0.9 volts is attainable with a greater
than 20% margin.
In selecting the pass transistor, one must decide between
a NPN and a PNP. Since this design is from a single supply,
the NPN drop out voltage minimum is the greater of the
saturation voltage from collector to emitter or the base to
emitter on voltage. For the PNP the minimum voltage drop
from input to output is the saturation voltage of emitter to
collector. To maximize the output current and minimize the
voltage drop, the MBT35200MT1 PNP is selected for this
design. This PNP has a 2.0 ampere collector current,
maximum emitter to collector saturation voltage of 0.31
volts, typical DC current gain of 200, and maximum emitter
to base voltage of 0.875 volts.
With the voltage reference and the pass element selected,
the remaining components in the circuit are placed. Refer to
http://onsemi.com
APPLICATION NOTE
Figure 1 for the circuit schematic. Since a PNP transistor is
implemented, the signal from the precision reference needs
to be inverted. A small signal NPN, Q2, is used for this
purpose. There is a voltage differential issue with driving
the base of a NPN, 0.6 to 0.7 volt turn on, from the NCP100
cathode, 0.9 volts minimum. To level shift the voltages, a
diode will be placed between the cathode and the gate. A
resistor, R5, from gate to ground is used for two purposes:
pull the gate to ground for turn off and provide a bias
current through the diode to set a minimum voltage drop. If
the minimum voltage drop is not set properly, the NPN will
have a small base current that will be amplified by the NPN
and the PNP providing a voltage runaway condition at the
output voltage during light or no load.
In addition to Q2 to invert the control signal from the
NCP100, R6 is used to pull up the gate to the input voltage
to turn off the MBT35200MT1. R4 is an over current
protection resistor. R4 is determined by subtracting the
minimum input voltage from the maximum Vbe of Q1 and
maximum Vsat of Q2, then dividing by the base current
of Q1.
C3, a 1 microfarad capacitor, is necessary to support the
NCP100 for normal operation. Its purpose is to stabilize the
operation of the precision reference. It has a negligible
effect on the response time of the system. R1 and R2 are the
resistor divider feedback network. C4 is used for fast
transient response of the system. R3 provides the DC bias
for the NCP100. The value of R3 is limited by the response
of the system at low line and low load. If the value of R3 is
too large, oscillations occur on the output. If R3 is too
small, the output voltage will run away at high line and
low load.
© Semiconductor Components Industries, LLC, 2000
1
August, 2000 – Rev. 0
Publication Order Number:
AND8028/D