With the trend toward decreasing voltages,
power management must keep pace to supply the necessary voltage and
power levels. Achieving a precise sub-1V output with current
devices is not feasible because most have reference voltage levels
of either 1.25V or 2.5V. A sub-1V LDO (low dropout) regulator has
become a vital component.
Figure 1: A precision sub-bandgap reference U1 in the
feedback loop of pass transistor Q1 provides the necessary voltage
margin to realize a 0.9V, 1.7A output LDO regulator.
System Design
The two main circuit elements in an LDO are the feedback network
and the pass element. The typical feedback network requires a
voltage reference and an op-amp to control the pass element. By
combining the reference and the op-amp into one component, one can
utilize a programmable precision reference. The minimum voltage
drop and maximum output current determines the pass element.
For an accurate output voltage, the circuit's feedback network
in Figure 1 uses a precision voltage reference. A voltage reference
should not be used if its output voltage is within a 20% margin of
the reference voltage. Because the LDO's output voltage is below
1.0V, the traditional TL431 and TLV431 programmable precision
references are unacceptable. Instead, the ON Semiconductor NCP100
is selected for U1 because it supplies a 0.7V reference. With a low
reference level, 0.9V is attainable with a greater than 20%
margin.
In selecting the pass transistor, the choice between an NPN or a
PNP must be made. Because this design is powered from a single
supply, the NPN's dropout voltage minimum is the greater of either
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,
this design uses the ON Semiconductor MBT35200MT1 PNP for Q1. This
PNP has a 2.0A collector current, maximum emitter-to-collector
saturation voltage of 0.31V, typical DC current gain of 200, and
maximum emitter-to-base voltage of 0.875V.
Figure 1 illustrates the circuit schematic. Inverting the signal
from the precision reference is necessary because of the PNP pass
element. A small-signal NPN (Q2) inverts the signal. There is a
voltage-differential issue with driving the base of an NPN, 0.6V to
0.7V turn on, from the NCP100's cathode, 0.9V minimum. To
accommodate the differential, a diode (D1) shifts the voltage
level.
A resistor (R5) from gate to ground performs two functions:
pulls the gate to ground for turn off and provides 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, causing a
voltage runaway at the output during light or no load. Because of
the possibility of double amplification, the NPN’s base to
emitter voltage needs to be very low.
In addition to Q2, to invert the control signal from the NCP100,
R6 pulls up the gate to the input voltage to turn off the
MBT35200MT1. R4 is an overcurrent protection resistor. R4 is
determined by subtracting the minimum input voltage from the
maximum VBE of Q1 and the maximum VSAT of Q2,
then dividing by the base current of Q1.
A 1-µF capacitor (C3) is necessary for the NCP100's normal
operation. It stabilizes the operation of the precision reference
and has a negligible effect on the response time of the system. R1
and R2 comprise the resistor divider feedback network. C4 is
necessary for the system’s fast transient response.
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.
Results
Utilizing the circuit illustrated in Figure 1, the output
voltage varies 40-mV between 1.5V to 3.0V and 0A to 1.7A for an
output voltage centered at 0.945V, which equates to a ±2.5%
variation for load and line. The voltage drop arising from load
transients is small as seen in Figure 2. If R1 is varied to change
the output voltage to 1.8V, the minimum dropout voltage is 34-mV
under light load conditions and 230-mV for 1.7A. The minimum input
voltage to operate the circuit in Figure 1 is 1.42V for 1.7A and
1.37V for 0.5A. As discussed previously, the operating range of
this circuit can vary depending upon the value of R3.
Figure 2: A 0.5A to 1.5A load transient applied to the
circuit in Figure 1 results in small output voltage
variation.
