© Semiconductor Components Industries, LLC, 2000

August, 2000 – Rev. 0

1

Publication Order Number:

AND8028/D

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

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.

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APPLICATION NOTE