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Precision sub-1V, 1.7A output LDO

Posted: 12 Dec 2000     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2000DEC/2000DEC12_AMD_AN4.PDF

) 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.

http://onsemi.com

APPLICATION NOTE

AND8028/D

http://onsemi.com

2

Vout Vin

R1

1.5 k

R2

4.3 k

R4

10

R3

18

R6

1 k

C2

100 m

C1

100 m

C3

1 m

Q2 MBT3904

D1

MRA4003T3

R5

300

NCP100

U1

Q1 MBT35200MT1

C4

1 n

Figure 1. Circuit Schematic for 0.9 Volt 1.7 Ampere LDO

RESULTS

Utilizing the circuit in Figure 1, layout in Figure 3 and

values in Table 1, the output voltage varies 40 millivolts

between 1.5 volts to 3.0 volts and 0 amperes to 1.7 amperes

for an output voltage centered at 0.945 volts, which equates

to a +/- 2.5 % variation for load and line. The voltage droop

due to load transient is small as seen in Figure 2. If R1 is

varied to change the output voltage to 1.8 volts, the

minimum drop out voltage is 34 millivolts under light load

conditions and 230 millivolts for 1.7 amperes. The

minimum input voltage to operate of the circuit in Figure 1

is 1.42 volts for 1.7 amperes and 1.37 volts for 0.5 amperes.

As discussed previously, the operating range of this circuit

can vary depending upon R3.

Vout: 200 mV/div

Iout: 0.5 A/div

Time Scale: 100 us/div

Figure 2. Load Transient for 1.8 Volts In and Load

from 0.5 Amperes to 1.5 Amperes

AND8028/D

http://onsemi.com

3

C2

+ +

C1

Out GND In

Q1

R6

R3

R2

R1

R5

R4

C4

D1

C3

Q2

U1

Figure 3. Layout of NCP100/MBT35200 LDO (4x)

Table 1. Part Description for LDO Circuit

Reference Part Description

C1 100 u AVX Tantalum SMT

C2 100 u AVX Tantalum SMT

C3 1 u 0805 Ceramic Chip Cap

C4 1 n 0805 Ceramic Chip Cap

D1 MRA4003T3 ON Semiconductor SMA Diode

JP1 Connector 3 Pin Connector

Q1 MBT35200 ON Semiconductor TSOP-6 PNP

Q2 MBT3904 ON Semiconductor SOT-23 NPN

R1 1.5 k 1206 Chip Resistor

R2 4.3 k 0805 Chip Resistor

R3 18 0805 Chip Resistor

R4 10 0805 Chip Resistor

R5 300 0805 Chip Resistor

R6 1 k 0805 Chip Resistor

U1 NCP100 ON Semiconductor Precision Reference

AND8028/D

http://onsemi.com

4

ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes

without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular

purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability,

including without limitation special, consequential or incidental damages. "Typical" parameters which may be provided in SCILLC data sheets and/or

specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals" must be

validated for each customer application by customer's technical experts. SCILLC does not convey any license under its patent rights nor the rights of others.

SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications

intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or

death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold

SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable

attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim

alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer.

PUBLICATION ORDERING INFORMATION

CENTRAL/SOUTH AMERICA:

Spanish Phone: 303-308-7143 (Mon-Fri 8:00am to 5:00pm MST)

Email: ONlit-spanish@hibbertco.com

ASIA/PACIFIC: LDC for ON Semiconductor - Asia Support

Phone: 303-675-2121 (Tue-Fri 9:00am to 1:00pm, Hong Kong Time)

Toll Free from Hong Kong & Singapore:

001-800-4422-3781

Email: ONlit-asia@hibbertco.com

JAPAN: ON Semiconductor, Japan Customer Focus Center

4-32-1 Nishi-Gotanda, Shinagawa-ku, Tokyo, Japan 141-0031

Phone: 81-3-5740-2700

Email: r14525@onsemi.com

ON Semiconductor Website: http://onsemi.com

For additional information, please contact your local

Sales Representative.

AND8028/D

NORTH AMERICA Literature Fulfillment:

Literature Distribution Center for ON Semiconductor

P.O. Box 5163, Denver, Colorado 80217 USA

Phone: 303-675-2175 or 800-344-3860 Toll Free USA/Canada

Fax: 303-675-2176 or 800-344-3867 Toll Free USA/Canada

Email: ONlit@hibbertco.com

Fax Response Line: 303-675-2167 or 800-344-3810 Toll Free USA/Canada

N. American Technical Support: 800-282-9855 Toll Free USA/Canada

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German Phone: (+1) 303-308-7140 (Mon-Fri 2:30pm to 7:00pm CET)

Email: ONlit-german@hibbertco.com

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Email: ONlit-french@hibbertco.com

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Email: ONlit@hibbertco.com

EUROPEAN TOLL-FREE ACCESS*: 00-800-4422-3781

*Available from Germany, France, Italy, UK, Ireland





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