Analog Devices AN-392 Application Notes

AN-392

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Circuit Design and Applications of the ADM663A/ADM666A

Micropower Linear Voltage Regulators

GENERAL INFORMATION

The ADM663A/ADM666A contains a micropower band­gap reference voltage source; an error amplifier, A1;
three comparators, C1, C2, C3, and a series pass output
transistor. A P-channel FET and an NPN transistor are
used on the ADM663A while the ADM666A uses an NPN
output transistor.

CIRCUIT DESCRIPTION

The internal bandgap reference is trimmed to 1.3 V
± 30 mV. This is used as a reference input to the error
amplifier A1. The feedback signal from the regulator out­put is supplied to the other input by an on-chip voltage
divider or by two external resistors. When V
ground, the internal divider tap between R1 and R2, pro­vides the error amplifier’s feedback signal giving a +5 V
output. When V
tween R2 and R3 provides the error amplifier’s feedback
signal giving a +3.3 V output. When V
50 mV above ground and less than 50 mV below V
error amplifier’s input is switched directly to the V
pin, and external resistors are used to set the output
voltage. The external resistors are selected so that the
desired output voltage gives 1.3 V at V

Comparator C1 monitors the output current via the
SENSE input. This input, referenced to V
the voltage drop across a load sense resistor. If the volt­age drop exceeds 0.5 V, then the error amplifier A1 is
disabled and the output current is limited.

The ADM663A has an additional amplifier, A2, which
provides a temperature proportional output, V
is summed into the inverting input of the error amplifier,
a negative temperature coefficient results at the output.
This is useful when powering liquid crystal displays over
wide temperature ranges.

The ADM666A has an additional comparator, C4, that
compares the voltage on the low battery input, LBI, pin
to the internal +1.3 V reference. The output from the
comparator drives an open drain FET connected to the
low battery output pin, LBO. The low battery threshold

is at VIN, the internal divider tap be-

SET

is at more than

SET

.

SET

OUT(2)

•

by Khy Vijeh, Matt Smith

is at

SET

, the

IN

, monitors

. If this

TC

APPLICATION NOTE

NORWOOD, MASSACHUSETTS 02062-9106

may be set using a suitable voltage divider connected to
LBI. When the voltage on LBI falls below 1.3 V, the open
drain output LBO is pulled low.

22k

1.3V

A1

D
E
C
O
D
E
R

R1

R2

0.9V

R3

1.3V

A1

D
E
C
O
D
E
R

R1

R2

1.3V

R3

SET

V

IN

SHDN

GND

Figure 1. ADM663A Functional Block Diagram

V

IN

SHDN

GND

Figure 2. ADM666A Functional Block Diagram

C1

VIN–50mV

C2

C3

50mV

A2

ADM663A

C1

VIN–50mV

C2

C3

50mV

C4

ADM666A

•

617/329-4700

V

OUT2

V

OUT1

0.5V
SENSE

V

SET

V

TC

V

OUT

0.5V
SENSE

V

SET

LBI

LBO

Both the ADM663A and the ADM666A contain a shut­down (SHDN) input that can be used to disable the error
amplifier and hence the voltage output. The power con­sumption in shutdown reduces to less than 9 µA.

Circuit Configurations

For a fixed +5 V output the V

input is grounded and no

SET

external resistors are necessary. This basic configura­tion is shown in Figure 3. For a fixed +3.3 V output, the
V

input is connected to VIN as shown in Figure 4. Cur-

SET

rent limiting is not being utilized so the SENSE input is
connected to V

+6V TO +16V

INPUT

OUT(2)

0.1µF

.

V

IN

ADM663A
ADM666A

V

SET

SENSE

V

OUT2

GND SHDN

0.1µF

+5V

OUTPUT

Figure 3. A Fixed +5 V Output

+4.5V TO +16V

INPUT

0.1µF

V

IN

ADM663A
ADM666A

V

SET

SENSE

V

OUT2

GND SHDN

0.1µF

+3.3V

OUTPUT

+2V TO +16V

INPUT

V

IN

ADM663A
ADM666A

SHDN

GND

SENSE

V

OUT2

V

SET

R

CL

+1.3V TO +15V

OUTPUT

R2

R1

Figure 5. Adjustable Output

Table I. Output Voltage Selection

V

SET

V

OUT

GND +5 V
V

IN

+3 V

R1/R2 ADJ

Current Limiting

Current limiting may be achieved by using an external
current sense resistor in series with V

OUT(2)

. When the

voltage across the sense resistor exceeds the internal

0.5 V threshold, current limiting is activated. The sense
resistor is therefore chosen such that the voltage across
it will be 0.5 V when the desired current limit is reached.

0.5

R

=

CL

I

CL

where RCL is the current sense resistor, ICL is the maxi­mum current limit.

Figure 4. A Fixed +3.3 V Output

Output Voltage Setting

If V

is not connected to GND or to VIN, the output volt-

SET

age is set according to the following equation:

(

R1+R

2)

R

1

where V

= 1.30 V.

SET

V

=

V

OUT

SET

×

The resistor values may be selected by first choosing a
value for R1 and then selecting R2 according to the fol­lowing equation:

V

R

2 =R1×






1. 3 0

The input leakage current on V



OUT

–1





is 10 nA maximum.

SET

This allows large resistor values to be chosen for R1 and
R2 with little degradation in accuracy. For example, a
1 MΩ resistor may be selected for R1, and then R2 may
be calculated accordingly. The tolerance on V

is guar-

SET

anteed at less than ±30 mV so in most applications, fixed
resistors will be suitable.

The value chosen for R

should also ensure that the cur-

CL

rent is limited to less than the 100 mA absolute maxi­mum rating and also that the power dissipation will also
be within the package maximum ratings.

If current limiting is employed, there will be an addi­tional voltage drop across the external sense resistor
that must be considered when determining the regula­tors dropout voltage.

If current limiting is not used, the SENSE input should
be connected to V

. In this case, input current should

OUT(2)

be limited so that in case of short circuited output,
device power dissipation does not exceed the rated
maximum.

Shutdown Input (SHDN)

The SHDN input allows the regulator to be turned off
with a logic level signal. This will disable the output and
reduce the current drain to a low quiescent (9 µA maxi­mum) current. This is very useful for low power applica­tions. The SHDN input should be driven with a CMOS
logic level signal since the input threshold is 0.3 V. In
TTL systems, an open collector driver with a pull-up re­sistor may be used.

If the shutdown function is not being used, then it
should be connected to GND.

–2–

Low Supply or Low Battery Detection

The ADM666A contains on-chip circuitry for low power
supply or battery detection. If the voltage on the LBI pin
falls below the internal 1.3 V reference, then the open
drain output LBO will go low. The low threshold voltage
may be set to any voltage above 1.3 V by appropriate
resistor divider selection.

R

3 =R4



V

BATT



1. 3





–1



V



where R3 and R4 are the resistive divider resistors and
V

is the desired low voltage threshold.

BATT

Since the LBI input leakage current is less than 10 nA,
large values may be selected for R3 and R4 in order to
minimize loading. For example, a 6 V low threshold may
be set using 10 M Ω for R3 and 2.7 M Ω for R4.

+2V TO +16V

INPUT

V

SENSE

IN

R3

ADM666A

LBI

R4

SHDN

GND

V

V

OUT

SET

LBO

R

CL

LOW
BATTERY
OUTPUT

+1.3V TO +15V

OUTPUT

R2

R1

Figure 6. ADM666A Adjustable Output with Low
Battery Detection

Low Output Detection

The circuit in Figure 7 will generate a low LBO when out­put voltage drops below a preset value determined by
the following equations:

V

for V

R

2 +R3 =R1

R

3= (R1+R2)

= 5.0 V nominal, VOL = 3% of V

OUT






1. 3

V






OUT

OL

1. 3



–1






–1




= 4.85 V and R1

OUT

= 1 MΩ solving the equations simultaneously we will get
R2 = 31 k Ω and R3 = 2.82 M Ω.

V

= 5V

OUT

0.1µF

0.1µF

V

IN

LBO

SHDN

GND

V

OUT

SENSE

LBI

V

SET

R3

2.82MΩ
R2

31kΩ
R1

1MΩ

High Current Operation

The ADM663A contains an additional output, V

OUT1

, suit­able for directly driving the base of an external NPN
transistor. Figure 8 shows a configuration which can be
used to provide +5 V with boosted current drive. A 1 Ω
current sensing resistor limits the current at 0.5 A.

V

IN

+

10µF

SHUTDOWN

V

IN

ADM663A

SHDN

GND

V

SET

V

OUT1

V

OUT2

SENSE

100Ω

+

10µF

2N4237

1.0Ω

+5V, 0.5A
OUTPUT

Figure 8. ADM663A Boosted Output Current (0.5 A)

Temperature Proportional Output

The ADM663A contains a VTC output with a positive tem­perature coefficient of +2.5 mV/ °C typ. This may be con­nected to the summing junction of the error amplifier
(V

) through a resistor resulting in a negative tempera-

SET

ture coefficient at the output of the regulator. This is
especially useful in multiplexed LCD displays to com­pensate for the inherent negative temperature coeffi­cient of the LCD threshold. At +25 °C, the voltage at the
VTC output is typically 0.9 V. The equations for setting
both the output voltage and the tempco are given be­low. If this function is not being used, then V

should be

TC

left unconnected.

R

2

R

2



(

V

–

V

+




1
–

R

2

R

3

R

3

(

TCV

R2

R1R3

TC

SET

V

OUT

)

TC

)

where V

V

=

OUT



V

1+



SET



R

TCV

= +1.3 V, VTC = +0.9 V, TCVTC = +2.5 mV/ °C

SET

ADM663A

OUT

SENSE

V

OUT2

V

SET

V

=

TC

Figure 9. ADM663A Temperature Proportional Output

Figure 7. Voltage Regulator Circuit with Low Output
Detector

–3–