LT1764A Series

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SHDN (Pin 1/1/10): Shutdown. The SHDN pin is used to

put the LT1764A regulators into a low power shutdown

state. The output will be off when the SHDN pin is pulled

low. The SHDN pin can be driven either by 5V logic or

open-collector logic with a pull-up resistor. The pull-up

resistor is required to supply the pull-up current of the

open-collector gate, normally several microamperes, and

the SHDN pin current, typically 7µA. If unused, the SHDN

pin must be connected to V

. The device will be in

the low power shutdown state if the SHDN pin is not

connected.

IN (Pin 2/Pin 2/Pins 12, 13, 14): Input. Power is supplied

to the device through the IN pin. A bypass capacitor is

required on this pin if the device is more than six inches

away from the main input filter capacitor. In general, the

output impedance of a battery rises with frequency, so it

is advisable to include a bypass capacitor in battery-

powered circuits. A bypass capacitor in the range of 1µF to

10µF is sufficient. The LT1764A regulators are designed to

withstand reverse voltages on the IN pin with respect to

ground and the OUT pin. In the case of a reverse input,

which can happen if a battery is plugged in backwards, the

device will act as if there is a diode in series with its input.

There will be no reverse current flow into the regulator and

no reverse voltage will appear at the load. The device will

protect both itself and the load.

NC (Pins 2, 11, 15) TSSOP Only: No Connect.

GND (Pin 3/Pin 3/Pins 1, 7, 8, 9, 16, 17): Ground.

OUT (Pin 4/Pin 4/Pins 3, 4, 5): Output. The output

supplies power to the load. A minimum output capacitor

of 10µF is required to prevent oscillations. Larger output

capacitors will be required for applications with large

transient loads to limit peak voltage transients. See the

Figure 1. Kelvin Sense Connection

SHDN

1764 F01

OUT

SENSE

GND

LT1764A

LOAD

PI FU CTIO S

Applications Information section for more information on

output capacitance and reverse output characteristics.

SENSE (Pin 5/Pin 5/Pin 6): Sense. For fixed voltage

versions of the LT1764A (LT1764A-1.5/LT1764A-1.8/

LT1764A-2.5/LT1764A-3.3), the SENSE pin is the input

to the error amplifier. Optimum regulation will be ob-

tained at the point where the SENSE pin is connected to the

OUT pin of the regulator. In critical applications, small

voltage drops are caused by the resistance (R

) of PC

traces between the regulator and the load. These may be

eliminated by connecting the SENSE pin to the output at

the load as shown in Figure 1 (Kelvin Sense Connection).

Note that the voltage drop across the external PC traces

will add to the dropout voltage of the regulator. The SENSE

pin bias current is 600µA at the nominal rated output

voltage. The SENSE pin can be pulled below ground (as in

a dual supply system where the regulator load is returned

to a negative supply) and still allow the device to start

and operate.

ADJ (Pin 5/Pin 5/Pin 6): Adjust. For the adjustable LT1764A,

this is the input to the error amplifier. This pin is internally

clamped to ±7V. It has a bias current of 3µA which flows

into the pin. The ADJ pin voltage is 1.21V referenced to

ground and the output voltage range is 1.21V to 20V.

DD/TO-220/TSSOP

LT1764A Series

1764afb

The LT1764A series are 3A low dropout regulators opti-

mized for fast transient response. The devices are capable

of supplying 3A at a dropout voltage of 340mV. The low

operating quiescent current (1mA) drops to less than 1µA

in shutdown. In addition to the low quiescent current, the

LT1764A regulators incorporate several protection fea-

tures which make them ideal for use in battery-powered

systems. The devices are protected against both reverse

input and reverse output voltages. In battery backup

applications where the output can be held up by a backup

battery when the input is pulled to ground, the LT1764A-X

acts like it has a diode in series with its output and prevents

reverse current flow. Additionally, in dual supply applica-

tions where the regulator load is returned to a negative

supply, the output can be pulled below ground by as much

as 20V and still allow the device to start and operate.

Adjustable Operation

The adjustable version of the LT1764A has an output

voltage range of 1.21V to 20V. The output voltage is set by

the ratio of two external resistors as shown in Figure 2. The

device servos the output to maintain the voltage at the ADJ

pin at 1.21V referenced to ground. The current in R1 is

then equal to 1.21V/R1 and the current in R2 is the current

in R1 plus the ADJ pin bias current. The ADJ pin bias

current, 3µA at 25°C, flows through R2 into the ADJ pin.

The output voltage can be calculated using the formula in

Figure 2. The value of R1 should be less than 4.17k to

minimize errors in the output voltage caused by the ADJ

pin bias current. Note that in shutdown the output is turned

off and the divider current will be zero.

The adjustable device is tested and specified with the ADJ

pin tied to the OUT pin for an output voltage of 1.21V.

Specifications for output voltages greater than 1.21V will

be proportional to the ratio of the desired output voltage to

1.21V: V

OUT

/1.21V. For example, load regulation for an

output current change of 1mA to 3A is –3mV typical at

OUT

= 1.21V. At V

OUT

= 5V, load regulation is:

(5V/1.21V)(–3mV) = –12.4mV

APPLICATIO S I FOR ATIO

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Output Capacitors and Stability

The LT1764A regulator is a feedback circuit. Like any

feedback circuit, frequency compensation is needed to

make it stable. For the LT1764A, the frequency compensa-

tion is both internal and external—the output capacitor.

The size of the output capacitor, the type of the output

capacitor, and the ESR of the particular output capacitor all

affect the stability.

In addition to stability, the output capacitor also affects the

high frequency transient response. The regulator loop has

a finite band width. For high frequency transient loads,

recovery from a transient is a combination of the output

capacitor and the bandwidth of the regulator. The

LT1764A was designed to be easy to use and accept a

wide variety of output capacitors. However, the frequency

compensation is affected by the output capacitor and

optimum frequency stability may require some ESR, espe-

cially with ceramic capacitors.

For ease of use, low ESR polytantalum capacitors (POSCAP)

are a good choice for both the transient response and

stability of the regulator. These capacitors have intrinsic

ESR that improves the stability. Ceramic capacitors have

extremely low ESR, and while they are a good choice in

many cases, placing a small series resistance element will

sometimes achieve optimum stability and minimize ring-

ing. In all cases, a minimum of 10µF is required while the

maximum ESR allowable is 3Ω.

The place where ESR is most helpful with ceramics is low

output voltage. At low output voltages, below 2.5V, some

ESR helps the stability when ceramic output capacitors

are used. Also, some ESR allows a smaller capacitor

value to be used. When small signal ringing occurs with

ceramics due to insufficient ESR, adding ESR or increas-

Figure 2. Adjustable Operation

1764 F02

OUT

ADJ

GND

LT1764A

OUT ADJ

ADJ

⎛

⎝

⎜

⎞

⎠

⎟

()()

=°

121 1

121

µ AT 25 C

OUTPUT RANGE = 1.21V TO 20V

LT1764A Series

1764afb

ing the capacitor value improves the stability and reduces

the ringing. Table 1 gives some recommended values of

ESR to minimize ringing caused by fast, hard current

transitions.

Table 1. Capacitor Minimum ESR

OUT

F22

F47

F 100

1.2V 10mΩ 5mΩ 3mΩ 0mΩ

1.5V 7mΩ 5mΩ 3mΩ 0mΩ

1.8V 5mΩ 5mΩ 3mΩ 0mΩ

2.5V 0mΩ 0mΩ 0mΩ 0mΩ

3.3V 0mΩ 0mΩ 0mΩ 0mΩ

≥ 5V 0mΩ 0mΩ 0mΩ 0mΩ

Figures 3 through 8 show the effect of ESR on the transient

response of the regulator. These scope photos show the

transient response for the LT1764A at three different

output voltages with various capacitors and various val-

ues of ESR. The output load conditions are the same for all

traces. In all cases there is a DC load of 1A. The load steps

up to 2A at the first transition and steps back to 1A at the

second transition.

At the worst case point of 1.2V

OUT

with 10µF C

OUT

(Figure 3), a minimum amount of ESR is required. While

5mΩ is enough to eliminate most of the ringing, a value

closer to 20mΩ provides a more optimum response. At

2.5V output with 10µF C

OUT

(Figure 4) the output rings

at the transitions with 0Ω ESR but still settles to within

10mV in 20µs after the 1A load step. Once again a small

value of ESR will provide a more optimum response.

At 5V

OUT

with 10µF C

OUT

(Figure 5) the response is well

damped with 0Ω ESR.

With a C

OUT

of 100µF at 0Ω ESR and an output of 1.2V

(Figure 6), the output rings although the amplitude is only

10mV

p-p

. With C

OUT

of 100µF it takes only 5mΩ to 20mΩ

of ESR to provide good damping at 1.2V output. Perfor-

mance at 2.5V and 5V output with 100µF C

OUT

shows sim-

ilar characteristics to the 10µF case (see Figures 7-8).

2.5V

OUT

5mΩ to 20mΩ can improve transient response.

At 5V

OUT

the response is well damped with 0Ω ESR.

Capacitor types with inherently higher ESR can be com-

bined with 0mΩ ESR ceramic capacitors to achieve both

good high frequency bypassing and fast settling time.

Figure 9 illustrates the improvement in transient response

that can be seen when a parallel combination of ceramic

and POSCAP capacitors are used. The output voltage is at

the worst case value of 1.2V. Trace A, is with a 10µF

ceramic output capacitor and shows significant ringing

with a peak amplitude of 25mV. For Trace B, a 22µF/45mΩ

POSCAP is added in parallel with the 10µF ceramic. The

output is well damped and settles to within 10mV in less

than 5µs.

For Trace C, a 100µF/35mΩ POSCAP is connected in

parallel with the 10µF ceramic capacitor. In this case the

peak output deviation is less than 20mV and the output

settles in about 5µs. For improved transient response the

value of the bulk capacitor (tantalum or aluminum electro-

lytic) should be greater than twice the value of the ceramic

capacitor.

Tantalum and Polytantalum Capacitors

There is a variety of tantalum capacitor types available,

with a wide range of ESR specifications. Older types have

ESR specifications in the hundreds of mΩ to several

Ohms. Some newer types of polytantalum with multi-

electrodes have maximum ESR specifications as low as

5mΩ. In general the lower the ESR specification, the larger

the size and the higher the price. Polytantalum capacitors

have better surge capability than older types and generally

lower ESR. Some types such as the Sanyo TPE and TPB

series have ESR specifications in the 20mΩ to 50mΩ

range, which provide near optimum transient response.

Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors can also be used with the

LT1764. These capacitors can also be used in conjunction

with ceramic capacitors. These tend to be the cheapest

and lowest performance type of capacitors. Care must be

used in selecting these capacitors as some types can have

ESR which can easily exceed the 3Ω maximum value.

APPLICATIO S I FOR ATIO

WUUU

Part #	LT1764AEQ
Description	LDO Regulator Pos 1.21V to 20V 3A 6-Pin(5+Tab) DDPAK
Category	RECTIFIER
Availability	Out of Stock
Qty	0

LT1764AEQ

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Technical Document