LTC1844 datasheet(14/28 Pages) LINER

14 / 28 page

LT1963A Series

1963aff

For more information www.linear.com/LT1963A

The LT1963A series are 1.5A low dropout regulators opti-

mized for fast transient response. The devices are capable

of supplying 1.5A at a dropout voltage of 350mV. The low

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

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

LT1963Aregulatorsincorporateseveralprotectionfeatures

whichmakethemidealforuseinbattery-poweredsystems.

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 LT1963A-X acts like it

has a diode in series with its output and prevents reverse

current flow. Additionally, in dual supply applications

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 LT1963A 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: VOUT/1.21V. For example, load regulation for an

output current change of 1mA to 1.5A is –3mV typical at

VOUT = 1.21V. At VOUT = 5V, load regulation is:

(5V/1.21V)(–3mV) = –12.4mV

Output Capacitors and Stability

The LT1963A regulator is a feedback circuit. Like any

feedback circuit, frequency compensation is needed to

make it stable. For the LT1963A, 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

LT1963A 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 opti-

mumfrequencystabilitymayrequiresomeESR,especially

with ceramic capacitors.

Foreaseofuse,lowESRpolytantalumcapacitors(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

ringing. 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 capaci-

tors are used. Also, some ESR allows a smaller capaci-

tor value to be used. When small signal ringing occurs

with ceramics due to insufficient ESR, adding ESR or

increasing 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.

1963A F02

OUT

VIN

VOUT

ADJ

GND

LT1963A

Figure 2. Adjustable Operation

VOUT =1.21V 1+











+ IADJ

(

) R2

( )

VADJ =1.21V

IADJ =3µA AT 25°C

OUTPUT RANGE = 1.21V TO 20V

applicaTions inForMaTion

LTC1844 Datasheet(PDF) 14 Page - Linear Technology

LTC1844 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL