6

LTC1550L/LTC1551L

SHDN: Shutdown (TTL Compatible). This pin is active low

(SHDN) for the LTC1550L and active high (SHDN) for the

the LTC1550L operates normally. When SHDN is pulled

low (SHDN pulled high for LTC1551L), the LTC1550L

enters shutdown mode. In shutdown, the charge pump

stops, the output collapses to 0V, and the quiescent

current drops typically to 0.2

µA. The SHDN pin for the

LTC1550L is a high impedance input and has no internal

pull-up. The user must supply a resistor or current source

pull-up to default the LTC1550L into normal operation.

The SHDN pin for the LTC1551L has an internal 5

µA

typical pull-down that defaults the LTC1551L into normal

operation.

2.7V and 5.25V. Certain combinations of output voltage

and output load current may place additional restrictions

on the required input voltage. Consult the Electrical Char-

acteristics table and Typical Performance Characteristics

for guaranteed test points. The difference between the

input voltage and output should not exceed 10.5V or

directly to PGND (GND for 8-pin packages) with at least a

0.1

µFcapacitorplacedincloseproximitytothechip.A1µF

or larger low ESR bypass capacitor is recommended to

minimize noise and ripple at the output. A surface mount

ceramic capacitor is recommended.

µFcapacitorbetween

with a 4.7

µF or larger capacitor to ensure regulator loop

stability. LTC recommends at least 10

µF to achieve the

specified output ripple. The output capacitor should be a

moderate ESR capacitor, and not a very low ESR capaci-

tor, as the zero in the feedback loop (formed by the ESR

and the output capacitor) provides phase lead to the linear

regulator feedback loop. Using very low ESR output ca-

pacitors will result in the output oscillating. A low ESR

0.1

µF capacitor is recommended in parallel with the main

output capacitor to minimize high frequency spikes at the

output. The ground connection for the output capacitor

capacitors, as well as to the GND of the LTC1550L/

capacitor ground connection to minimize noise.

µF capacitor from

GND: Ground. Connect to a low impedance ground. A

ground plane will help minimize regulation errors.

0.1

µF storage capacitor to ground. In order to achieve

ripple on the output voltage of less than 1mV, the ground

the LTC1550L/LTC1551L. This minimizes the AC current

path for the charge pump.

REG: This is an open-drain output that pulls low when the

output voltage is within 5% of the set value. It will sink 4mA

to ground with a 5V supply. The external circuitry must

provide a pull-up or REG will not swing high. The voltage

ground without damage. For the LTC1550L adjustable

voltage version, the REG pin is only available in the 16-lead

GN package.

ADJ (for adjustable versions): This is the feedback point

for the external resistor divider string. Connect a divider

ADJ. Note that the resistor string needs to be connected

“upside-down” from a negative regulator. See the Applica-

tions Information section for hook-up details.

GN PACKAGE ONLY

PGND: Power Ground. Connect to a low impedance ground.

PGND should be connected to the same potential as

AGND.

AGND: Analog Ground. Connect to a low impedance

ground. AGND should be connected to a ground plane to

minimize regulation errors.

NC: No Internal Connection.