6

LTC1555/LTC1556

LTC1556 Only

EN (Pin 5): Auxiliary LDO/Power Switch Enable Pin. A

logic high on this pin from the controller will enable the

auxiliary LDO output. When the LDO is disabled, the LDO

output will float or be pulled to ground by the load. If left

floating, the EN pin will be pulled down to GND by an

internal 1

µA current source.

FB (Pin 6): Auxiliary LDO Feedback Pin. When FB is

connected to the LDO pin (Pin 17), the LDO output is

regulated to 4.3V (typ). If the FB pin is left open or tied to

ground, the regulator acts as a

and LDO.

LDO (Pin 17): LDO Output Pin. This pin should be tied to

the FB pin for 4.3V LDO operation. The 4.3V LDO output

used as a

≤30Ω power switch if the FB pin is grounded

or left floating. When used as a regulator, LDO must be

bypassed to GND with a

≥3.3µF capacitor. The LDO

1.23V

61k

153k

FB

EN

1555/56 BD

DDRV

OPTIONAL

1

µA

1

µA

LDO

I/O

FREQUENCY

SYNTHESIZER

POWER

LTC1555/LTC1556

LTC1556 ONLY

4.3V

DATA

20k

10k

CLK

CIN

RST

SS

3V

RIN

GND

M0

M1

10

µF

I/O

CLK

RST

GND

0.1

µF

GND

STEP-UP/

STEP-DOWN

CHARGE PUMP

DC/DC

CONVERTER

SIM

CONTROLLER

10

µF

10

µF

+

+