SC900 datasheet(11/23 Pages) SEMTECH

11 / 23 page

 2005 Semtech Corp.

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POWER MANAGEMENT

SC900

Applications Information (Cont.)

Active Shutdown

The shutdown control bits determine how the on-chip

active shutdown switches behave. Each LDO register

uses bit 5 of the LDO output voltage data byte to control

the shutdown behavior. When the active shutdown bit is

enabled (set to 1), the capacitance on the LDO output

will be discharged by an on-chip FET after the LDO is dis-

abled. When the active shutdown bit is disabled (set to

0), the output capacitance on the LDO output is dis-

charged by the load. The default active shutdown state

for all LDOs is on.

ON/OFF Control Register

Each individual LDO may be turned on or off by access-

ing the ON/OFF control register. LDOs are turned on by

setting their respective on/off bit to 1. Likewise, they

can be turned off by setting the on/off bit to 0. This

allows for on/off control with a single write command. If

the enable (EN) pin is high and data is written to the LDO

voltage registers, the LDO outputs will go to the voltage

prescribed by the Output Voltage Code bits (0-4). Data

will not be lost when toggling the on/off bit from 0 to 1.

However, if the EN pin is forced low, all circuitry in the

device is disabled. All programmed information is lost

when the enable bit is subsequently pulled high.

VASEL Pin

The VASEL pin sets the default voltage of LDO A, the

core supply. When this pin is set to VIN, the default volt-

age is 2.80V. When this pin is set to GND, the default

voltage is 1.80V. The voltage can be changed from its

default state after start up by writing to the LDO voltage

code register.

Device Addressing

Following a start condition, the master must output the

address of the slave it is accessing. The most significant

six bits of the slave are the device type identifier (ID).

For the SC900 this is fixed at 000100[B]. The next sig-

nificant bit addresses a particular device. A system can

have up to two SC900 devices on the bus. The two

addresses are defined by the state of the A0 input (see

Figure 1).

000

Figure 1 - Slave Address Structure

When the A0 pin is tied to GND, device 1 has an address

of 0 and the combination of device type ID and address

is 0x08H. When the A0 pin is tied to VIN, device 2 has

and address of 1 and the combination of device type ID

and address is 0x09H.

The last bit of the slave address defines the operation to

be performed. When set to a one a read operation is

selected; when set to a zero a write operation is selected.

Following a start condition, the SC900 monitors the SDA

line comparing the slave address being transmitted with

its slave address (device type ID and state of A0 input).

Upon a correct compare the SC900 outputs an acknowl-

edge on the SDA line. Depending on the state of the R/

W bit, the SC900 will execute a read or write operation.

Protection Circuitry

The SC900 provides protection circuitry that prevents

the device from operating in an unspecified state. These

include Under-voltage Lockout Protection, Over-tempera-

ture Protection and Short-circuit Protection.

Under Voltage Lockout

The SC900 provides an Under Voltage Lockout (UVLO)

circuit to protect the device from operating in an unknown

state if the input voltage supply is too low. When the

battery voltage drops below the UVLO threshold, the LDOs

are disabled. As the battery voltage increases above

the hysteresis level, the LDOs are re-enabled into their

previous states, provided ENABLE has remained high. If

ENABLE goes low, the SC900 will shut down.

Over-temperature Protection

The SC900 provides an internal Over-temperature (OT)

protection circuit that monitors the internal junction tem-

perature. When the temperature exceeds the OT thresh-

old, the OT protection disables all the LDO outputs. As

the junction temperature drops below the hysteresis level

the OT protection re-enables all the LDOs in their previ-

ous states, provided ENABLE has remained high. If EN-

ABLE goes low, the SC900 will shut down.

Short-circuit Protection

Each LDO output has short-circuit protection. If a short

is applied to any output, the output voltage will drop and

the output current will be limited to the short circuit cur-

rent until the short is removed.

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