SC905 datasheet(12/21 Pages) SEMTECH

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POWER MANAGEMENT

SC905

Over-Temperature Protection

The SC905 provides an internal over-temperature (OT)

protection circuit that monitors the internal junction tem-

perature. When the temperature exceeds the OT threshold

as deﬁned in the Electrical Characteristics section, the OT

protection disables all the LDO outputs, holds the RESB

signal low and sets the OTF bit low in the status register.

When the junction temperature drops below the hysteresis

level, the OT protection resets the OTF bit high and re-en-

ables all the LDOs in their previous states, provided PGOOD

has remained high. If PGOOD goes low, the SC905 will shut

down. This is only useful if the MSM is not powered by the

SC905, since during an OT fault the MSM will lose power.

An external switching regulator such as the SC190A could

power the MSM in the case where monitoring the OTF bit

is desired.

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 current

until the short is removed.

Interfacing to Semtech Battery Chargers

The SC905 is designed to interface with Semtech battery

chargers by providing three control inputs that map the

state of the controls to register bits so the host processor

can monitor the charger’s status via the I2C interface. For

open-drain control outputs from the charger, pull-up resis-

tors must be connected to the lines for the SC905 registers

to display the correct status.

Status Register

The status register monitors these inputs for changes, and

the MSM can periodically poll this register to determine

the status of the charger. This is a read-only register. This

ing status before performing an LCD update.

The MSM can control other aspects of the charger with

general purpose I/O. These include enable/disable and

charge time-out functions. Many of the chargers functions

can be statically set and do not need MSM intervention.

The amount of MSM intervention is determined by the

intended application.

The following table is a summary of the status register

inputs and their functions:

CONDITION

FUNCTION

FAULTB is Low

Charger Fault

CPB is Low

USB or AC

Charger Present

CHRGB is Low

Charging in Progress

CHRGB & FAULTB is Low

Battery Fault

Applications Information (Cont.)

Layout Considerations

The PCB layout associated with the SC905 is straight for-

ward, with the main consideration being given to the value

and position of the input bypass capacitors. The device

itself has eight input voltage pins which can be powered

from a single supply or from a number of individual supplies

depending on how much copper is available on the input

voltage feed track and how much real estate is available

on the PCB for components. If all the supply inputs are

fed from one single supply trace or from a power plane, a

10μF low ESR capacitor or two 4.7μF low ESR capacitors

should be used. Larger input capacitance and lower ESR

provide better supply noise rejection and line transient

response. The copper trace to the inputs should be fairly

thick in order to keep trace inductance to a minimum and

the capacitors should be located as close to the SC905

as possible. If the supply trace is thin then the inputs

should be treated as if they were powered from individual

supplies, each input should be bypassed by at least one

1μF low ESR capacitor located very close to each input pin.

The SC905 is designed to have excellent stability with

a minimum output capacitance of 1μF. Low ESR ce-

ramic capacitors are recommended and should be

located as close to the LDO output pins as possible.

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