SC905 データシートの表示（PDF） - Semtech Corporation
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SC905 Datasheet PDF : 21 Pages
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SC905
POWER MANAGEMENT
Applications Information (Cont.)
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
register is useful when the MSM needs to determine charg-
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
FAULTB is Low
CPB is Low
CHRGB is Low
FUNCTION
Charger Fault
USB or AC
Charger Present
Charging in Progress
CHRGB & FAULTB is Low
Battery Fault
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.
© 2006 Semtech Corp.
12
www.semtech.com
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