SC905A View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC905A' PDF : 23 Pages View PDF
SC905A
POWER MANAGEMENT
Applications Information (Cont.)
Protection Circuitry
The SC905A contains protection circuitry that prevents
the device from operating in an unspecified state. These
include Under-voltage Lockout Protection, Over-temperature
Protection and Short-circuit Protection.
Under-Voltage Lockout
The SC905A 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,
as defined in the Electrical Characteristics section, the
LDOs are disabled and RESB is held low. When the battery
voltage is increased above the hysteresis level, the LDOs
are re-enabled into their previous states, provided PGOOD
has remained high. If PGOOD goes low, the SC905A will
shut down. When powering-up with a battery voltage below
the UVLO threshold, RESB will be held low.
Over-Temperature Protection
The SC905A provides an internal over-temperature (OT)
protection circuit that monitors the internal junction
temperature. 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-enables all the LDOs in their previous states,
provided PGOOD has remained high. If PGOOD goes low,
the SC905A will shut down. This is only useful if the MSM
is not powered by the SC905A, since during an OT fault
the MSM will lose power. An external switching regulator
such as Semtech’s SC190A could power the MSM in the
case where monitoring the OTF bit is desired.
Layout Considerations
The PCB layout associated with the SC905A is
straightforward, with the main consideration being given
to the value and position of the input bypass capacitors.
The device 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 SC905A
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 SC905A is designed to have excellent stability
with a minimum output capacitance of 1μF. Low ESR
ceramic capacitors are recommended and should be
located as close to the LDO output pins as possible.
Short-Circuit Protection
Each LDO output has short-circuit protection. If any output
is short-circuited to ground, the output voltage will drop
and the output current will be limited to the short circuit
current until the short is removed.
Status Register
The status register monitors the OTF and the state of the
external enable pins dedicated to the TCXO and MOT LDOs.
The MSM can periodically poll this register to determine
their status. This is a read-only register.
© 2006 Semtech Corp.
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