SC905AEVB datasheet(12/23 Pages) SEMTECH

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POWER MANAGEMENT

SC905A

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.

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.

Applications Information (Cont.)

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.

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