SC4612 Ver la hoja de datos (PDF) - Semtech Corporation
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SC4612 Datasheet PDF : 24 Pages
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SC4612
POWER MANAGEMENT
Applications Information (Cont.)
The reference input of the error amplifier is effectively
ramped up with the soft-start signal. The error amp output
will vary between 100mV and 1.2V, depending on the duty
cycle. The error amp will be off until SS/EN reaches 0.7V
(TYP) and will move the output up to its desired voltage by
the time SS/EN reaches 1.3V. The gate drivers will be in
asynchronous mode until the FB pin reaches 500mV.
The intention for the asynchronous start up is to keep the
low side MOSFET from being switched on which forces the
low side MOSFETs body diode or the parallel Schottky di-
ode to conduct. The conduction by the diode prevents any
dips in an existing output voltage that might be present,
allowing for a glitch free start up in applications that are
sensitive to any bus disturbances.
During the asynchronous start up SC4612 monitors the
output and if within 10 cycles the FB has not reached the
internal soft start ramp level, the device switches to syn-
chronous mode. This provides an added protection in case
of short circuit at the output during the asynchronous start
when the bottom MOSFET is not being switched to provide
the RDS-ON sensing current limit protection.
In case of a current limit, the gate drives will be held off
until the soft-start is initiated. The soft-start cycle defined
by the SS cap being charged from 800mV to 1.3V and
slowly discharged to achieve an approximate hiccup duty
cycle of 1% to minimize excessive power dissipation.
The part will try to restart on the next softstart cycle. If the
fault has cleared, the outputs will start . If the fault still
remains, the part will repeat the soft-start cycle above in-
definitely until the fault has been removed.
The soft-start time is determined by the value of the
softstart capacitor (see formula below).
TSS
≈
CSS X 1.2
ISS
Oscillator Frequency Selection
The internal oscillator sawtooth signal is generated by
charging an external capacitor with a current source of
100µA charge current.
See Table 1 “Frequency vs. COSC” on page 14 to determine
oscillator frequency.
OVERCURRENT PROTECTION
SC4612 features low side MOSFET on-state Rds current
sensing and hiccup mode over current protection. ILIM pin
would be connected to DRV or PHASE via programming
resistors to adjust the over current trip point to meet
different customer requirements.
The sampling of the current thru the bottom FET is set at
~150ns after the bottom FET drive comes ON. It is done to
prevent a false tripping of the current limit circuit due to
the ringing at the phase node when the top FET is turned
OFF.
Internally overcurrent threshold is set to 100mV_typ. If
voltage magnitude at the phase node during sampling is
such that the current comparator meets this condition then
the OCP occurs.
Connecting a resistor from external voltage source such
as VDD, DRV, etc. to ILIM increases the current limit.
Connecting a resistor from ILIM to PHASE lowers the current
limit (see the block diagram in page 9).
Internal current source at ILIM node is ~20µA. External
programming resistors add to or subtract from that source
and hence vary the threshold.
The tolerance of the collective current sink at ILIM node is
fairly loose when combined with variations of the FET’s
Rds(on). Therefore when setting current limit some iteration
might be required to get to the wanted trip point.
Nonetheless, this circuit does serve the purpose of a hard
fault protection of the power switches. When choosing the
current limit one should consider the cumulative effect of
the load and inductor ripple current. As a rule of thumb,
the limit should be set at least x10 greater then the pk-pk
ripple current. Whenever a high current peak is detected,
SC4612 would first block the driving of the high side and
low side MOSFET, and then discharge the soft-start
capacitor. Discharge rate of the SS capacitor is 1/25 of
the charge rate.
Under Voltage Lock Out
Under Voltage Lock Out (UVLO) circuitry senses the VDD
through a voltage divider. If this signal falls below 4.5V (typi-
cal) with a 400mV hysteresis (typical), the output drivers
are disabled . During the thermal shutdown, the output
drivers are disabled.
2007 Semtech Corp.
10
www.semtech.com
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