FA7764 Datasheet, PDF (13/18 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7764
The delay time of the timer and latch is fixed
internally and cannot be configured from outside.
Fig. 4
In order to resetting from latch stoppage, resetting
by using the ENB pin or setting the input voltage
down to the UVLO threshold voltage are required.
(7) Overcurrent limitation circuit
This IC has a pulse-by-pulse overcurrent limitation
function that detects and limits each peak current of
the built-in MOSFET.
The current of the built-in MOSFET is transferred to
the voltage signal at current detect circuit, and the
voltage signal is input into the overcurrent limitation
comparator.
If the current transferred voltage signal is higher
than the reference voltage, the built-in MOSFET is
turned off and the current is limited.
The current limitation will be reset in the next
switching period, and it will be repeated in each
period along with overcurrent limitation.
Furthermore, when the pulse-by-pulse overcurrent
limitation continues for five times, the IC latch stops
the switching in order to avoid inductor saturation
and the destruction of the MOSFET.
If the output voltage of the DC-DC converter drops
to a level lower than 75% (typ) due to line
impedance or some other factor, the IC will be shut
down by the same way as timer latch short-circuit
protection described in (6).
(8) Thermal shut down circuit
The IC has an thermal shut down function that
stops switching operation when the IC overheats
due to overcurrent or other error.
The IC stops switching when the chip temperature
is heated up to 135 degrees (typ.), and the IC
resumes switching when the chip temperature is
cooled down to 115 degrees.
The latching function does not work when thermal
shut down is operated.
(9) ON/OFF controller
This IC can be on/off controlled by the input of
external signals to the ENB pin.
To turn the output on, set the ENB pin voltage less
than 1.0V. Then the output voltage gradually
increases by the soft start (Fig. 3).
The output can be turned off by either opening the
ENB pin or input an external voltage of at least 2V.
When the ENB pin is open, the ENB pin voltage will
clamps to the self-limited voltage near 5V. In this
case, the IC keeps standby mode where the
regulated voltage CREG (3.0V typ.) is shut down,
and also the current consumption of the IC is
controlled less than 100 μA(typ.).
(10) Under voltage lockout circuit (UVLO)
This IC has a UVLO function to prevent circuit
malfunction in case of a lower input power supply.
When increasing the power supply voltage Vcc
from 0V, the IC will be start operation over 6.9V
(typ).
When decreasing the power supply voltage Vcc
from operation state, the IC will be shut down under
5.9V (typ).
(11) Output circuit
The output circuit consists of P-channel MOSFET,
with ON resistance of 0.8 Ω (typically).
(12)Opened rectifier diode protection
Generally when a rectifier diode in the buck
converter is removed by some kind of abnormality
states, large voltage occurs on switching MOSFET
by the energy of the inductor and it has a possibility
of destruction the MOSFET with emitting smoke and
ignition.
Therefore, the IC will shut down the operation
when the OUT pin voltage falls under -9V with
synchronized switching.
Fuji Electric Systems Co., Ltd.
AN-059E Rev.1.0
13
June.2010
http://www.fujielectric.co.jp/fdt/scd/