LTC3780
OPERATION
FOLDBACK CURRENT
SHORT-CIRCUIT PROTECTION AND CURRENT LIMIT
Foldback current limiting is activated when the output
voltage falls below 70% of its nominal level, reducing
power waste. During start-up, foldback current limiting
is disabled.
SwitchAon-timeislimitedbyoutputvoltage.Whenoutput
voltage is reduced and is lower than its nominal level,
switch A on-time will be reduced.
In every boost mode cycle, current is limited by a voltage
reference, which is proportional to the I pin voltage. The
TH
INPUT UNDERVOLTAGE RESET
maximum sensed current is limited to 160mV. In every
buck mode cycle, the maximum sensed current is limited
to 130mV.
The SS capacitor will be reset if the input voltage is al-
lowed to fall below approximately 4V. The SS capacitor
will attempt to charge through a normal soft-start ramp
after the input voltage rises above 4V.
STANDBY MODE PIN
TheSTBYMDpinisathree-stateinputthatcontrolscircuitry
within the IC as follows: When the STBYMD pin is held at
ground, the SS pin is pulled to ground. When the pin is
left open, the internal SS current source charges the SS
capacitor, allowing turn-on of the controller and activat-
ing necessary internal biasing. When the STBYMD pin is
taken above 2V, the internal linear regulator is turned on
independentofthestateontheRUNandSSpins,providing
an output power source for “wake-up” circuitry. Bypass
the pin with a small capacitor (0.1μF) to ground if the pin
is not connected to a DC potential.
OUTPUT OVERVOLTAGE PROTECTION
An overvoltage comparator guards against transient over-
shoots (>7.5%) as well as other more serious conditions
thatmayovervoltagetheoutput.Inthiscase,synchronous
switch B and synchronous switch D are turned on until the
overvoltage condition is cleared or the maximum negative
current limit is reached. When inductor current is lower
than the maximum negative current limit, synchronous
switch B and synchronous switch D are turned off, and
switch A and switch C are turned on until the inductor
current reaches another negative current limit. If the
comparator still detects an overvoltage condition, switch
A and switch C are turned off, and synchronous switch B
and synchronous switch D are turned on again.
3780fe
15
Linear Systems [ Linear Systems ]
