Datasheet

LTC4413-1/LTC4413-2
9
441312fd
OPERATION
The LTC4413-1/LTC4413-2 are described with the aid of the
Block Diagram. Operation begins when the power source at
V
INA
or V
INB
rises above the undervoltage lockout (UVLO)
voltage of 2.4V and the corresponding control pin ENBA or
ENBB is low. If only the voltage at the V
INA
pin is present,
the internal power source (V
DD
) is supplied from the V
INA
pin. The amplifi er (A) pulls a current proportional to the
difference between V
INA
and V
OUTA
from the gate (V
GATEA
)
of the internal PFET (PA), driving this gate voltage below
V
INA
. This turns on PA. As V
OUTA
pulls up to a forward
voltage drop (V
FWD
) of 15mV below V
INA
, the LTC4413
regulates V
GATEA
to maintain the small forward voltage
drop. The system is now in forward regulation and the
load at V
OUTA
is powered from the supply at V
INA
. As the
load current varies, V
GATEA
is controlled to maintain V
FWD
until the load current exceeds the transistor’s (PA) ability
to deliver the current as V
GATEA
approaches GND. At this
point, the PFET behaves as a fi xed resistor, R
ON
, whereby
the forward voltage increases slightly with increased load
current. As the magnitude of I
OUT
increases further, (such
that I
LOAD
> I
OC
) the LTC4413-1/LTC4413-2 fi xes the load
current to the constant value I
OC
to protect the device.
The characteristics for parameters R
FWD
, R
ON
, V
FWD
and
I
OC
are specifi ed with the aid of Figure 1, illustrating the
LTC4413-1/LTC4413-2 forward voltage drop versus that
of a Schottky.
If another supply is provided at V
INB
, the LTC4413-1/
LTC4413-2 likewise regulate the gate voltage on PB to
maintain the output voltage, V
OUTB
, just below the input
voltage V
INB
. If this alternate supply, V
INB
, exceeds the
voltage at V
INA
, the LTC4413-1/LTC4413-2 selects this
input voltage as the internal supply (V
DD
). This second
ideal diode operates independently of the fi rst ideal diode
function.
When an alternate power source is connected to the load
at V
OUTA
(or V
OUTB
), the LTC4413-1/LTC4413-2 sense the
increased voltage at V
OUTA
, and amplifi er A increases the
voltage V
GATEA
, reducing the current through PA. When
V
OUTA
is higher than V
INA
+ V
RTO
, V
GATEA
will be pulled up
to V
DD
, turning off PA. The internal power source for the
LTC4413-1/LTC4413-2 (V
DD
) then diverts to draw current
from the V
OUTA
pin, only if V
OUTA
is larger than V
INB
(or
V
OUTB
). The system is now in the reverse turn-off mode.
Power to the load is being delivered from an alternate
supply, and only a small current (I
LEAK
) is drawn from or
sourced to V
INA
to sense the potential at V
INA
.
When the selected channel of the LTC4413-1/LTC4413-2
is in reverse turn-off mode or both channels are disabled,
the STAT pin sinks 11μA of current (I
SON
) if connected.
Channel selection is accomplished using the two ENB
pins, ENBA and ENBB. When the ENBA input is asserted
(high), PA has its gate voltage pulled to V
DD
, turning off
PA. A 3.5μA pull-down current on the ENB pins ensures
a low level at these inputs if left fl oating.
FORWARD VOLTAGE (V)
0
0
CURRENT (A)
I
OC
I
FWD
LTC4413-1
LTC4413-2
SLOPE: 1/R
ON
V
FWD
441312 F01
SLOPE: 1/R
FWD
1N5817
Figure 1. The LTC4413 vs the 1N5817