LTC4413
8
4413fd
For more information www.linear.com/LTC4413
operaTion
The LTC4413 is described with the aid of the Block Diagram
(Figure 1). Operation begins when the power source at
V
INA
or V
INB
rises above the undervoltage lockout (UVLO)
voltage of 2.4V and either of the ENBA or ENBB control
pins is low. If only the voltage at the V
INA
pin is present, the
power source to the LTC4413 (V
DD
) will be supplied from
the V
INA
pin. The amplifier (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
is pulled up to
a forward voltage drop (V
FWD
) of 20mV 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 fixed resistor with
resistance 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
fixes 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 specified with the aid of Figure 2,
illustrating the LTC4413 forward voltage drop versus that
of a Schottky diode.
If another supply is provided at V
INB
, the LTC4413 likewise
regulates 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 selects this input voltage as the internal supply
(V
DD
). This second ideal diode operates independently of
the first ideal diode function.
When an alternate power source is connected to the load
at V
OUTA
(or V
OUTB
), the LTC4413 senses the increased
voltage at V
OUTA
and amplifier A increases the voltage
V
GATEA
, reducing the current through PA. When V
OUTA
is
higher than V
INA
+ V
RTO
, V
GATEA
is pulled up to V
DD
, which
turns off PA. The internal power source for the LTC4413
(V
DD
) is then diverted to source 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 is drawn from V
INA
to sense the potential at V
INA
.
When the selected channel of the LTC4413 is in reverse
turn-off mode or both channels are disabled, the STAT pin
sinks 9µA of current (I
SON
) if connected.
Channel selection is accomplished using the two ENB pins,
ENBA and ENBB. For example with channel A, when the
ENBA input is asserted (high), PA’s gate voltage is pulled
to V
DD
at a controlled rate, limiting the turn-off time to
avoid voltage spiking at the input when being driven by an
inductive source impedance. A 3µA pull-down current on
the ENBA, ENBB pins ensures a low level at these inputs
if left floating.
Slow Response Time
The LTC4413-1 (or LTC4413-2) is recommended for
applications with demanding load step or fast slew rate
requirements. The LTC4413-1 and LTC4413-2 provide bet
-
ter load regulation in these environments at the expense
of higher quiescent current. The LTC4413 is optimized
for lower power consumption and should not be used in
high slew rate environments or when large and fast load
transients are anticipated.
Overcurrent and Short-Circuit Protection
During an overcurrent condition, the output voltage droops
as the load current exceeds the amount of current that
the LTC4413 can supply. At the time when an overcurrent
condition is first detected, the LTC4413 takes some time to
Figure 2
FORWARD VOLTAGE (V)
0
0
I
FWD
OC
SLOPE
1/R
ON
SLOPE
1/R
FWD
LTC4413
SCHOTTKY
DIODE
