Datasheet

LTC4411
5
4411fa
BLOCK DIAGRA
W
Figure 2. Detailed Block Diagram
–
+
–
+
+
–
P1
V
B
V
B
OVERTEMP
UVLO
OUT
MAX
10µA
3µA
V
REF
OFF
SHDB
+
–
3
1 5
4
2
STAT
OUT
CTL
IN
GND
4411 F02
A
V
GATE
Figure 3. LTC4411 vs Schottky Diode
Forward Conduction Characteristics
OPERATIO
U
The LTC4411 operation is described with the aid of
Figure 3. Forward regulation for the LTC4411 has three
operation modes depending on the magnitude of the load
current. For small load currents, the LTC4411 will provide
a constant voltage drop; this operating mode is referred to
as “constant V
ON
” regulation. As the current exceeds I
FWD
the voltage drop will increase linearly with the current with
a slope of 1/R
ON
; this operating mode is referred to as
“constant R
ON
” regulation. As the current increases fur-
ther, exceeding I
MAX
, the forward voltage drop will in-
crease rapidly; this operating mode is referred to as
“constant I
ON
” regulation. The characteristics for the
following parameters: R
FWD
, R
ON
, V
FWD
, I
FWD
, and I
MAX
are specified with the aid of Figure 3.
Operation begins when the power source at IN rises above
the UVLO voltage of 2.4V (typ) and the CTL (control) pin
is low. If only the voltage at the IN pin is present, the power
source to LTC4411 (V
DD
) will be supplied from the IN pin.
The amplifier (A) will deliver a voltage proportional to the
difference between V
IN
and V
OUT
to the gate (V
GATE
) of the
internal P-channel MOSFET (P1), driving this gate voltage
below V
IN
. This will turn on P1. As P1 conducts, V
OUT
will
be pulled up towards V
IN
. The LTC4411 will then control
V
GATE
to maintain a low forward voltage drop. The system
is now in forward regulation and the load at OUT will be
powered from the supply at IN. As the load current varies,
V
GATE
will be controlled to maintain a low forward voltage
drop. If the load current exceeds P1’s ability to deliver the
current, as V
GATE
approaches GND, the P1 will behave as
a fixed resistor, with resistance R
ON
, whereby the forward
voltage will increase with increased load current. As I
LOAD
increases further (I
LOAD
> I
MAX
), the LTC4411 will regulate
the load current as described below. During the forward
regulation mode of operation the STAT pin will be an open
circuit.
V
FWD
I
OC
I
MAX
I
FWD
FORWARD VOLTAGE (V)
0
LOAD CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
0.25 0.5 0.75 1.0
4411 F03
T
A
= 40°C
SLOPE
1/R
ON
SLOPE
1/R
FWD
SCHOTTKY
DIODE
LTC4411
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