Datasheet

LT3055
14
3055f
For more information www.linear.com/LT3055
OPERATION
I
MON
Pin Operation (Current Monitor)
The I
MON
pin is the collector of a PNP which mirrors the
LT3055 output PNP at a ratio of 1:500 (see Block Diagram).
There is additional circuitry which compensates for early
voltage variation by regulating the collector of the I
MON
mirror PNP at the output voltage. This circuitry is active
for V
IMON
≤ (V
OUT
– 500mV). For the range where the
early voltage compensation circuit is active, calculate the
output current from the simple equation:
I
OUT
= 500 •
V
IMON
R
IMON
For V
IMON
> (V
OUT
-500mV), the I
MON
mirror PNP collec-
tor is V
IMON
+ VDS
SAT
(500mV at 500mA). Early voltage
effects increase the I
OUT
to I
MON
ratio as V
IMON
increases.
In addition, if V
IN
– V
IMON
< 1V, the I
MON
mirror PNP
saturates at high loads, causing the I
OUT
-to-I
MON
ratio
to increase quickly. The I
MON
mirror ratio is affected by
power dissipation in the LT3055; it increases at a rate of
approximately 0.5 percent per watt.
Open-Circuit Detection (I
MIN
Pin)
The I
MIN
pin is the collector of a PNP which mirrors
the LT3055 output PNP at a ratio of 1:2000 (see Block
Diagram). The I
MIN
fault comparator asserts the FAULT1
pin if the I
MIN
pin voltage is below 0.6V. This low output
current fault threshold voltage (I
OPEN
) is set by attaching
a resistor from I
MIN
to GND:
R
IMIN
= 2000 •
0.6
I
OPEN
If the open-circuit detection function is not needed, the
I
MIN
pin must be left floating (unconnected). A small de-
coupling capacitor (10nF minimum) from I
MIN
to GND is
required to improve I
MIN
pin power supply rejection and to
prevent FAULT1 pin glitches. See the Typical Performance
Characteristics section for additional information.
I
MAX
Pin Operation
The I
MAX
pin is the collector of a PNP which mirrors the
LT3055 output PNP at a ratio of 1:500 (see Block Diagram).
The I
MAX
pin is also the input to the precision current limit
amplifier. If the output load increases to the point where it
causes the I
MAX
pin voltage to reach 0.6V, the current limit
amplifier takes control of the output regulation so that the
I
MAX
pin regulates at 0.6V, regardless of the output voltage.
The current limit threshold (I
LIMIT
) is set by connecting a
resistor (R
IMAX
) from I
MAX
to GND:
R
IMAX
= 500 •
0.6V
I
LIMIT
In cases where the IN to OUT differential voltage exceeds
10V, fold-back current limit lowers the internal current
limit level, possibly causing it to override the external
programmable current limit. See the Internal Current Limit
vs V
IN
-V
OUT
graph in the Typical Performance Character-
istics section.
V
OUT
(V)
0
475
I
OUT
:I
IMON
RATIO (mA/mA)
485
495
505
1
2
3 4
3055 F03a
515
525
V
IN
= 6V
V
IMON
= 2.5V
I
OUT
= 500mA (IN CURRENT LIMIT)
480
490
500
510
520
5
EARLY VOLTAGE
EFFECTS
V
IMON
(V)
0
475
I
OUT
:I
IMON
RATIO (mA/mA)
485
495
505
1
2
3 4
3055 F03b
5
515
525
V
IN
= 6V
V
OUT
= 5V
I
OUT
= 500mA
480
490
500
510
520
6
I
MON
MIRROR
PNP SATURATING
EARLY VOLTAGE
EFFECTS
Figure 3a. I
OUT
:I
IMON
Ratio vs V
OUT
Figure 3b. I
OUT
:I
IMON
Ratio vs V
IMON