Datasheet

LT3050 Series
15
3050fa
OPERATION
I
MON
Pin Operation (Current Monitor)
The I
MON
pin is the collector of a PNP which mirrors the
LT3050 output PNP at a ratio of 1:100 (see block diagram
on page 11). The current sourced by the I
MON
pin is
~1/100
th
of the current sourced by the OUT pin when the
I
MON
and OUT pin voltages are equal and the device is not
operating in dropout. If the I
MON
and OUT pin voltages
are not the same, the ratio deviates from 1/100 due to
the Early voltages of the I
MON
and OUT PNPs according
to the equation:
I
IMON
I
OUT

1
I
MONR
v
70  V
IN
 V
IMON
70  V
IN
 V
OUT
¥
§
¦
´
¶
µ
Early Voltage Compensation
124443444
where the Early voltage of the PNPs is 70V and I
MONR
is
a variable which represents the I
OUT
to I
MON
current ratio.
I
MONR
varies with V
IN
to V
OUT
voltage according to the
empirically derived equation:
I
MONR
= 97 + 5 • log
10
(1+V
IN
– V
OUT
) for (V
IN
– V
OUT
)
≥0.5
I
MONR
= 96 + 2 • (V
IN
– V
OUT
) for (V
IN
– V
OUT
) < 0.5
The I
MON
pin current can be converted into a voltage for
use by monitoring circuitry simply by connecting the I
MON
pin to a resistor.
Connecting a resistor from I
MON
to GND converts the
I
MON
pin current into a voltage that can be monitored by
circuitry such as an ADC.
For example, a 1.2k resistor results in a I
MON
pin voltage
of 1.2V for an output current of 100mA and an output
voltage of 1.2V.
The output current of the device can be calculated from
the I
MON
pin voltage by the following equation:
I
OUT
 I
MONR
I
OUT
I
MON
Ratio
{
v
V
IMON
R
IMON
I
IMON
123
v
70  V
IN
 V
OUT
70  V
IN
 V
IMON
¥
§
¦
´
¶
µ
Early VoltageCompensation
124443444
A small decoupling capacitor (22nF minimum) from I
MON
to GND is required to improve I
MON
pin power supply
rejection. If the current monitor is not needed, it must be
tied to GND.
Open Circuit Detection (I
MIN
Pin)
The I
MIN
pin is the collector of a PNP which mirrors the
LT3050 output at a ratio of approximately 1:200 (see block
diagram on page 11). The I
MIN
fault comparator asserts
the FAULT 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
IV
I
IMIN
OPEN OUT
OPEN
=
119 85 1 68 36 8.–(.–.• )•
This equation is empirically derived and partially
compensates for early voltage effects in the I
MIN
current
mirror. It is valid for an input voltage range from 0.6V
above the output to 10V above the output. It is valid for
output voltages up to 12V. The accuracy of this equation
for setting the resistor value is approximately ±2%. Unit
values are Amps, Volts, and Ohms.
If the open circuit detection function is not needed, the I
MIN
pin must be left fl oating (unconnected). A small decoupling
capacitor (10nF minimum) from I
MIN
to GND is required
to improve I
MIN
pin power supply rejection and to prevent
FAULT pin glitches.
See the Typical Performance Characteristics section for
additional information.