Datasheet

LT6109-1/LT6109-2
21
610912fa
APPLICATIONS INFORMATION
until the EN/RST pin is transitioned high. When the part
is disabled, both the amplifier and comparator outputs
are high impedance.
When the EN/RST pin is transitioned from low to high
to enable the part, the amplifier output PMOS can turn
on momentarily causing typically 1mA of current to flow
into the SENSEHI pin and out of the OUTA pin. Once the
amplifier is fully on, the output will go to the correct cur-
rent. Figure16 shows this behavior and the impact it has
on V
OUTA
. Circuitry connected to OUTA can be protected
from these transients by using an external diode to clamp
V
OUTA
or a capacitor to filter V
OUTA
.
Power Up
After powering on the LT6109, the comparators must be
reset in order to guarantee a valid state at their outputs.
Fast supply ramps may cause a supply current transient
during start-up as shown in the Typical Performance
Characteristics. This current can be lowered by reducing
the edge speed of the supply.
Reverse-Supply Protection
The LT6109 is not protected internally from external rever-
sal of supply polarity. To prevent damage that may occur
during this condition, a Schottky diode should be added
in series with V
—
(Figure 17). This will limit the reverse
current through the LT6109. Note that this diode will limit
the low voltage operation of the LT6109 by effectively
reducing the supply voltage to the part by V
D
.
Also note that the comparator reference, comparator output
and EN/RST input are referenced to the V
–
pin. In order to
preserve the precision of the reference and to avoid driving
the comparator inputs below V
–
, R2 must connect to the
V
–
pin. This will shift the amplifier output voltage up by
V
D
. V
OUTA
can be accurately measured differentially across
R1 and R2. The comparator output low voltage will also be
shifted up by V
D
. The EN/RST pin threshold is referenced
to the V
–
pin. In order to provide valid input levels to the
LT6109 and avoid driving EN/RST below V
–
the negative
supply of the driving circuit should be tied to V
–
.
Figure 16. Amplifier Enable Response
Figure 15. Comparator Reset Functionality
EN/RST
OUTC1
OUTC2
t
RESET
0.5µs (TYPICAL)
610912 F15
t
RPW(MAX)
15µs
COMPARATOR
RESET
RESET PULSE WIDTH LIMITS
t
RPW(MIN)
2µs
50µs/DIV
0V
V
EN/RST
2V/DIV
0V
V
OUTA
2V/DIV
610912 F14
V
+
= 60V
R
IN
= 100Ω
R
OUT
= 10k