LT6109-2_15 Datasheet, PDF (21/28 Pages) Linear Technology – High Side Current Sense Amplifier with Reference and Comparators

LT6109-1/LT6109-2
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
EN/RST
RESET PULSE WIDTH LIMITS
t_RPW(MIN)
2µs
COMPARATOR
RESET
t_RPW(MAX)
15µs
OUTC1
OUTC2
t_RESET
0.5µs (TYPICAL)
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Figure 15. Comparator Reset Functionality
V_EN/RST
2V/DIV
0V
V_OUTA
2V/DIV
V⁺ = 60V
R_IN = 100Ω
R_OUT = 10k
0V
50µs/DIV
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Figure 16. Amplifier Enable Response
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^–.
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