LT6108-1/LT6108-2
21
610812fa
Figure 16. Comparator Reset Functionality
The approximate total hysteresis is:
V
HYS
= 10mV +R1•
V
DD
– 390mV
R2
For example, to achieve I
OVER
= 900µA with 50mV of total
hysteresis, R5 = 442Ω. Choosing R1 = 4.42k, R3 = 10k
and V
DD
= 5V results in R2 = 513k.
The analog output voltage will also be affected when the
comparator trips due to the current injected into R5 by
the positive feedback. Because of this, it is desirable to
have (R1 + R2) >> R5. The maximum V
OUTA
error caused
by this can be calculated as:
∆V
OUTA
= V
DD
•
R5
R1+R2+ R5
In the previous example, this is an error of 4.3mV at the
output of the amplifier or 43µV at the input of the amplifier
assuming a gain of 100.
When using the comparator with its input decoupled from
the output of the amplifier it may be driven directly by a
voltage source. It is useful to know the threshold voltage
equations with additional hysteresis. The input rising edge
threshold which causes the output to transition from high
to low is:
V
TH R
( )
= 400mV • 1+
R1
R2
The input falling edge threshold which causes the output
to transition from low to high is:
V
TH F
( )
= 390mV 1+
R1
R2
– V
DD
R1
R2
Comparator Output
The comparator output can maintain a logic-low level of
150mV while sinking 500µA. The output can sink higher
currents at elevated V
OL
levels as shown in the Typical
Performance Characteristics. Load currents are conducted
to the V
–
pin. The output off-state voltage may range
between 0V and 60V with respect to V
–
, regardless of the
supply voltage used.
APPLICATIONS INFORMATION
EN/RST Pin (LT6108-1 Only)
The EN/RST pin performs the two functions of resetting
the latch on the comparator as well as shutting down the
LT6108-1. When this pin is pulled high the LT6108-1 is
enabled. After powering on the LT6108-1, the comparator
must be reset in order to guarantee a valid state at its output.
Applying a pulse to the EN/RST pin will reset the compara-
tor from its tripped low state as long as the input on the
comparator is below the threshold and hysteresis. For
example, if V
INC
is pulled higher than 400mV and latches
the comparator, a reset pulse will not reset that comparator
unless its input is held below the threshold by a voltage
greater than the 10mV typical hysteresis. The comparator
output typically unlatches in 0.5µs with 2pF of capacitive
load. Increased capacitive loading on the comparator
output will cause an increased unlatch time.
Figure 16 shows the reset functionality of the EN/RST
pin. The width of the pulse applied to reset the compara-
tor must be greater than t
RPW(MIN)
(2µs) but less than
t
RPW(MAX)
(15µs). Applying a pulse that is longer than
40µs typically (or tying the pin low) will cause the part
to enter shutdown. Once the part has entered shutdown,
the supply current will be reduced to 3µA typically and the
amplifier, comparator and reference will cease to function
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
EN/RST
OUTC
t
RESET
0.5µs (TYPICAL)
610812 F16
t
RPW(MAX)
15µs
COMPARATOR
RESET
t
RPW(MIN)
2µs
