LTC1923 LINER [Linear Technology], LTC1923 Datasheet - Page 12

• Manufacturer Part Number: LTC1923
• Description: High Efficiency Thermoelectric Cooler Controller
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC1923
PROTECTION FEATURES
Many protection features have been integrated into the
LTC1923 to ensure that the TEC is not overstressed or the
system does not thermally run away. These features
include pulse-by-pulse current limiting, TEC voltage clamp-
ing and open/shorted thermistor detection.
Current Limit
The peak current in the full bridge during each switching
cycle can be limited by placing a sense resistor, R
the common NMOS source connections of MNA and MNB
to ground. The CS
as shown in Figure 1. Current limit is comprised of a fixed
gain of ten differential amplifier, an attenuator (resistor
divider) and a current limit comparator. A detailed diagram
of the circuitry is shown in Figure 3. The differential
amplifier output, I
ability to monitor the instantaneous current flowing in the
bridge. If an average current is desired, an external RC
filter can be used to filter the I
50ns of leading edge blanking is also internally integrated
to prevent nuisance tripping of the current sense circuitry.
It relieves the filtering requirements for the CS input pins.
During a switching cycle, current limit occurs when the
voltage on I
conditions: 1) 1.5 times the voltage on the SS pin, 2) 1.5
times the voltage on the I
current limit condition is sensed, all four external FETs are
immediately shut off. These devices are turned back on
only after C
12
TEC
T
reaches the same state (either charging or
exceeds the lowest of the following three
CS
CS
I
I
TEC
LIM
SS
U
+
TEC
+
–
and CS
, is provided to allow the user the
TEC
INPUT SELECT
+
–
TEC
LIM
connections should be made
NDRVA
–
TEC
pin or 3) 1.5V. When a
1.5 A
NDRVB
output. Approximately
+
–
A = 10
CURRENT SENSE
Figure 3. Current Sense Circuitry
AMPLIFIER
S
, from
R
2R
NDRVA
LEB
discharging) as when the current limit condition oc-
curred. For instance, if C
occurs, the outputs are forced off for the remainder of this
charging time, the entire C
re-enabled when C
charging again. An analogous sequence of events occurs
if current limit is tripped while C
The full-bridge current can be soft-started (gradually
increased) by placing a capacitor from the SS pin to
ground. A 1.5 A current is sourced from the chip and will
charge the capacitor. This limits the inrush current at start-
up and allows the current delivered to the TEC to be linearly
increased from zero.
The LTC1923 features a dedicated pin, I
current limit. If the voltage placed on I
1V, the default current limit, I
where R
Utilizing the I
be easily set and adjusted (the current limit threshold can
also be adjusted by changing R
facilitates independent setting of the heating and cooling
current limits with the addition of one transistor. Figure 4
shows how to implement this using three resistors and an
external NMOS, M1. In many applications, a higher cool-
ing capability is desired. When TEC
TEC
system is being cooled (this is typical for most lasers).
NDRVB
I
LIMIT
–
1V
, the H/C output is in a low state signifying that the
S
= 150mV/R
+
–
–
–
= the current sense resistor.
LIM
PEAK/VALLEY
PULSE-BY-PULSE
OSCILLATOR
CURRENT LIMIT
pin allows the current limit threshold to
T
reaches its valley voltage and begins
S
T
S
R
is charging when current limit
T
discharge time, and are only
Q
LIMIT
1923 F03
T
S
). More importantly, it
is being discharged.
, is:
SHUT
OUTPUTS
OFF
LIM
+
is greater than
is greater than
LIM
, to adjust
1923f