Datasheet

11
LTC1923
1923f
OPERATIO
U
MAIN CONTROL LOOP
The LTC1923 uses a constant frequency, voltage mode
architecture to control temperature. The relative duty
cycles of two pairs of N-/P-channel external MOSFETs, set
up in a full-bridge (also referred to as an H-bridge)
configuration are adjusted to control the system tempera-
ture. The full-bridge architecture facilitates bidirectional
current flow through a thermoelectric cooler (TEC) or
other heating element. The direction of the current flow
determines whether the system is being heated or cooled.
Typically a thermistor, platinum RTD or other appropriate
element is used to sense the system temperature. The
control loop is closed around this sense element and TEC.
The voltage on the output of the error amplifier, EAOUT,
relative to the triangle wave on C
T
, controls whether the
TEC will be heating or cooling. A schematic of the external
full bridge is shown in Figure 1. The “A” side of the bridge
is comprised of the top left PMOS, MPA, and lower right
NMOS, MNA. The gates of these devices are attached to
the PDRVA and NDRVA outputs of the LTC1923, respec-
tively. The “B” side of the bridge is comprised of PMOS,
MPB and NMOS, MNB. The gates of these MOSFETs are
controlled by the PDRVB and NDRVB outputs of the
LTC1923.
The “A” side of the bridge is turned on (NDRVA is high and
PDRVA is low) when the output of the error amplifier is
less than the voltage on the C
T
pin as shown in Figure 2.
For this condition, the state of each output driver is as
follows: PDRVA is low, NDRVA is high, PDRVB is high and
NDRVB is low. When the voltage on EAOUT is greater than
the voltage on the C
T
pin, the “B” side of the bridge is
turned on. The average voltage across the TEC, V
TECOOLER
,
is approximately:
V
TECOOLER
= V
TEC
+
– V
TEC
–
= V
DD
•␣ (D
A
– D
B
)
where
V
DD
= the full-bridge supply voltage
V
TECOOLER
= V
TEC
+
– V
TEC
–
D
A
= the duty cycle of the “A” side of the bridge or the
amount of time the “A” side is on divided by the
oscillator period
D
B
= the duty cycle of the “B” side of the bridge
Duty cycle terms D
A
and D
B
are related by the following
equation:
D
A
= 1 – D
B
In steady-state, the polarity of V
TECOOLER
indicates whether
the system is being heated or cooled. Typically, when
current flows into the TEC
+
side of the cooler, the system
is being cooled and heated when current flows out of this
terminal. Note: Do not confuse the TEC
+
side of the TEC
with the TEC
+
input of the LTC1923, although these two
points should be connected together .
4
3
2
1
MNA
1923 F01
MPB
MPA
–+
MNBNDRVB
NDRVA
CS
+
CS
–
TEC
+
TEC
–
PDRVA
PDRVB
TEC
V
TECOOLER
V
DD
R
S
NDRVA
A
SIDE
ON
B
SIDE
ON
EAOUT
C
T
PDRVA
NDRVB
PDRVB
1923 F02
Figure 1. Full-Bridge Schematic
Figure 2. Error Amplifier Output, C
T
and Output Driver Waveforms