Datasheet

17
LTC1923
1923f
APPLICATIO S I FOR ATIO
WUUU
The thermistor may be isolated from the control circuitry.
It has a relatively high input impedance and is therefore
susceptible to noise pick up. Extreme care should be taken
to ensure this signal is noise free by shielding the line
(coaxially). A lowpass filter can be added between the
thermistor and the input to the LTC2053, but since it is in
the signal path, there are limitations on how much filtering
can be added.
Inductor Ripple Current
The current that flows in the bridge can be separated into
two components, the DC current that flows through the
TEC and the inductor ripple current that is present due to
the switchmode nature of the controller. Although the TEC
current has its own ripple component, proper filtering will
minimize this ripple relative to the inductor ripple current,
validating this assumption that the TEC current is constant
(see TEC Ripple Current section). A simplified half-circuit
of the bridge in steady-state is shown in Figure 10. The
current, I
L
, through the inductor (L) consists of the ripple
current (∆I
1
) and static TEC current (I
TEC
). The ripple
current magnitude, ∆I
1
, can be calculated using the fol-
lowing equation:
∆I
1
= (V
BRIDGE
2
– V
TEC
2
)/(4 • f
OSC
• L • V
BRIDGE
)
where
V
BRIDGE
is the full-bridge supply voltage (typically V
DD
)
f
OSC
is the oscillator frequency
L is the filter inductor value
V
TEC
is the DC voltage drop across the TEC
The peak inductor current is equal to I
TEC
+ ∆I
1
/2 and is
the current level that trips the current limit comparator.
Keeping the ripple current component small relative to
I
TEC
keeps the current limit trip level equal to the current
flowing through the TEC.
Example: V
BRIDGE
= 5V, R
TEC
= 2.5Ω, V
TEC
= 2.5V,
I
TEC
= 1A, L = 22µH, f
OSC
= 250kHz. The peak-to-peak
ripple current using the above equation is:
∆I
1
= 170mA
The peak inductor current is therefore 1.085A in order to
get 1A of DC TEC current.
TEC Ripple Current
Every TEC has a fundamental limitation (based mainly on
the TEC’s physical characteristics) on the maximum
temperature differential that it can create between sides.
The ability to create this maximum temperature differen-
tial is affected by the amount of ripple current that flows
through the device, relative to the DC component. An
approxima tion of this degradation due to TEC ripple cur-
rent is given by the following equation:
dT/dT
MAX
= 1/(1 + N
2
)
where:
dT is the adjusted achievable temperature differential
dT
MAX
is the maximum possible temperature differen-
tial when the TEC is fed strictly by DC current and is
typically specified by the manufacturer
N is the ratio of TEC ripple current to DC current
TEC manufacturers typically state that N should be no
greater than 10%.
MPA
MNB
PDRVA
NDRVB
ESR
V
TEC
/2
C
1923 F10
+
L
I
L
I
TEC
TEC
1/2 V
BRIDGE
V
BRIDGE
Figure 10. Full-Bridge Half Circuit