Datasheet

LT3692A
29
3692afc
For more information www.linear.com/3692A
applicaTions inForMaTion
Die Temperature and Thermal Shutdown
The LT3692A T
J
pin outputs a voltage proportional to the
internal junction temperature. The T
J
pin typically outputs
250mV for 25°C and has a slope of 10mV/°C. Without the
aid of external circuitry, the T
J
pin output is valid from 20°C
to 150°C (200mV to 1.5V) with a maximum load of 100µA.
Full Temperature Range Measurement
To extend the operating temperature range of the T
J
out-
put below 20°C, connect a resistor from the T
J
pin to a
negative supply as shown in Figure 20. The negative rail
voltage and T
J
pin resistor may be calculated using the
following equations:
V
NEG
≤
2• TEMP(MIN)°C
100
R1≤
V
NEG
33µA
where:
TEMP(MIN)°C is the minimum temperature where a
valid T
J
pin output is required.
V
NEG
= Regulated negative voltage supply.
For example:
TEMP(MIN)°C = –40°C
V
NEG
≤ –0.8V
V
NEG
= –1, R1 ≤ |V
NEG
|/33µA = 30.2kΩ
Figure 20. Circuit to Extend the T
J
Pin Operating Range
Figure 21. Circuit to Generate the Negative Voltage Rail to
Extend the T
J
Pin Operating Range
+
LT3692A
R1
V
NEG
GND
3692 F20
T
J
LT3692A
30k
330pF
0.1µF
D3, D4: ZETEX BAT54S
D4
D3
GND
3692 F21
T
J
CLKOUT
Generating a Negative Regulated Voltage
The simple charge pump circuit in Figure 21 uses the
CLKOUT pin output to generate a negative voltage, elimi-
nating the need for an external regulated supply. Surface
mount capacitors and dual-package Schottky diodes
minimize the board area needed to implement the nega
-
tive voltage supply.
As a safeguard, the LT3692A has an additional thermal
shutdown threshold set at a typical value of 163°C for each
channel. Each time the threshold is exceeded, a power-on
sequence for that channel will be initiated. The sequence
will then repeat until the thermal overload is removed.
It should be noted that the T
J
pin voltage represents
a steady-state temperature and should not be used to
guarantee that maximum junction temperatures are
not exceeded. Instantaneous power along with thermal
gradients and time constants may cause portions of the
die to exceed maximum ratings and thermal shutdown
thresholds. Be sure to calculate die temperature rise for
steady state (>1Min) as well as impulse conditions.