Datasheet

LT3692
26
3692fa
For more information www.linear.com/3692
applicaTions inForMaTion
The power dissipation in the other power components
such as catch diodes, boost diodes and inductors, cause
additional copper heating and can further increase what
the IC sees as ambient temperature. See the LT1767 data
sheet’s Thermal Considerations section.
Die Temperature and Thermal Shutdown
The LT3692 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 21. 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
Generating a Negative Regulated Voltage
The simple charge pump circuit in Figure 22 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
.
Figure 21. Circuit to Extend the T
J
Pin Operating Range
Figure 22. Circuit to Generate the Negative Voltage Rail to
Extend the T
J
Pin Operating Range
+
LT3692
R1
V
NEG
GND
3692 F23
T
J
LT3692
30k
330pF
0.1µF
D3, D4: ZETEX BAT54S
D4
D3
GND
3692 F23
T
J
CLKOUT