lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 22

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3695
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3695
cool. The Exposed Pad on the bottom of the package
may be soldered to a copper area which should be tied
to large copper layers below with thermal vias; these lay-
ers will spread the heat dissipated by the LT3695. Place
additional vias to reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
to ambient can be reduced to θ
100 LFPM airfl ow, this resistance can fall by another 25%.
Further increases in airfl ow will lead to lower thermal
resistance. Because of the large output current capability
of the LT3695, it is possible to dissipate enough heat to
raise the junction temperature beyond the absolute maxi-
mum. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches these maximums. If the junction
temperature reaches the thermal shutdown threshold, the
part will stop switching to prevent internal damage due
to overheating.
Table 5: Effects of Pin Shorts
PINS
PGND-DA
SW-RUN/SS
RUN/SS-RT
RT -SYNC
SYNC-V
PG-GND
GND-BD
BD-BOOST
22
IN
EFFECT
No effect if V
The result of this short depends on the load resistance and on R3 (Figure 10). See the following discussion.
No effect or V
No effect or V
No effect if V
No effect.
V
voltage source supplying BD. Make sure it is safe to short the supply for BD to ground. For this reason BD should not be connected to
V
If diode D2 (see Figure 10) is used, no effect or V
OUT
IN
, but it is safe to connect it to V
may fall below regulation voltage, power dissipation of the power switch will be increased. Note that this short also grounds the
IN
IN
OUT
OUT
< V
does not exceed the absolute maximum voltage of SYNC (20V).
will fall below regulation voltage if I
will fall below regulation voltage if the current into the RT pin is less than 1mA.
IN(MAX)
JA
. See Input Voltage Range section for description of V
= 40°C/W or less. With
OUT
.
OUT
R3
may fall below regulation voltage. Otherwise the device may be damaged.
(Figure 10) < 1mA.
Power dissipation within the LT3695 can be estimated
by calculating the total power loss from an effi ciency
measurement. The die temperature rise is calculated by
multiplying the power dissipation of the LT3695 by the
thermal resistance from junction to ambient. Die tempera-
ture rise was measured on a 2-layer, 10cm × 10cm circuit
board in still air at a load current of 1A (f
For a 12V input to 5V output the die temperature elevation
above ambient was 22°C with the exposed pad soldered
and 44°C without the exposed pad soldered.
Fault Tolerance
The LT3695 is designed to tolerate single fault conditions.
Shorting two adjacent pins together or leaving one single
pin fl oating does not raise V
LT3695. However, the application circuit must meet the
requirements discussed in this section in order to achieve
this tolerance level.
Tables 5 and 6 show the effects that result from shorting
adjacent pins or from a fl oating pin, respectively.
IN(MAX)
.
OUT
or cause damage to the
SW
= 800kHz).
3695f