LT1176/LT1176-5
4
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Application Hints
Although the LT1176 has a peak switch rating of 1.2A and a maximum duty
cycle of 85%, it must be used cautiously in applications which require high
switch current and high duty cycle simultaneously, to avoid excessive chip
temperature. Thermal resistance is 90°C/W for the 8-pin DIP package and
50°C/W for the 20-pin SO. This limits continuous chip power dissipation to
the 0.5W to 1W range. These numbers assume typical mounting techniques.
Extra or thick copper connected to the leads can reduce thermal resistance.
Bonding the package to the board or using a clip style heat sink can also help.
The following formulas will give chip power dissipation and peak switch current
for the standard buck converter. Note that surges less than 30 seconds do not
need to be considered from a thermal standpoint, but for proper regulation,
they must not result in peak switch currents exceeding the 1.2A limit.
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: To calculate maximum switch ON voltage at current between low
and high conditions, a linear interpolation may be used.
Note 3: A feedback pin voltage (V
FB
) of 2.5V forces the V
C
pin to its low
clamp level and the switch duty cycle to zero. This approximates the
zero load condition where duty cycle approaches zero. The LT1176-5 has
V
SENSE
= 5.5V.
Note 4: Total voltage from V
IN
pin to ground pin must be ≥ 8V after
start-up for proper regulation.
Note 5: Switch frequency is internally scaled down when the feedback pin
voltage is less than 1.3V to avoid extremely short switch-on times. During
testing, V
FB
or V
SENSE
is adjusted to give a minimum switch-on time of
1µs.
Note 6: I
LM
= (R
LIM
– 1k)/7.65k
Note 7: Switch to input voltage limitation must also be observed.
Note 8: V
MAX
= 35V
Note 9: Does not include switch leakage.
Note 10: Error amplifier voltage gain and transconductance are
specified relative to the internal feedback node. To calculate gain and
transconductance from the sense pin (Output) to the V
C
pin on the
LT1176-5, multiply by 0.44.
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. T
J
= 25°C, V
IN
25V, unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Status Low Level I
STATUS
= 1.6mA Sinking
l
0.25 0.4 V
Status Delay Time 9 µs
Status Minimum Width 30 µs
Power = I
LOAD
(V
OUT
/V
IN
) + V
IN
[7mA + 3mA (V
OUT
/V
IN
) + 0.012 (I
LOAD
)]
I
PEAK
= I
LOAD(PEAK)
+ [V
OUT
(V
IN
– V
OUT
)]/2E
5
(V
IN
)(L)
Example: V
IN
= 15V, V
OUT
= 5V, I
LOAD
= 0.5A Continuous, 0.8A Peak,
L = 100µH
Power (I
LOAD
= 0.5A) = 0.38W
I
PEAK
(I
LOAD
= 0.8A) = 0.97A
Where component size or height is critical, we suggest using solid tantalum
capacitors (singly or in parallel), but be sure to use units rated for switching
applications. Coiltronics is a good source for low profile surface mount
inductors and AVX makes high quality surface mount tantalum capacitors. For
further help, use Application Notes 19 and 44,LTC’s SwitcherCAD
®
computer
design program, and our knowledgeable application department.
