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LT1176/LT1176-5

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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.

Thefollowingformulaswillgivechippowerdissipationandpeakswitchcurrent

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 (VFB) of 2.5V forces the VC 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

VSENSE = 5.5V.

Note 4: Total voltage from VIN 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, VFB or VSENSE is adjusted to give a minimum switch-on time of

1µs.

Note 6: ILM = (RLIM – 1k)/7.65k

Note 7: Switch to input voltage limitation must also be observed.

Note 8: VMAX = 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 VC pin on the

LT1176-5, multiply by 0.44.

ELECTRICAL CHARACTERISTICS The

denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. TJ = 25°C, VIN 25V, unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Status Low Level

ISTATUS = 1.6mA Sinking

0.25

0.4

Status Delay Time

µs

Status Minimum Width

µs

Power = ILOAD(VOUT/VIN) + VIN [7mA + 3mA (VOUT/VIN) + 0.012 (ILOAD)]

IPEAK = ILOAD(PEAK) + [VOUT(VIN – VOUT)]/2E5(VIN)(L)

Example: VIN = 15V, VOUT = 5V, ILOAD = 0.5A Continuous, 0.8A Peak,

L = 100µH

Power (ILOAD = 0.5A) = 0.38W

IPEAK (ILOAD = 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 AVXmakeshighqualitysurfacemounttantalumcapacitors.For

further help, use Application Notes 19 and 44,LTC’s SwitcherCAD® computer

design program, and our knowledgeable application department.

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