PD(MAX) = ((VIN(MAX) ± VOUT) × IOUT(MAX))

LP3985

SNVS087AE – OCTOBER 2000 – REVISED MAY 2015

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The LP3985 regulator has internal thermal limiting designed to protect the device during overload conditions. For

continuous normal conditions, the recommended maximum operating junction temperature is 125°C. It is

important to give careful consideration to all sources of thermal resistance from junction to ambient. Additional

heat sources mounted nearby must also be considered.

For surface-mount devices, heat sinking is accomplished by using the heat-spreading capabilities of the PC

board and its copper traces. Copper board stiffeners and plated through-holes can also be used to spread the

heat generated by power devices. Example: Given an output voltage of 3.3 V, an input voltage range of 4 V to 6

V, a maximum output current of 100 mA, and a maximum ambient temperature of 50°C, what is the maximum

operating junction temperature? The power dissipated by the device is found using the formula:

where

•

IOUT(MAX) = 100 mA

•

VIN(MAX) = 6 V

•

VOUT = 3.3 V

(1)

For example, PD(MAX) = ((6 V – 3.3 V) × 100 mA ) = 0.27 W.

Using the 5-pin SOT-23 (DBV) package, the LP3985 junction-to-ambient thermal resistance (R

θJA) has a rating

of 220°C/W using the standard JEDEC JESD51-7 PCB (High-K) circuit board. The junction temperature rise

above ambient is found using the formula:

TRISE = PD(MAX) × RθJA;

for example, TJ(MAX) = 50°C + 59.4°C = 109.4°C.

8.2.3 Application Curves

VIN = 4.2 V

VIN = 4.2 V

Figure 39. Start-up Time

Figure 40. Enable Response

9 Power Supply Recommendations

The LP3985 is designed to operate from an input voltage supply range between 2.5 V and 6 V. The input-voltage

range provides adequate headroom in order for the device to have a regulated output. This input supply must be

well regulated. If the input supply is noisy, additional input capacitors with low ESR can help to improve the

output noise performance.

18

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