L1 =

SM74301

SNVS719A – JULY 2011

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Thermal Protection

The SM74301 should be operated so the junction temperature does not exceed 125°C during normal operation.

An internal Thermal Shutdown circuit is provided to shutdown the SM74301 in the event of a higher than normal

junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state by

disabling the buck switch. This feature prevents catastrophic failures from accidental device overheating. When

the junction temperature reduces below 140°C (typical hysteresis = 25°C) normal operation is resumed.

Applications Information

SELECTION OF EXTERNAL COMPONENTS

A guide for determining the component values will be illustrated with a design example. Refer to the Block

Diagram. The following steps will configure the SM74301 for:

•

Input voltage range (Vin): 12V to 95V

•

•

Load current (for continuous conduction mode): 100 mA to 300 mA

Standard values of 3.01 k

Ω and 1.00 kΩ are chosen. Other values could be used as long as the 3:1 ratio is

maintained.

application requires a specific frequency, the choice of frequency is generally a compromise since it affects the

size of L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time of

400 ns, is calculated from:

(6)

will be used to allow for tolerances in equation 1, resulting in a frequency of 223 kHz.

L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor

value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum

ripple current occurs at maximum Vin.

a) Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude

using the following equation:

(7)

At Vin = 95V, L1(min) calculates to 200 µH. The next larger standard value (220 µH) is chosen and with this

b) Maximum load current: At a load current of 300 mA, the peak of the ripple waveform must not reach the

minimum value of the SM74301’s current limit threshold (410 mA). Therefore the ripple amplitude must be less

than 220 mA p-p, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum Vin and Io,

the peak of the ripple will be 391 mA. While L1 must carry this peak current without saturating or exceeding its

temperature rating, it also must be capable of carrying the maximum value of the SM74301’s current limit

threshold (610 mA) without saturating, since the current limit is reached during startup.

The DC resistance of the inductor should be as low as possible. For example, if the inductor’s DCR is one ohm,

the power dissipated at maximum load current is 0.09W. While small, it is not insignificant compared to the load

power of 3W.

µF.

C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its

ESR, ripple voltage due to its capacitance, and the nature of the load.

10

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