SC284 datasheet(13/20 Pages) SEMTECH

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SC284

Applications Information (continued)

Maximum Power Dissipation

Each channel of SC284 has its own Θ

JA of 40°C/W when

only one channel is in operation. Since both channels are

withinsamepackage,thereisabout50%heatwhichwillbe

transferred to the adjacent channel. The equivalent total

thermal impedance will be higher when the neighboring

channel is also in operation. To guarantee an operating

junction temperature of less than 25°C, Figure 2 shows

the maximum allowable power loss of each channel. The

curve is based upon the junction temperature of either

channel reaching a maximum of 25°C. Each channel of

SC284 can support up to .8A load current. Figures 3a

and 3b show the maximum allowable load current based

upon the limit of maximum loss for V

IN=3.3V and VIN=5.0V,

respectively. The curves are drawn for high duty-cycle

operation. If the operating duty-cycle is lower, the loss is

lower allowing higher load current.

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

2.2

2.4

2.6

2.8

0 0.2 0.4 0.6 0.8 1

1.2 1.4 1.6 1.8 2

2.2 2.4 2.6 2.8

Loss of Channel A (W)

TA = 25°C

TA = 55°C

TA = 85°C

Figure 2 — Maximum allowable loss for each channel

for a maximum junction temperature of 125°C

Protection Features

The SC284 provides the following protection features:

Current Limit

Over-Voltage Protection

Soft-Start Operation

Thermal Shutdown

•

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

Load Current Channel A (A)

TA = 85°C

TA = 68°C

TA = 75°C

(a) V

IN= 3.3V, VOUT=2.5V

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

Load Current A (A)

TA < 76°C

TA < 85°C

(b) V

IN= 5.0V, VOUT=3.3V

Figure 3 — Maximum allowable Load Current for each

channel for a maximum junction temperature of 125°C

Current Limit and Protection

The internal PMOS power device in the switching stage

is protected by a current limit feature. If the inductor

current is above the PMOS current limit for 6 consecutive

cycles, the part enters foldback current limit mode and

the output current is limited to the current limit holding

current (I

CL_HOLD) of a few hundred milliampere. Under

this condition, the output voltage will be the product

of I

CL_HOLD and the load resistance. The

current limit

holding current will decrease when the output voltage

increases. The load presented must fall below the current

limit holding current for the part to exit foldback current

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