SC189CEVB datasheet(17/23 Pages) SEMTECH

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SC189

the current limit holding current for the SC189 to exit

foldback current limit mode. Figure 5 shows the typical

current limit holding current decreasing rate over differ-

ent output voltage. The SC189 is capable of sustaining an

indefinite short circuit without damage and will resume

normal operation when the fault is removed. The foldback

current limit mode will be disabled during the soft-start.

Over-Voltage Protection

In the event of a 15% over-voltage on the output, the

PWM drive is disabled with LX pin floating.

Soft-Start

The soft-start mode is activated after VIN reaches its

UVLO and EN signal is set high to enable the part. An over

temperature shutdown event will also activate the soft

start sequence. Soft-start mode controls the maximum

current during startup thus limiting in-rush current. The

PMOS current limit is stepped through four soft start

levels of approximately 20%, 25%, 40%, & 100%. Each

step is maintained for 20μs following internal reference

start up of 20μs giving the total nominal startup period

of 100μs. During startup, the chip operates in controlling

the inductor current swings between 0A and current

limit. If V

OUT reaches 90% of the target within the first 2

current levels, the chip continues in hysteretic mode till

the end of the soft-start time period before switching to

PWM mode. If V

OUT does not reach 90% by the end of the

second current limit level, soft start will continue to level 3

or level 4 till the output voltage reaches 96% and will then

transition into PWM mode. After the full soft start time

period, the SC189 will switch into PWM mode operation

regardless of the V

OUT level.

The SC189 is capable of starting up into a pre-biased

output. When the output is precharged by another supply

rail, the SC189 will not discharge the output during the

soft start interval.

Shut Down

When the EN pin voltage goes low, the SC189 will run in

shutdown mode, drawing less than 1μA from the input

power supply. The internal switches and bandgap voltage

will be immediately turned off.

Inductor Selection

The SC189 converter has internal loop compensation. The

Applications Information (continued)

Figure 3 — Typical maximum output current over

temperature of SOT23-5 package, V

IN= 5.0V

Figure 4 — Typical maximum output current over

temperature of SOT23-5 package, V

IN= 3.3V

Maximum Output Current for TJ

 125°C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-40

-25

-10

110

125

Ambient Temperature (°C)

VIN= 5.0V



JA= 90°C/W

VOUT=1.2V

VOUT=2.5V

VOUT= 3.3V

Maximum Output Current for TJ

 125°C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-40

-25

-10

110

125

Ambient Temperature (°C)

VIN= 3.3V



JA= 90°C/W

VOUT=1.2V

VOUT=2.5V

Maximum Power Dissiption for TJ

 125°C

SOT23-5 Package

300

600

900

1200

1500

-40

-25

-10

110

125

Ambient Temperature (°C)



JA= 90°C/W

Figure 2 — Maximum power dissiption of SOT23-5

package over temperature

Current Limit Holding Current over Vout

120

150

1.0

1.5

2.0

2.5

3.0

3.5

Output Voltage (V)

TA= 25°C

VIN= 5.0V

VIN= 3.6V

Figure 5 — Current limit holding current decreasing

rate vs. output voltage

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