SC189ASKTRT Semtech, SC189ASKTRT Datasheet - Page 17

• Manufacturer Part Number: SC189ASKTRT
• Description: 2.5MHz, 1.5A Synchronous Step Down Regulator In SOT-23_5
• Manufacturer: Semtech

Applications Information (continued)
© 2009 Semtech Corp.
Figure 5 — Current limit holding current decreasing
Figure 2 — Maximum power dissiption of SOT23-5
Figure 4 — Typical maximum output current over
Figure 3 — Typical maximum output current over
temperature of SOT23-5 package, V
temperature of SOT23-5 package, V
1500
1200
900
600
300
150
120
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
90
60
30
0
0
-40
-40
-40
1.0
V

T
V

package over temperature
IN
JA
A

-25
-25
-25
IN
JA
= 25° C
= 5.0V
= 90° C/W
= 3.3V
JA
= 90° C/W
rate vs. output voltage
Maximum Power Dissiption for T
= 90° C/W
Maximum Output Current for T
Maximum Output Current for T
Current Limit Holding Current over Vout
-10
-10
-10
1.5
5
5
5
Ambient Temperature (° C)
Ambient Temperature (° C)
Ambient Temperature (° C)
SOT23-5 Package
20
20
20
Output Voltage (V)
V
2.0
V
IN
OUT
= 3.6V
35
35
35
V
=2.5V
OUT
V
OUT
50
50
50
=2.5V
= 3.3V
2.5
V
65
65
65
V
IN
J
J
OUT
J
 125° C
 125° C
= 5.0V
 125° C
V
=1.2V
80
80
80
OUT
=1.2V
3.0
95
95
95
110 125
110 125
110 125
IN
IN
= 3.3V
= 5.0V
3.5
17
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
current levels, the chip continues in hysteretic mode till
the end of the soft-start time period before switching to
PWM mode. If V
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
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
OUT
reaches 90% of the target within the first 2
OUT
OUT
does not reach 90% by the end of the
level.
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