sc183c Semtech Corporation, sc183c Datasheet - Page 15

• Manufacturer Part Number: sc183c
• Description: 2.5mhz, 2a Synchronous Step-down Regulator
• Manufacturer: Semtech Corporation

© 2009 Semtech Corp.
Applications Information (continued)
output voltage. The SC83C 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.
The typical operation of current limit protection is shown
in figure 3.
Over-Voltage Protection
In the event of a 5% over-voltage on the output, the
PWM drive is disabled with LX pin floating.
Sof-Start
The soft-start mode is activated after AVIN reaches its
UVLO and one or more CTL pins are set high to enable the
part. The thermal 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%, & 00%. Each step is
maintained for 200μs following internal reference start up
of 50μs giving the total nominal startup period of 850μs.
During startup, the chip operates in controlling the induc-
tor current swings between 0A and current limit. If V
reaches 86% of the target or at the end of soft-start period,
the SC83C will switch to PWM mode operation. Figure 2
shows the typical diagram of soft start operation.
Figure 4 — Typical Current Limit Holding Current
300
250
200
150
100
50
0
1.0
Decreasing Rate vs. Output Voltage
Current Limit Holding Current over Vout
1.5
V
IN
= 3.3V
Output Voltage (V)
2.0
V
IN
= 3.6V
2.5
V
IN
= 5.0V
T
3.0
A
= 25° C
3.5
OUT
5
The SC83C is capable of starting up into a pre-biased
output. When the output is precharged by another
supply rail, the SC83C will not discharge the output
during the soft start interval.
Shut Down
When all CTL pins are low, the SC83C will run in shut-
down mode, drawing less than μA from the input power
supply. The internal switches and bandgap voltage will
be immediately turned off.
Thermal Shutdown
The device has a thermal shutdown feature to protect the
SC83C if the junction temperature exceeds 60°C.
During thermal shutdown, the on-chip power devices are
disabled, tri-stating the LX output. When the temperature
drops by 0°C, it will initial the soft start cycle to resume
normal operation.
Inductor Selection
The SC83C converter has internal loop compensation.
The compensation is designed to work with a output
filter corner frequency of less than 40kHz for V
50KHz for V
corner frequency of output filter can be defined by the
equation 2.
Values outside this range may lead to instability, malfunc-
tion, or out-of-specification performance.
In general, the inductance is chosen by making the induc-
tor ripple current to be less than 30% of maximum load
current. When choosing an inductor, it is important to
consider the change in inductance with DC bias current.
The inductor saturation current is specified as the current
at which the inductance drops a specific percentage from
the nominal value. This is approximately 30%. Except for
short-circuit or other fault conditions, the peak current
must always be less than the saturation current specified
by the manufacturer. The peak current is the maximum
load current plus one half of the inductor ripple current
at the maximum input voltage. Load and/or line tran-
sients can cause the peak current to exceed this level for
f
C
=
2
π
L
1
IN
⋅
C
of 3.3V over any operating condition. The
OUT
www.semtech.com
SC183C
IN
of 5V and
(2)