SC189ASKTRT Semtech, SC189ASKTRT Datasheet - Page 16

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

© 2009 Semtech Corp.
Applications Information
Detailed Description
The SC189 is a synchronous step-down pulse width
modulated (PWM) voltage mode DC-DC regulator
operating at 2.5MHz fixed-frequency. The switching
frequency is chosen to minimize the size of the external
inductor and capacitors while maintaining high
efficiency.
Operation
During
side
ing edge of the internal oscillator.
feedback loop uses an internal feedback resistor divider.
The period is set by the on board oscillator when in PWM
mode at average to high loads. The device has an internal
low-side synchronous NMOS device and does not require a
Schottky diode on the LX pin. The device operates as a buck
converter in PWM mode with a fixed frequency of 2.5MHz.
Output Voltage Selection
The SC189 is designed for fixed output voltage. There
are some options for preset output voltage shown in
Table 1. If the voltage desired is not shown in the Table
1, it can be programmed via an external resistor di-
vider. There will be typical 1uA current flowing into the
VOUT pin. The typical schematic of adjustable output
voltage option from the part with standard 1.0V, the
SC189A, is shown in Figure 1. The C
tain the performance of the transient response. The
proper value of C
To simplify the design, it is recommended to program the
desired output voltage from standard 1.0V as shown in
Figure 1 with a proper C
Schematic of Adjustable V
,where the V
Enable
C
Figure 1 — Typical schematic for adjustable output
V
FF
IN
voltage option from standard 1.0V of SC189A
[
nF
PMOS
Note: (1) R
]
R
normal
=
EN
(2) R
10
OSTD
FB2
EN
×
C
is optional.
=10k and C
device
IN
R
is the standard voltage shown in Table 1.
FB
FF
1
[
operation,
can be calculated by the equation
k
(
FF
V
Ω
VIN
GND
=10nF for standard design.
EN
OUT
SC189A
]
is
⋅
FF
(
V
OUT
−
calculated from the equation
OUT
VOUT
0
activated
5 .
LX
from SC189A (Std V
−
)
2
V
10k
R
R
the
OSTD
FB1
FB2
FF
L
C
is needed for main-
)
FF
×
(
internal
R
on
V
FB
OSTD
1
The voltage
V
=
C
OSTD
(
OUT
V
each
−
OUT
0
−
5 .
) 1
OUT
V
high-
)
×
OUT
R
ris-
=1.0V)
FB
2
16
shown above. For programming the output voltage from
other standard voltage, the R
adjusted to meet the equation shown above.
Maximum Power Dissiption of SOT23-5 Package
The maximum power dissiption for junction tempera-
ture of less than 125°C on SOT23-5 package is shown
in figure 2. The curve is drawn based on the Θ
90°C/W which is measured in free convection, mounted
on 10mm x 10mm, 2 layer FR4 PCB shown in figure 8
with copper of 1oz for each layer. The maximum power
dissiption may limit the maximum output current over
temperature. The figure 3 and figure 4 show the typi-
cal maximum output current for T
perature of V
inductor with higher loss (i.e., chip inductor), due to
the higher board temperature, the Θ
higher.
Protection Features
The SC189 provides the following protection features:
Thermal Shutdown
The device has a thermal shutdown feature to protect
the SC189 if the junction temperature exceeds 160°C.
During thermal shutdown, the on-chip power devices
are disabled with the LX output floating. When the die
temperature drops by 10°C, the part will initiate a soft
start recovery to normal operation.
Current Limit
The internal PMOS power device in the switching stage is
protected by current limit feature. If the output is loaded
above the PMOS current limit for 32 consecutive cycles,
the SC189 enters foldback current limit mode and the
output current is limited to the current limit holding
current (I
these conditions the output voltage will be the product
of I
holding current (I
voltage is increased. The load presented must fall below
•
•
•
•
CL_HOLD
Thermal Shutdown
Current Limit
Over-Voltage Protection
Soft-Start Operation
CL_HOLD
and the load resistance. The current limit
IN
=5.0V and V
) of a few hundred milliampere. Under
CL_HOLD
) will be decreased when output
IN
=3.3V, respectively. If using
FB1
, R
J
FB2
≤ 125°C over tem-
JA
and C
will be a little bit
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FF
need to be
JA
SC189
of