SC189ASKTRT Semtech, SC189ASKTRT Datasheet - Page 20

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

© 2009 Semtech Corp.
Applications Information (continued)
capacitor’s capacitance, verify the voltage derating effect
from the capacitor vendors data sheet.
Capacitors with X7R or X5R ceramic dielectric are
recommended for their low ESR and superior temperature
and voltage characteristics. Y5V capacitors should not
be used as their temperature coefficients make them
unsuitable for this application.
The output voltage droop due to a load transient is
determined by the capacitance of the ceramic output
capacitor. The ceramic capacitor supplies the load current
initially until the loop responds. Within a few switching
cycles the loop will respond and the inductor current will
increase to match the required load. The output voltage
droop during the period prior to the loop responding
can be related to the choice of output capacitor by the
relationship.
The output capacitor RMS current ripple may be calcu-
lated from the equation
Table 3 lists the manufacturers of recommended output
capacitor options.
C
The SC189 source input current is a DC supply current
with a triangular ripple imposed on it. To prevent large
input voltage ripple, a low ESR ceramic capacitor is
required. A minimum value of 4.7μF should be used. It is
important to consider the DC voltage coefficient charac-
teristics when determining the actual required value. To
estimate the required input capacitor, determine the
acceptable input ripple voltage and calculate the
minimum value required for C
IN
C
I
COUT
C
Selection
OUT
IN
(
=
=
RMS
V

 

DROOP
)
3
I
∆
V
OUT
V
=
∆ ⋅
OUT
V
IN
2
I
1
−
LOAD
⋅

 

3
1
f
ESR
OSC

 

−
V
V
V
OUT
OUT

⋅  

IN
f
⋅
L
OSC

 

(
V
⋅
IN
f
OSC
(
MAX
⋅
IN
V
)
−
IN
from the equation
V
OUT
)

 

20
The input capacitor RMS ripple current varies with the
input and output voltage. The maximum input capacitor
RMS current is found from the equation
The input voltage ripple and RMS current ripple are at
maximum when the input voltage is twice the output
voltage or 50% duty cycle.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the PMOS switch.
Low ESR/ESL X5R ceramic capacitors are recommended
for this function. To minimise stray inductance ,the
capacitor should be placed as closely as possible to the
VIN and GND pins of the SC189.
Manufacturer
Part Nunber
Murata
GRM21BR61A106K
Murata
GRM21BR71A106K
Murata
GRM21BR60J106K
Murata
GRM21BR70J106K
Murata
GRM21BR60J226M
I
CIN
(
RMS
)
=
Table 3 – Recommended Capacitors
V
V
OUT
IN
10±10%
10±10%
10±10%
10±10%
22±20%
Value

 

(μF)
1
−
V
V
OUT
IN
Type

 

X5R
X7R
X5R
X7R
X5R
Voltage
Rated
(VDC)
6.3
6.3
6.3
10
10
Value
www.semtech.com
3.3V
(μF)
4.42
4.88
4.05
4.91
6.57
at
SC189
2.0x1.25x1.25
2.0x1.25x1.25
2.0x1.25x1.25
2.0x1.25x1.25
2.0x1.25x1.25
Dimensions
(EIA:0805)
(EIA:0805)
(EIA:0805)
(EIA:0805)
(EIA:0805)
LxWxH
(mm)