sc202amltrt Semtech Corporation, sc202amltrt Datasheet - Page 15

• Manufacturer Part Number: sc202amltrt
• Description: 3.5mhz, 500ma Step-down Regulator With Integrated Inductor And Digital Programmable Output
• Manufacturer: Semtech Corporation

Applications Information (continued)
the device goes through the soft-start process and
resumes normal operation.
Under-Voltage Lockout
UVLO (Under-Voltage Lockout) activates when the supply
voltage drops below the falling UVLO threshold. This pre-
vents the device from entering an ambiguous state in
which regulation cannot be maintained. Hysteresis of
approximately 200mV is included to prevent chattering
near the threshold.
C
The internal voltage loop compensation in the SC202A
limits the minimum output capacitor value to 10μF. This
is due to its influence on the the loop crossover frequency,
phase margin, and gain margin. Increasing the output
capacitor above this minimum value will reduce the cross-
over frequency and provide greater phase margin.
Capacitors with X7R or X5R ceramic dielectric are recom-
mended 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.
In addition to ensuring stability, the output capacitor
serves other important functions. This capacitor deter-
mines the output voltage ripple — as capacitance
increases, ripple voltage decreases. It also supplies current
during a large load step for a few switching cycles until
the control loop responds (typically 3 switching cycles).
Once the loop responds, regulation is restored and the
desired output is reached. During the period prior to PWM
operation resuming, the relationship between output
voltage and output capacitance can be approximated
using the following equation.
This equation can be used to approximate the minimum
output capacitance needed to ensure voltage does not
droop below an acceptable level. For example, a load step
from 50mA to 400mA requiring droop less than 50mV
would require the minimum output capacitance as shown
by the following equation.
OUT
C
Selection
OUT
3
V
DROOP
I
LOAD
f
In this example, using a standard 10µF capacitor would be
adequate to keep voltage droop less than the desired
limit. Note that if the voltage droop limit were decreased
from 50mV to 25mV, the output capacitance would need
to be increased to at least 12µF (twice as much capaci-
tance for half the droop). Capacitance will decrease from
the nominal value when a ceramic capacitor is biased with
a DC current, so it is important to select a capacitor whose
value exceeds the necessary capacitance value at the pro-
grammed output voltage. Check the manufacturer’s
capacitance vs. DC voltage graphs when selecting an
output capacitor to ensure the capacitance will be
adequate.
Table 2 lists the manufacturers of recommended output
capacitor options.
C
The SC202A input source current will appear as 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 coeffi-
cient characteristics when determining the actual required
value. For example, a 10μF, 6.3V, X5R ceramic capacitor
with 5V DC applied may exhibit a capacitance as low as
4.5μF. The value of required input capacitance is estimated
by determining the acceptable input ripple voltage and
Manufacturer
Part Nunber
Murata
GRM188R60J106ME47D
Murata
GRM21BR60J106K
Taiyo Yuden
JMK107BJ106MA-T
TDK
C1608X5R0J106MT
IN
Selection
C
Table 2 — Recommended Output Capacitors
OUT
. 0
05
3
3
0
5 .
4 .
10
10±20%
10±10%
10±20%
10±20%
Value
(μF)
6
6
0 .
Type
X5R
X5R
X5R
X5R
F
Voltage
Rated
(VDC)
6.3
6.3
6.3
6.3
SC202A
LxWxH (mm)
2.0x1.25x1.25
Dimensions
1.6x0.8x0.8
1.6x0.8x0.8
1.6x0.8x0.8
Case Size
0603
0805
0603
0603
15