SC202 Datasheet, PDF (15/19 Pages) Semtech Corporation – 3.5MHz, 500mA Step-down Regulator with Integrated Inductor

SC202
Applications Information (continued)
C_OUT Selection
The internal voltage loop compensation in the SC202
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 equation
C_OUT
3 u 'I_LOAD
V_DROOP u f
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 to be
C_OUT
3 u 0.4
0.05 u 3.5 u10⁶
6.0PF
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.
Table 2 — Recommended Output Capacitors
Manufacturer
Part Nunber
Murata
GRM188R60J106ME47D
Murata
GRM21BR60J106K
Taiyo Yuden
JMK107BJ106MA-T
TDK
C1608X5R0J106MT
Value
(μF)
Rated Dimensions
Type Voltage LxWxH (mm)
(VDC) Case Size
10±20% X5R
10±10% X5R
10±20% X5R
10±20% X5R
6.3
1.6x0.8x0.8
0603
6.3
2.0x1.25x1.25
0805
6.3
1.6x0.8x0.8
0603
6.3
1.6x0.8x0.8
0603
C_IN Selection
The SC202 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 coefficient charac-
teristics 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 deter-
mining the acceptable input ripple voltage and calculating
the minimum value required for C_IN using the equation
C_IN
V_OUT
V_IN
¨¨©§^1
V_OUT
V_IN
¸¸¹·
¨¨©§
'V
I_OUT
 ESR_{¸¸¹·}f
The input voltage ripple is at maximum level when the
input voltage is twice the output voltage (50% duty cycle
scenario).
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