SC4626CSKTRT Datasheet, PDF (18/22 Pages) Semtech Corporation – 2.5MHz, 1A Synchronous Step Down Regulator in SOT23-5

SC4626
Applications Information (continued)
The SC4626 is compatible with small shielded chip
inductors for low cost, low profile applications. The
inductance roll off characteristic of chip inductor is worse
resulting in high ripple current and increased output
voltage ripple at heavy load operation. SC4626 has OCP
peak inductor current threshold of 1.5A minimum, to
support 1A DC load current, the inductor ripple current at
1A DC load current needs to be less than 1A.
Final inductor selection depends on various design
considerations such as efficiency, EMI, size, and cost.
Table 2a and 2b list the manufacturers of recommended
inductor and output capacitors options. Chip inductors
provide smaller footprint and height with lower efficiency
and increased output voltage ripple. Transient load
performance is equivalent to wire wound inductors.
Figure 3 shows the typical efficiency curves for different
inductors.
Efficiency
100%
95%
L=1071AS-2R2N (50m_typ)
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.
The output voltage droop due to a load transient is deter-
mined 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
C_OUT
=
3⋅ ∆I _LOAD
V_DROOP ⋅ f_OSC
The output capacitor RMS current ripple may be calculated
from the equation
90%
85%
L=1071AS-1R0N (33m_typ)
80%
L=MDT2520-CR1R0M (60m_typ)
75%
70%
65%
V_IN= 5.0V
V_OUT= 3.3V
T_A=25°C
L=LQM2HP1R0MG0 (55m_typ)
60%
0.0
0.2
0.4
0.6
0.8
1.0
Output Current (A)
Figure 3 — Typical efficiency curves
(V_IN=5.0V, V_OUT=3.3V)
C_OUT Selection
The internal voltage loop compensation in the SC4626
limits the minimum output capacitor value to 10µF if
using the inductor of 2.2µH. 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 crossover frequency and
provide greater phase margin. A total output capacintance
should not exceed 30uF to avoid any start-up problems.
For most typical applications, it is recommended to use
output capacitance of 10uF to 22uF. When choosing
output capacitor’s capacitance, verify the voltage derating
effect from the capacitor vendors data sheet.
( ) _{ICOUT (RMS )}
=
1
23
 ^VOUT
⋅ V_{IN ( MAX )} − V_OUT
L ⋅ f_OSC ⋅V_IN

Table 3 lists the manufacturers of recommended output
capacitor options.
C_IN Selection
The SC4626 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 from the equation
C_IN
=

V_OUT
V_IN
¹
−
V_OUT
V_IN

∆V
I _OUT
− ESR_{} ⋅
f_OSC
The input capacitor RMS ripple current varies with the
© 2009 Semtech Corp.
18
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