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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 .5A minimum, to

support A DC load current, the inductor ripple current at

A DC load current needs to be less than A.

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

60%

65%

70%

75%

80%

85%

90%

95%

100%

0.0

0.2

0.4

0.6

0.8

1.0

Output Current (A)

VIN= 5.0V

VOUT= 3.3V

TA=25°C

L=1071AS-1R0N (33m

_typ)

L=1071AS-2R2N (50m

_typ)

L=LQM2HP1R0MG0 (55m

_typ)

L=MDT2520-CR1R0M (60m

_typ)

Figure 3 — Typical efficiency curves

IN=5.0V, VOUT=3.3V)

OUT Selection

The internal voltage loop compensation in the SC4626

limits the minimum output capacitor value to 0µF 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 0uF to 22uF. When choosing

output capacitor’s capacitance, verify the voltage derating

effect from the capacitor vendors data sheet.

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

OSC

DROOP

LOAD

OUT

⋅

∆

⋅

= 3

The outputcapacitorRMScurrentripplemaybecalculated

from the equation

(

)













⋅

−

⋅

OSC

OUT

MAX

OUT

RMS

COUT

)

(

)

(

Table 3 lists the manufacturers of recommended output

capacitor options.

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

OSC

OUT

ESR

⋅













−

∆













−

The input capacitor RMS ripple current varies with the

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