SC189AEVB datasheet(20/23 Pages) SEMTECH

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SC189

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.

OSC

DROOP

LOAD

OUT

⋅

∆

⋅

= 3

The output capacitor RMS current ripple may be calcu-

lated from the equation

(

)













⋅

−

⋅

OSC

OUT

MAX

OUT

RMS

COUT

)

(

)

(

Table 3 lists the manufacturers of recommended output

capacitor options.

IN Selection

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 from the equation

OSC

OUT

ESR

⋅













−

∆













−

Applications Information (continued)

The input capacitor RMS ripple current varies with the

input and output voltage. The maximum input capacitor

RMS current is found from the equation













−

OUT

RMS

CIN

)

(

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

Value

(μF)

Type

Rated

Voltage

(VDC)

Value

3.3V

(μF)

Dimensions

LxWxH

(mm)

Murata

GRM21BR61A106K

10±10%

X5R

4.42

2.0x1.25x1.25

(EIA:0805)

Murata

GRM21BR71A106K

10±10%

X7R

4.88

2.0x1.25x1.25

(EIA:0805)

Murata

GRM21BR60J106K

10±10%

X5R

6.3

4.05

2.0x1.25x1.25

(EIA:0805)

Murata

GRM21BR70J106K

10±10%

X7R

6.3

4.91

2.0x1.25x1.25

(EIA:0805)

Murata

GRM21BR60J226M

22±20%

X5R

6.3

6.57

2.0x1.25x1.25

(EIA:0805)

Table 3 – Recommended Capacitors

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