SC4503 datasheet(11/22 Pages) SEMTECH

11 / 22 page

 2007 Semtech Corp.

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SC4503

POWER MANAGEMENT

Switch on duty ratio in DCM is given by,

−

(11)

Higher input current ripples and lower output current are

the drawbacks of DCM operation.

Input Capacitor

The input current in a boost converter is the inductor cur-

rent, which is continuous with low RMS current ripples. A

2.2-4.7

µF ceramic input capacitor is adequate for most

applications.

Output Capacitor

Both ceramic and low ESR tantalum capacitors can be

used as output ﬁltering capacitors. Multi-layer ceramic

capacitors, due to their extremely low ESR (<5mΩ), are

the best choice. Use ceramic capacitors with stable

temperature and voltage characteristics. One may be

tempted to use Z5U and Y5V ceramic capacitors for output

ﬁltering because of their high capacitance density and

small sizes. However these types of capacitors have high

temperature and high voltage coefﬁcients. For example,

the capacitance of a Z5U capacitor can drop below 60%

of its room temperature value at –25°C and 90°C. X5R

ceramic capacitors, which have stable temperature and

voltage coefﬁcients, are the preferred type.

The diode current waveform in Figure 4 is discontinuous

with high ripple-content. Unlike a buck converter in which

the inductor ripple current

∆I

L determines the output ripple

voltage. The output ripple voltage of a boost regulator is

much higher and is determined by the absolute inductor

current. Decreasing the inductor ripple current does not

reduce the output ripple voltage appreciably. The current

flowing in the output filter capacitor is the difference

between the diode current and the output current. This

capacitor current has a RMS value of:

−

(12)

If a tantalum capacitor is used, then its ripple current rating

in addition to its ESR will need to be considered.

Applications Information (Cont.)

When the switch is turned on, the output capacitor supplies

the load current I

OUT (Figure 4). The output ripple voltage

due to charging and discharging of the output capacitor

is therefore:

∆

(13)

For most applications, a 10-22

µF ceramic capacitor is suf-

ﬁcient for output ﬁltering. It is worth noting that the output

ripple voltage due to discharging of a 10

µF ceramic capaci-

tor (13) is higher than that due to its ESR.

Rectifying Diode

For high efﬁciency, Schottky barrier diodes should be used

as rectifying diodes for the SC4503. These diodes should

have an average forward current rating at least equal to the

output current and a reverse blocking voltage of at least

a few volts higher than the output voltage. For switching

regulators operating at low duty cycles (i.e. low output

voltage to input voltage conversion ratios), it is beneﬁcial

to use rectifying diodes with somewhat higher average cur-

rent ratings (thus lower forward voltages). This is because

the diode conduction interval is much longer than that of

the transistor. Converter efﬁciency will be improved if the

voltage drop across the diode is lower.

The rectifying diodes should be placed close to the SW

pin of the SC4503 to minimize ringing due to trace induc-

tance. Surface-mount equivalents of 1N5817 and 1N5818,

MBRM120, MBR0520L, ZHCS400, 10BQ015 and equiva-

lent are suitable.

Shutdown and Soft-Start

The shutdown (

SHDN SS

SHDN

) pin is a dual function pin. When

driven from a logic gate with V

OH>2V, the

SHDN SS

SHDN

functions as an on/off input to the SC4503. When the

shutdown pin is below 2V, it clamps the error ampliﬁer

output to

and reduces the switch current limit.

Connecting R

SS and CSS to the

SHDN SS

SHDN

pin (Figure 5) slows

the voltage rise at the pin during start-up. This forces the

peak inductor current (hence the input current) to follow a

slow ramp, thus achieving soft-start.

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