SC4503EVB SEMTECH [Semtech Corporation], SC4503EVB Datasheet - Page 11

• Manufacturer Part Number: SC4503EVB
• Description: 1.3MHz Step-Up Switching Regulator with 1.4A Switch
• Manufacturer: SEMTECH [Semtech Corporation]

 2007 Semtech Corp.
Switch on duty ratio in DCM is given by,
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
2.2-4.7µF ceramic input capacitor is adequate for most
applications.
Output Capacitor
Both ceramic and low ESR tantalum capacitors can be
used as output fi ltering 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
fi ltering because of their high capacitance density and
small sizes. However these types of capacitors have high
temperature and high voltage coeffi cients. 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 coeffi cients, 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
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:
If a tantalum capacitor is used, then its ripple current rating
in addition to its ESR will need to be considered.
POWER MANAGEMENT
Applications Information (Cont.)
=
−
−
∆I
L
determines the output ripple
(11)
(12)
11
When the switch is turned on, the output capacitor supplies
the load current I
due to charging and discharging of the output capacitor
is therefore:
For most applications, a 10-22µF ceramic capacitor is suf-
fi cient for output fi ltering. It is worth noting that the output
ripple voltage due to discharging of a 10µF ceramic capaci-
tor (13) is higher than that due to its ESR.
Rectifying Diode
For high effi ciency, 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 benefi cial
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 effi ciency 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 (
driven from a logic gate with V
functions as an on/off input to the SC4503. When the
shutdown pin is below 2V, it clamps the error amplifi er
output to
Connecting R
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.
∆
=
SS
and C
SHDN SS
SHDN
OUT
and reduces the switch current limit.
(Figure 4). The output ripple voltage
SS
SS
to the
) pin is a dual function pin. When
SHDN SS
SHDN
OH
>2V, the
SS
pin (Figure 5) slows
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SC4503
SHDN SS
SHDN
SS
(13)
pin