SC4501STDEVB Semtech, SC4501STDEVB Datasheet - Page 11

• Manufacturer Part Number: SC4501STDEVB
• Description: EVALUATION BOARD
• Manufacturer: Semtech

Specifications of SC4501STDEVB

• Silicon Manufacturer: Semtech
• Application Sub Type: Step Up Switching Regulator
• Kit Application Type: Power Management - Voltage Regulator
• Silicon Core Number: SC4501
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 5 is discontinuous
with high ripple-content. In a buck converter the inductor
ripple current I
The output ripple voltage of a boost regulator is however
much higher and is determined by the absolute inductor
current. Decreasing the inductor ripple current does not
appreciably reduce the output ripple voltage. 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.
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
sufficient for output filtering. It is worth noting that the
output ripple voltage due to discharging of a 10µF ceramic
capacitor (9) is higher than that due to its ESR.
Rectifying Diode
For high efficiency, Schottky barrier diodes should be used
as rectifying diodes for the SC4501. These diodes should
have a RMS current rating of at least 1A 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 RMS current ratings (thus lower
Application Information
POWER MANAGEMENT
2005 Semtech Corp.
I
OUT
V
OUT
V
V
OUT
IN
I
OUT
C
OUT
DT
1
L
OUT
determines the output ripple voltage.
(Figure 5). The output ripple voltage
(8)
(9)
11
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
pins of the SC4501 to minimize ringing due to trace
inductance. Surface-mount equivalents of 1N5817,
1N5818, MBRM120 (ON Semi) and 10BQ015 (IRF) are
all suitable.
Soft-Start
Soft-start prevents a DC-DC converter from drawing
excessive current (equal to the switch current limit) from
the power source during start up. If the soft-start time is
made sufficiently long, then the output will enter regulation
without overshoot. An external capacitor from the SS pin
to the ground and an internal 1.5µA charging current
source set the soft-start time. The soft-start voltage ramp
at the SS pin clamps the error amplifier output. During
regulator start-up, COMP voltage follows the SS voltage.
The converter starts to switch when its COMP voltage
exceeds 0.7V. The peak inductor current is gradually
increased until the converter output comes into regulation.
If the shutdown pin is forced below 1.1V or if fault is
detected, then the soft-start capacitor will be discharged
to ground immediately.
The SS pin can be left open if soft-start is not required.
Shutdown
The input voltage and shutdown pin voltage must be greater
than 1.4V and 1.1V respectively to enable the SC4501.
Forcing the shutdown pin below 1.1V stops switching.
Pulling this pin below 0.1V completely shuts off the SC4501.
The total V
shows several ways of interfacing the control logic to the
shutdown pin. Beware that the shutdown pin is a high
impedance pin. It should always be driven from a low-
impedance source or tied to a resistive divider. Floating
the shutdown pin will result in undefined voltage. In Figure
6(c) the shutdown pin is driven from a logic gate whose
V
diode clamps the maximum shutdown pin voltage to one
diode voltage above the input power supply.
OH
is higher than the supply voltage of the SC4501. The
IN
current decreases to 10µA at 2V. Figure 6
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