SC4501 datasheet(11/21 Pages) SEMTECH

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POWER MANAGEMENT

SC4501

Application Information

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

L determines the output ripple voltage.

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:

OUT

−

(8)

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

OUT (Figure 5). The output ripple voltage

due to charging and discharging of the output capacitor is

therefore:

OUT

∆

(9)

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

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

IN current decreases to 10µA at 2V. Figure 6

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

OH is higher than the supply voltage of the SC4501. The

diode clamps the maximum shutdown pin voltage to one

diode voltage above the input power supply.

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