SC2440 datasheet(13/26 Pages) SEMTECH

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SC2440

POWER MANAGEMENT

Applications Information

results from the ESR of the output capacitor while the

second term is due to the charging and discharging of

OUT by the inductor ripple current. Substituting ∆IL = 0.6A,

f = 1MHz and C

OUT = 10µF ceramic with ESR = 3mΩ in

(7),

)

(

VOUT

Ω

⋅

∆

Depending on operating frequency and the type of

capacitor, ripple voltage resulting from charging and

discharging of C

OUT may be higer than that due to ESR. A

10µF or 22µF X5R ceramic capacitor is found adequate

for output filtering in most applications. Ripple current

in the output capacitor is not a concern because the

inductor current of a buck converter directly feeds C

OUT,

resulting in very low ripple current. Avoid using Z5U and

Y5V ceramic capacitors for output filtering because these

types of capacitors have high temperature and high

voltage coefficients.

Freewheeling Diode

Use of Schottky barrier diodes as freewheeling rectifiers

reduces diode reverse recovery input current spikes,

easing high-side current sensing in the SC2440. These

diodes should have a RMS current rating between 1A

and 2A and a reverse blocking voltage of at least 5V

higher than the input voltage. For switching regulators

operating at low duty cycles (i.e. low output voltage to

input voltage conversion ratios), it is beneficial to use

freewheeling 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 freewheeling diodes should be placed close to the

SW pins of the SC2440 to minimize ringing due to trace

inductance. Surface-mount equivalents of 1N5817 and

1N5819, MBRM120LT3 (ON Semi), UPS120 and

UPS140 (Micro-Semi) are all suitable.

Bootstrapping the Power Transistors

To maximize efficiency, the turn-on voltage across the

internal power NPN transistors should be minimized. If

these transistors are to be driven into saturation, then

their bases will have to be driven from a power supply

higher in voltage than V

IN. The required driver supply

voltage (at least 2.5V higher than the SW voltage over

the industrial temperature range) is generated with a

bootstrap circuit (the diode D

BST and the capacitor CBST in

Figure 8). The bootstrapped output (the common node

between D

BST and CBST) is connected to the BOOST pin of

the SC2440. The power transistor in the SC2440 is first

switched on to build up current in the inductor. When

the transistor is switched off, the inductor current pulls

the SW node low, allowing C

BST to be charged through

BST. When the power switch is again turned on, the SW

voltage goes high. This brings the BOOST voltage to

BST

V +

, thus back-biasing D

BST. CBST voltage increases

with each subsequent switching cycle, as does the

bootstrapped voltage at the BOOST pin. After a number

of switching cycles, C

BST will be fully charged to a voltage

approximately equal to that applied to the anode of D

BST.

Figure 7 shows the typical minimum BOOST to SW voltage

required to fully saturate the power transistor. This

differential voltage (

BST

) must be at least 1.8V at

room temperature. This is also specified in the “Electrical

Characteristics” as “Minimum Bootstrap Voltage”. The

minimum required V

CBST increases as temperature

decreases. The bootstrap circuit reaches equilibrium

when the base charge drawn from C

BST during transistor

on time is equal to the charge replenished during the off

interval.

Figure 7. Typical Minimum Bootstrap Voltage Re-

quired to Maintain Saturation at I

SW = 2A.

Minimum Bootstrap Voltage

vs Temperature

1.4

1.6

1.8

2.0

2.2

2.4

-50

-25

100

Temperature (°C)

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