SC2440AEVB datasheet(17/28 Pages) SEMTECH

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Applications Information (Continued)

(d)

(b)

SC2440A

BST

GND

DBST

VOUT

CBST

VIN

DFW

(c)

SC2440A

BST

GND

DBST

VOUT

CBST

VIN

+ VZ -

DFW

(a)

SC2440A

BST

GND

DBST

VOUT

CBST

VIN

DFW

- VZ +

SC2440A

BST

GND

DBST

VOUT

CBST

VIN

DFW

Figure 7(a)-(d). Bootstrap Configurations for the SC2440A

SC2440A

terval 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 SC2440A to minimize ringing due to trace

inductance.

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 2.4V higher than the SW voltage) is generated with

a bootstrap circuit (the diode D

BST and the capacitor CBST

in Figure 7). The bootstrapped output (the common node

between D

BST and CBST) is connected to the BST pin of the

SC2440A.

The minimum BST to SW voltage required to fully saturate

the power transistor is shown in the “Typical Character-

istics” (pages 8-9). 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 dur-

ing the off interval.

Figure 7 summarizes various ways of bootstrapping the

SC2440A. A fast switching PN diode (such as N448 or

N94) and a small (0.μF – 0.47mF) ceramic capacitor can

be used.

In Figure 7(a) the power switch is bootstrapped from the

output. This is the most efficient configuration and it also

results in the least voltage stress at the BST pin. The maxi-

mum BST pin voltage is about V

IN + VOUT. The minimum

OUT required for this bootstrap configuration is 2.5V. If the

output voltage is 2.5V, then D

BST will preferably be a small

Schottky diode (such as BAT54) to maximize the boot-

strap voltage. A 0.33-0.47mF bootstrap capacitor may also

be needed to reduce droop. Bench measurement shows

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