ZXSC380

Issue 3 - September 2007

7

www.zetex.com

© Zetex Semiconductors plc 2007

Figure 4 Switching frequency vs the supply voltage for various inductor values

Bootstrap operation

The ZXSC380 can be operated in bootstrap mode, as shown in Figure 5, to operate down to 0.7V

typical after the initial successful start up. The operation down to 0.7V typical allows further cell

energy to be extracted beyond typically quoted end of battery voltage of approx. 0.9. This

prolongs the battery use time. (Note: Some batteries may have low voltage protections and may

switch the supply off.) The schotkky diodes, D1 and D2, should have a very low forward voltage,

e.g. of such part is Zetex ZHCS400 (SOD323 package) or Zetex BAT54C (dual schotkky diode with

common Cathode). The start up voltage is 0.9V typical without bootstrap, however, due to the

additional schotkky, D1, being in the path of the supply to the Vcc pin, the start up voltage can be

1.1V typical for the bootstrap configuration.

Figure 5 ZXSC380 in bootstrap mode

Frequency vs Supply Voltage

for L= 22, 47, 68 & 100uH

0

50

100

150

200

250

300

350

11.5

22.5

Supply Voltage Vcc (V)

3

L = 100uH

L = 68uH

L = 47uH

L = 22uH

GND

GND

L1

100uH

D1

C1

D2

LED1

ZXSC380

1

2

3

1uF