Zxsc380, Gnd v – Diodes ZXSC380 User Manual
Page 7
ZXSC380
© 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
1
1.5
2
2.5
Supply Voltage Vcc (V)
Fr
e
q
u
e
nc
y
(
k
H
z
)
3
L = 100uH
L = 68uH
L = 47uH
L = 22uH
V
CC
V
OUT
GND
V
IN
GND
L1
100uH
D1
C1
D2
LED1
ZXSC380
1
2
3
1uF