Kingbor Technology Co.,Ltd
TEL:(86)0755-26508846 FAX:(86)0755-26509052
kB3440
OPERATION
A detailed diagram of this function is shown in Figure 5.
The components RSS and CSS provide a slow ramping
voltage on the SHDN/SS pin to provide a soft-start
function.
SOFT-START
Thesoft-startfunctioniscombinedwithshutdown.When
the SHDN/SS pin is brought above typically 1V, the IC is
enabled but the EA duty cycle is clamped from the VC pin.
ERROR AMP
V
IN
15µA
V
OUT
1.22V
+
–
R1
FB
9
R2
C
P1
V
C
SOFT-START
CLAMP
10
V
CI
TO PWM
COMPARATORS
SHDN/SS
8
R
SS
ENABLE SIGNAL
C
SS
+
–
CHIP
ENABLE
1V
Figure 5. Soft-Start Circuitry
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the kB3440 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
R
kB3440
V
C
1
2
3
4
5
10
9
T
MODE/SYNC
FB
L1
SW1
SW2
8
SHDN/SS
R1
R2
D2
V
7
V
IN
IN
D1
V
GND
6
OUT
V
2 • VOUT – V
(
)
IN(MIN)
IN(MIN)
C1
L >
L >
µH,
2
MULTIPLE
VIAS
f •IOUT(MAX) •Ripple • VOUT
VOUT • VIN(MAX) – VOUT
C2
V
OUT
(
)
µH
GND
f •IOUT(MAX) •Ripple • V
IN(MAX)
where f = operating frequency, MHz
Figure 6. Recommended Component Placement. Traces Carrying
High Current are Direct. Trace Area at FB and VC Pins are Kept
Low. Lead Length to Battery Should be Kept Short
11
KINGBOR [ KINGBOR TECHNOLOGY CO ]
