Kingbor Technology Co.,Ltd
TEL:(86)0755-26508846 FAX:(86)0755-26509052
kB3440
APPLICATIONS INFORMATION
traces and external components. Following the recom-
mendations for output voltage >4.3V and input voltage
>4.5V will improve this condition. Additional short-circuit
protection can be accomplished with some external cir-
cuitry.
1
f
POLE1 5
Hz
2 • / •32e3 •R1• CP1
Whichis extremely close toDC
1
fZERO1
=
Hz
In an overload or short-circuit condition the kB3440
voltage loop opens and the error amp control voltage on
the VC pin slams to the upper clamp level. This condition
forcesboostmodeoperationinordertoattempttoprovide
more output voltage and the IC hits a peak switch current
limitof2.7A.Whenswitchcurrentlimitisreachedswitches
B and D turn on for the remainder of the cycle to reverse
thevolts•secondsontheinductor. Althoughthisprevents
current run away, this condition produces four switch
operation producing a current foldback characteristic and
the average input current drops. The IC is trimmed to
guarantee greater than 1A average input current to meet
the maximum load demand, but in a short-circuit or
overload condition the foldback characteristic will occur
producing higher peak switch currents. To minimize this
affect during this condition the following circuits can be
utilized.
2 • / •RZ •CP1
1
2 • / •R1 •CZ1
fZERO2
=
Hz
1
fPOLE2
=
Hz
2 • / •RZ •CP2
V
OUT
1.22V
+
ERROR
AMP
R1
FB
9
–
C
R2
P1
V
C
10
3440 F07
Figure 7. Error Amplifier with Type I Compensation
Restart Circuit
V
OUT
For a sustained short-circuit the circuit in Figure 9 will
force a soft-start condition. The only design constraint is
that R2/C2 time constant must be longer than the soft-
start components R1/C1 to ensure start-up.
1.22V
+
–
C
Z1
R1
ERROR
AMP
FB
9
C
P1
R2
V
C
R
Z
10
V
IN
C
P2
3440 F08
R1
1M
R2
1M
D1
1N4148
Figure 8. Error Amplifier with Type III Compensation
M2
SOFT-START
SO/SS
NMOS
VN2222
M1
NMOS
VN2222
C1
4.7nF
C2
10nF
V
OUT
Short-Circuit Improvements
The kB3440 is current limited to 2.7A peak to protect the
IC from damage. At input voltages above 4.5V a current
limitconditionmayproduceundesirablevoltagestotheIC
due to the series inductance of the package, as well as the
3440 F09
Figure 9. Soft-Start Reset Circuitry for a Sustained Short-Circuit
14
KINGBOR [ KINGBOR TECHNOLOGY CO ]
